New information technologies are being introduced in all industrial sectors, albeit to varying extents. In some cases, the costs of computerizing production processes may constitute an impediment to innovation, particularly in small and medium-sized companies and in developing countries. Computers make possible the rapid collection, storage, processing and dissemination of large quantities of information. Their utility is further enhanced by their integration into computer networks, which allow resources to be shared (Young 1993).

Computerization exerts significant effects on the nature of employment and on working conditions. Beginning about the mid-1980s, it was recognized that workplace computerization may lead to changes in task structure and work organization, and by extension to work requirements, career planning and stress suffered by production and management personnel. Computerization may exert positive or negative effects on occupational health and safety. In some cases, the introduction of computers has rendered work more interesting and resulted in improvements in the work environment and reductions of workload. In others, however, the result of technological innovation has been an increase in the repetitive nature and intensity of tasks, a reduction of the margin for individual initiative and the isolation of the worker. Furthermore, several companies have been reported to increase the number of work shifts in an attempt to extract the largest possible economic benefit from their financial investment (ILO 1984).

As far as we have been able to determine, as of 1994 statistics on the worldwide use of computers are available from one source only—The Computer Industry Almanac (Juliussen and Petska-Juliussen 1994). In addition to statistics on the current international distribution of computer use, this publication also reports the results of retrospective and prospective analyses. The figures reported in the latest edition indicate that the number of computers is increasing exponentially, with the increase becoming particularly marked at the beginning of the 1980s, the point at which personal computers began to attain great popularity. Since 1987, total computer processing power, measured in terms of the number of million instructions per second executed (MIPS) has increased 14-fold, thanks to the development of new microprocessors (transistor components of microcomputers which perform arithmetical and logical calculations). By the end of 1993, total computing power attained 357 million MIPS.

Unfortunately, available statistics do not differentiate between computers used for work and personal purposes, and statistics are unavailable for some industrial sectors. These knowledge gaps are most likely due to methodological problems related to the collection of valid and reliable data. However, reports of the International Labour Organization’s tripartite sectoral committees contain relevant and comprehensive information on the nature and extent of the penetration of new technologies in various industrial sectors.

In 1986, 66 million computers were in use throughout the world. Three years later, there were more than 100 million, and by 1997, it is estimated that 275–300 million computers will be in use, with this number reaching 400 million by 2000. These predictions assume the widespread adoption of multimedia, information highway, voice recognition and virtual reality technologies. The Almanac’s authors consider that most televisions will be equipped with personal computers within ten years of publication, in order to simplify access to the information highway.

According to the Almanac, in 1993 the overall computer: population ratio in 43 countries in 5 continents was 3.1 per 100. It should however be noted that South Africa was the only African country reporting and that Mexico was the only Central American country reporting. As the statistics indicate, there is a very wide international variation in the extent of computerization, the computer:population ratio ranging from 0.07 per 100 to 28.7 per 100.

The computer:population ratio of less than 1 per 100 in developing countries reflects the generally low level of computerization prevailing there (table 1) (Juliussen and Petska-Juliussen 1994). Not only do these countries produce few computers and little software, but lack of financial resources may in some cases prevent them from importing these products. Moreover, their often rudimentary telephone and electrical utilities are often barriers to more widespread computer use. Finally, little linguistically and culturally appropriate software is available, and training in computer-related fields is often problematic (Young 1993).

Table 1. Distribution of computers in various regions of the world

Source: Juliussen and Petska-Juliussen 1994.

Computerization has significantly increased in the countries of the former Soviet Union since the end of the Cold War. The Russian Federation, for example, is estimated to have increased its stock of computers from 0.3 million in 1989 to 1.2 million in 1993.

The largest concentration of computers is found in the industrialized countries, especially in North America, Australia, Scandinavia and Great Britain (Juliussen and Petska-Juliussen 1994). It was principally in these countries that the first reports of visual display unit (VDU) operators’ fears regarding health risks appeared and the initial research aimed at determining the prevalence of health effects and identifying risk factors undertaken. The health problems studied fall into the following categories: visual and ocular problems, musculoskeletal problems, skin problems, reproductive problems, and stress.

It soon became evident that the health effects observed among VDU operators were dependent not only on screen characteristics and workstation layout, but also on the nature and structure of tasks, organization of work and manner in which the technology was introduced (ILO 1989). Several studies have reported a higher prevalence of symptoms among female VDU operators than among male operators. According to recent studies, this difference is more reflective of the fact that female operators typically have less control over their work than do their male counterparts than of true biological differences. This lack of control is thought to result in higher stress levels, which in turn result in increased symptom prevalence in female VDU operators.

VDUs were first introduced on a widespread basis in the tertiary sector, where they were used essentially for office work, more specifically data entry and word processing. We should not therefore be surprised that most studies of VDUs have focused on office workers. In industrialized countries, however, computerization has spread to the primary and secondary sectors. In addition, although VDUs were used almost exclusively by production workers, they have now penetrated to all organizational levels. In recent years, researchers have therefore begun to study a wider range of VDU users, in an attempt to overcome the lack of adequate scientific information on these situations.

Most computerized workstations are equipped with a VDU and a keyboard or mouse with which to transmit information and instructions to the computer. Software mediates information exchange between the operator and the computer and defines the format with which information is displayed on the screen. In order to establish the potential hazards associated with VDU use, it is first necessary to understand not only the characteristics of the VDU but also those of the other components of the work environment. In 1979, Çakir, Hart and Stewart published the first comprehensive analysis in this field.

It is useful to visualize the hardware used by VDU operators as nested components that interact with each other (IRSST 1984). These components include the terminal itself, the workstation (including work tools and furniture), the room in which the work is carried out, and the lighting. The second article in this chapter reviews the main characteristics of workstations and their lighting. Several recommendations aimed at optimizing working conditions while taking into account individual variations and variations in tasks and work organization are offered. Appropriate emphasis is placed on the importance of choosing equipment and furniture which allow flexible layouts. This flexibility is extremely important in light of international competition and rapidly evolving technological development that are constantly driving companies to introduce innovations and while simultaneously forcing them to adapt to the changes these innovations bring.

The next six articles discuss health problems studied in response to fears expressed by VDU operators. The relevant scientific literature is reviewed and the value and limitations of research results highlighted. Research in this field draws upon numerous disciplines, including epidemiology, ergonomics, medicine, engineering, psychology, physics and sociology. Given the complexity of the problems and more specifically their multifactorial nature, the necessary research has often been conducted by multidisciplinary research teams. Since the 1980s, these research efforts have been complemented by regularly organized international congresses such as Human-Computer Interaction and Work with Display Units, which provide an opportunity to disseminate research results and promote the exchange of information between researchers, VDU designers, VDU producers and VDU users.

The eighth article discusses human-computer interaction specifically. The principles and methods underlying the development and evaluation of interface tools are presented. This article will prove useful not only to production personnel but also those interested in the criteria used to select interface tools.

Finally, the ninth article reviews international ergonomic standards as of 1995, related to the design and layout of computerized workstations. These standards have been produced in order to eliminate the hazards to which VDU operators can be exposed in the course of their work. The standards provide guidelines to companies producing VDU components, employers responsible for the purchase and layout of workstations, and employees with decision-making responsibilities. They may also prove useful as tools with which to evaluate existing workstations and identify modifications required in order to optimize operators’ working conditions.

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Workstation Design

On workstations with visual display units

Visual displays with electronically generated images (visual display units or VDUs) represent the most characteristic element of computerized work equipment both in the workplace and in private life. A workstation may be designed to accommodate just a VDU and an input device (normally a keyboard), as a minimum; however, it can also provide room for diverse technical equipment including numerous screens, input and output devices, etc. As recently as the early 1980s, data entry was the most typical task for computer users. In many industrialized countries, however, this type of work is now performed by a relatively small number of users. More and more, journalists, managers and even executives have become “VDU users”.

Most VDU workstations are designed for sedentary work, but working in standing postures may offer some benefits for the users. Thus, there is some need for generic design guidelines applicable to simple and complex workstations used both while sitting and standing. Such guidelines will be formulated below and then applied to some typical workplaces.

Design guidelines

Workplace design and equipment selection should consider not only the needs of the actual user for a given task and the variability of users’ tasks during the relatively long life cycle of furniture (lasting 15 years or longer), but also factors related to maintenance or change of equipment. ISO Standard 9241, part 5, introduces four guiding principles to be applied to workstation design:

Guideline 1: Versatility and flexibility.

A workstation should enable its user to perform a range of tasks comfortably and efficiently. This guideline takes into account the fact that users’ tasks may vary often; thus, the chance of a universal adoption of guidelines for the workplace will be small.

Guideline 2: Fit.

The design of a workstation and its components should ensure a “fit” to be achieved for a variety of users and a range of task requirements. The concept of fit concerns the extent to which furniture and equipment can accommodate an individual user’s various needs, that is, to remain comfortable, free from visual discomfort and postural strain. If not designed for a specific user population, e.g., male European control room operators younger than 40 years of age, the workstation concept should ensure fit for the entire working population including users with special needs, e.g., handicapped persons. Most existing standards for furniture or the design of workplaces take only parts of the working population into consideration (e.g., “healthy” workers between the 5th and 95th percentile, aged between 16 and 60, as in German standard DIN 33 402), neglecting those who may need more attention.

Moreover, though some design practices are still based on the idea of an “average” user, an emphasis on individual fit is needed. With regard to workstation furniture, the fit required may be achieved by providing adjustability, designing a range of sizes, or even by custom-made equipment. Ensuring a good fit is crucial for the health and safety of the individual user, since musculoskeletal problems associated with the use of VDUs are common and significant.

Guideline 3: Postural change.

The design of the workstation should encourage movement, since static muscular load leads to fatigue and discomfort and may induce chronic musculoskeletal problems. A chair that allows easy movement of the upper half of the body, and provision of sufficient space to place and use paper documents as well as keyboards at varying positions during the day, are typical strategies for facilitating body movement while working with a VDU.

Guideline 4: Maintainability—adaptability.

The design of the workstation should take into consideration factors such as maintenance, accessibility, and the ability of the workplace to adapt to changing requirements, such as the ability to move the work equipment if a different task is to be performed. The objectives of this guideline have not received much attention in the ergonomics literature, because problems related to them are assumed to have been solved before users start to work at a workstation. In reality, however, a workstation is an ever-changing environment, and cluttered workspaces, partly or fully unsuitable for the tasks at hand, are very often not the result of their initial design process but are the outcome of later changes.

Applying the guidelines

Task analysis.

Workplace design should be preceded by a task analysis, which provides information about the primary tasks to be performed at the workstation and the equipment needed for them. In such an analysis, the priority given to information sources (e.g., paper-based documents, VDUs, input devices), the frequency of their use and possible restrictions (e.g., limited space) should be determined. The analysis should include major tasks and their relationships in space and time, visual attention areas (how many visual objects are to be used?) and the position and use of the hands (writing, typing, pointing?).

General design recommendations

Height of the work surfaces.

If fixed-height work surfaces are to be used, the minimum clearance between the floor and the surface should be greater than the sum of the popliteal height (the distance between the floor and the back of the knee) and thigh clearance height (sitting), plus allowance for footwear (25 mm for male users and 45 mm for female users). If the workstation is designed for general use, the popliteal height and thigh clearance height should be selected for the 95th percentile male population. The resulting height for the clearance under the desk surface is 690 mm for the population of Northern Europe and for North American users of European origin. For other populations, the minimum clearance needed is to be determined according to the anthropometric characteristics of the specific population.

If the legroom height is selected this way, the top of the work surfaces will be too high for a large proportion of intended users, and at least 30 per cent of them will need a footrest.

If work surfaces are adjustable in height, the required range for adjustment can be calculated from the anthropometric dimensions of female users (5th or 2.5th percentile for minimum height) and male users (95th or 97.5th percentile for maximum height). A workstation with these dimensions will in general be able to accommodate a large proportion of persons with little or no change. The result of such a calculation yields a range between 600 mm to 800 mm for countries with an ethnically varied user population. Since the technical realization of this range may cause some mechanical problems, best fit can also be achieved, for example, by combining adjustability with different size equipment.

The minimum acceptable thickness of the work surface depends on the mechanical properties of the material. From a technical point of view, a thickness between 14 mm (durable plastic or metal) and 30 mm (wood) is achievable.

Size and form of the work surface.

The size and the form of a work surface are mainly determined by the tasks to be performed and the equipment needed for those tasks.

For data entry tasks, a rectangular surface of 800 mm by 1200 mm provides sufficient space to place the equipment (VDU, keyboard, source documents and copy holder) properly and to rearrange the layout according to personal needs. More complex tasks may require additional space. Therefore, the size of the work surface should exceed 800 mm by 1,600 mm. The depth of the surface should allow placing the VDU within the surface, which means that VDUs with cathode ray tubes may require a depth of up to 1,000 mm.

In principle, the layout displayed in figure 1 gives maximum flexibility for organizing the workspace for various tasks. However, workstations with this layout are not easy to construct. Thus, the best approximation of the ideal layout is as displayed in figure 2. This layout allows arrangements with one or two VDUs, additional input devices and so on. The minimum area of the work surface should be larger than 1.3 m².

Figure 1. Layout of a flexible workstation that can be adapted to fit the needs of users with different tasks

Figure 2. Flexible layout

Arranging the workspace.

The spatial distribution of equipment in the workspace should be planned after a task analysis determining the importance and use frequency of each element has been conducted (table 1). The most frequently used visual display should be located within the central visual space, which is the shaded area of figure 3, while the most important and frequently used controls (such as the keyboard) should be located within optimum reach. In the workplace represented by the task analysis (table 1), the keyboard and the mouse are by far the most frequently handled pieces of equipment. Therefore, they should be given the highest priority within the reach area. Documents which are frequently consulted but do not need much handling should be assigned priority according to their importance (e.g., handwritten corrections). Placing them on the right-hand side of the keyboard would solve the problem, but this would create a conflict with the frequent use of the mouse which is also to be located to the right of the keyboard. Since the VDU may not need adjustment frequently, it can be placed to the right or left of the central field of vision, allowing the documents to be set on a flat document holder behind the keyboard. This is one possible, though not perfect, “optimized” solution.

Table 1. Frequency and importance of elements of equipment for a given task

Figure 3. Visual workplace range

Since many elements of the equipment possess dimensions comparable to corresponding parts of the human body, using various elements within one task will always be associated with some problems. It also may require some movements between parts of the workstation; hence a layout like that shown in figure 1 is important for various tasks.

In the course of the last two decades, computer power that would have needed a ballroom at the beginning was successfully miniaturized and condensed into a simple box. However, contrary to the hopes of many practitioners that miniaturization of equipment would solve most problems associated with workplace layout, VDUs have continued to grow: in 1975, the most common screen size was 15"; in 1995 people bought 17" to 21”:monitors, and no keyboard has become much smaller than those designed in 1973. Carefully performed task analyses for designing complex workstations are still of growing importance. Moreover, although new input devices have emerged, they have not replaced the keyboard, and require even more space on the work surface, sometimes of substantial dimensions, e.g., graphic tablets in an A3-format.

Efficient space management within the limits of a workstation, as well as within work rooms, may help in developing acceptable workstations from an ergonomic point of view, thus preventing the emergence of various health and safety problems.

Efficient space management does not mean saving space at the expense of the usability of input devices and especially vision. Using extra furniture, such as a desk return, or a special monitor-holder clamped to the desk, may appear to be a good way to save desk space; however, it may be detrimental to posture (raised arms) and vision (raising the line of vision upwards from the relaxed position). Space-saving strategies should ensure that an adequate visual distance (approximately 600 mm to 800 mm) is maintained, as well as an optimum line-of-vision, obtained from an inclination of approximately 35º from the horizontal (20º head and 15º eyes).

New furniture concepts.

Traditionally, office furniture was adapted to the needs of businesses, supposedly reflecting the hierarchy of such organizations: large desks for executives working in “ceremonial” offices at one end of the scale, and small typists furniture for “functional” offices at the other. The basic design of office furniture did not change for decades. The situation changed substantially with the introduction of information technology, and a completely new furniture concept has emerged: that of systems furniture.

Systems furniture was developed when people realized that changes in working equipment and work organization could not be matched by the limited capabilities of existing furniture to adapt to new needs. Furniture today offers a tool-box that enables the user organizations to create workspace as needed, from a minimal space for just a VDU and a keyboard up to complex workstations that can accommodate various elements of equipment and possibly also groups of users. Such furniture is designed for change and incorporates efficient and flexible cable management facilities. While the first generation of systems furniture did not do much more than add an auxiliary desk for the VDU to an existing desk, the third generation has completely broken its ties to the traditional office. This new approach offers great flexibility in designing workspaces, limited only by the available space and the abilities of organizations to use this flexibility.

Radiation

Radiation in the context of VDU applications

Radiation is the emission or transfer of radiant energy. The emission of radiant energy in the form of light as the intended purpose for the use of VDUs may be accompanied by various unwanted by-products such as heat, sound, infrared and ultraviolet radiation, radio waves or x rays, to name a few. While some forms of radiation, like visible light, may affect humans in a positive way, some emissions of energy can have negative or even destructive biological effects, especially when the intensity is high and the duration of exposure is long. Some decades ago exposure limits for different forms of radiation were introduced to protect people. However, some of these exposure limits are questioned today, and, for low frequency alternating magnetic fields, no exposure limit can be given based on levels of natural background radiation.

Radiofrequency and microwave radiation from VDUs

Electromagnetic radiation with a frequency range from a few kHz to 10⁹ Hertz (the so-called radiofrequency, or RF, band, with wavelengths ranging from some km to 30 cm) can be emitted by VDUs; however, the total energy emitted depends on the characteristics of the circuitry. In practice, however, the field strength of this type of radiation is likely to be small and confined to the immediate vicinity of the source. A comparison of the strength of alternating electric fields in the range of 20 Hz to 400 kHz indicates that VDUs using cathode ray tube (CRT) technology emit, in general, higher levels than other displays.

“Microwave” radiation covers the region between 3x10⁸ Hz to 3x10¹¹ Hz (wavelengths 100 cm to 1 mm). There are no sources of microwave radiation in VDUs that emit a detectable amount of energy within this band.

Magnetic fields

Magnetic fields from a VDU originate from the same sources as alternating electric fields. Although magnetic fields are not “radiation”, alternating electric and magnetic fields cannot be separated in practice, since one induces the other. One reason why magnetic fields are discussed separately is that they are suspected to have teratogenic effects (see discussion later in this chapter).

Although the fields induced by VDUs are weaker than those induced by some other sources, such as high-voltage power lines, power plants, electrical locomotives, steel ovens and welding equipment, the total exposure produced by VDUs may be similar since people may work eight or more hours in the vicinity of a VDU but seldom near power lines or electric motors. The question of the relationship between electromagnetic fields and cancer, however, is still a matter for debate.

Optical radiation

“Optical” radiation covers visible radiation (i.e., light) with wavelengths from 380 nm (blue) to 780 nm (red), and the neighbouring bands in the electromagnetic spectrum (infrared from 3x10¹¹ Hz to 4x10¹⁴ Hz, wavelengths from 780 nm to 1 mm; ultraviolet from 8x10¹⁴ Hz to 3x10¹⁷ Hz). Visible radiation is emitted at moderate levels of intensity comparable with that emitted by room surfaces (»100 cd/m²). However, ultraviolet radiation is trapped by the glass of the tube face (CRTs) or not emitted at all (other display technologies). Levels of ultraviolet radiation, if detectable at all, stay well below occupational exposure standards, as do those of infrared radiation.

X rays

CRTs are well-known sources of x rays, while other technologies like liquid crystal displays (LCDs) do not emit any. The physical processes behind emissions of this type of radiation are well understood, and tubes and circuitry are designed to keep the emitted levels far below the occupational exposure limits, if not below detectable levels. Radiation emitted by a source can only be detected if its level exceeds the background level. In the case of x rays, as for other ionizing radiation, the background level is provided by cosmic radiation and by radiation from radioactive materials in the ground and in buildings. In normal operation, a VDU does not emit x rays exceeding the background level of radiation (50 nGy/h).

Radiation recommendations

In Sweden, the former MPR (Statens Mät och Provråd, the National Council for Metrology and Testing) organization, now SWEDAC, has worked out recommendations for evaluating VDUs. One of their main objectives was to limit any unwanted by-product to levels that can be achieved by reasonable technical means. This approach goes beyond the classical approach of limiting hazardous exposures to levels where the likelihood of an impairment of health and safety seems to be acceptably low.

At the beginning, some recommendations of MPR led to the unwanted effect of reducing the optical quality of CRT displays. However, at present, only very few products with extremely high resolution may suffer any degradation if the manufacturer attempts to comply with the MPR (now MPR-II). The recommendations include limits for static electricity, magnetic and electric alternating fields, visual parameters, etc.

Image Quality

Definitions for image quality

The term quality describes the fit of distinguishing attributes of an object for a defined purpose. Thus, the image quality of a display includes all properties of the optical representation regarding the perceptibility of symbols in general, and the legibility or readability of alphanumeric symbols. In this sense, optical terms used by tube manufacturers, like resolution or minimum spot size, describe basic quality criteria concerning the abilities of a given device for displaying thin lines or small characters. Such quality criteria are comparable with the thickness of a pencil or brush for a given task in writing or painting.

Some of the quality criteria used by ergonomists describe optical properties that are relevant for legibility, e.g., contrast, while others, like character size or stroke width, refer more to typographical features. In addition, some technology-dependent features like the flicker of images, the persistence of images, or the uniformity of contrast within a given display are also considered in ergonomics (see figure 4).

Figure 4. Criteria for image evaluation

Typography is the art of composing “type”, which is not only shaping the fonts, but also selecting and setting of type. Here, the term typography is used in the first meaning.

Basic characteristics

Resolution.

Resolution is defined as the smallest discernible or measurable detail in a visual presentation. For example, the resolution of a CRT display can be expressed by the maximum number of lines that can be displayed in a given space, as usually done with the resolution of photographic films. One can also describe the minimum spot size that a device can display at a given luminance (brightness). The smaller the minimum spot, the better the device. Thus, the number of dots of minimum size (picture elements—also known as pixels) per inch (dpi) represents the quality of the device, e.g., a 72 dpi device is inferior to a 200 dpi display.

In general, the resolution of most computer displays is well below 100 dpi: some graphic displays may achieve 150 dpi, however, only with limited brightness. This means, if a high contrast is required, the resolution will be lower. Compared with the resolution of print, e.g., 300 dpi or 600 dpi for laser printers, the quality of VDUs is inferior. (An image with 300 dpi has 9 times more elements in the same space than a 100 dpi image.)

Addressability.

Addressability describes the number of individual points in the field that the device is capable of specifying. Addressability, which is very often confused with resolution (sometimes deliberately), is one specification given for devices: “800 x 600” means that the graphic board can address 800 points on every one of 600 horizontal lines. Since one needs at least 15 elements in the vertical direction to write numbers, letters and other characters with ascenders and descenders, such a screen can display a maximum of 40 lines of text. Today, the best available screens can address 1,600 x 1,200 points; however, most displays used in industry address 800 x 600 points or even less.

On displays of the so-called “character-oriented” devices, it is not dots (points) of the screen that are addressed but character boxes. In most such devices, there are 25 lines with 80 character positions each in the display. On these screens, each symbol occupies the same space regardless of its width. In industry the lowest number of pixels in a box is 5 wide by 7 high. This box allows both upper and lower case characters, although the descenders in “p”, “q” and “g”, and the ascenders above “Ä” or “Á” cannot be displayed. Considerably better quality is provided with the 7 x 9 box, which has been “standard” since the mid-1980s. To achieve good legibility and reasonably good character shapes, the character box size should be at least 12 x 16.

Flicker and refresh rate.

The images on CRTs and on some other types of VDU are not persistent images, as on paper. They only appear to be steady by taking advantage of an artefact of the eye. This, however, is not without penalty, since the screen tends to flicker if the image is not refreshed constantly. Flicker can influence both performance and comfort of the user and should always be avoided.

Flicker is the perception of brightness varying over time. The severity of flicker depends on various factors such as the characteristics of the phosphor, size and brightness of the flickering image, etc. Recent research shows that refresh rates up to 90 Hz may be needed to satisfy 99 per cent of users, while in earlier research, refresh rates well below 50 Hz were thought to be satisfactory. Depending on various features of the display, a flicker-free image may be achieved by refresh rates between 70 Hz and 90 Hz; displays with a light background (positive polarity) need a minimum of 80 Hz to be perceived as flicker-free.

Some modern devices offer an adjustable refresh rate; unfortunately, higher refresh rates are coupled with lower resolution or addressability. The ability of a device to display high “resolution” images with high refresh rates can be assessed by its video bandwidth. For displays with high quality, the maximum video bandwidth lies above 150 MHz, while some displays offer less than 40 MHz.

To achieve a flicker-free image and a high resolution with devices with lower video bandwidth, the manufacturers apply a trick that stems from commercial TV: the interlace mode. In this case, every second line on the display is refreshed with a given frequency. The result, however, is not satisfactory if static images, such as text and graphics, are displayed and the refresh rate is below 2 x 45 Hz. Unfortunately, the attempt to suppress the disturbing effect of flicker may induce some other negative effects.

Jitter.

Jitter is the result of spatial instability of the image; a given picture element is not displayed at the same location on the screen after each refresh process. The perception of jitter cannot be separated from the perception of flicker.

Jitter may have its cause in the VDU itself, but it can also be induced by interaction with other equipment at the workplace, such as a printer or other VDUs or devices that generate magnetic fields.

Contrast.

Brightness contrast, the ratio of the luminance of a given object to its surroundings, represents the most important photometric feature for readability and legibility. While most standards require a minimum ratio of 3:1 (bright characters on dark background) or 1:3 (dark characters on bright background), optimum contrast is actually about 10:1, and devices of good quality achieve higher values even in bright environments.

The contrast of “active” displays is impaired when the ambient light is increased, while “passive” displays (e.g., LCDs) lose contrast in dark environments. Passive displays with background lighting may offer good visibility in all environments under which people may work.

Sharpness.

Sharpness of an image is a well-known, but still ill-defined feature. Hence, there is no agreed-upon method to measure sharpness as a relevant feature for legibility and readability.

Typographical features

Legibility and readability.

Readability refers to whether a text is understandable as a series of connected images, while legibility refers to the perception of single or grouped characters. Thus, good legibility is, in general, a precondition for readability.

Legibility of text depends on several factors: some have been investigated thoroughly, while other relevant factors like character shapes are yet to be classified. One of the reasons for this is that the human eye represents a very powerful and robust instrument, and the measures used for performance and error rates often do not help to distinguish between different fonts. Thus, to some extent, typography still remains an art rather than a science.

Fonts and readability.

A font is a family of characters, designed to yield either optimum readability on a given medium, e.g., paper, electronic display or projection display, or some desired aesthetic quality, or both. While the number of available fonts exceeds ten thousand, only a few fonts, numbered in tens, are believed to be “readable”. Since legibility and readability of a font are also affected by the experience of the reader—some “legible” fonts are believed to have become so because of decades or even centuries of use without changing their shape—the same font may be less legible on a screen than on paper, merely because its characters look “new”. This, however, is not the main reason for the poor legibility of screens.

In general, the design of screen fonts is restricted by shortcomings in technology. Some technologies impose very narrow limits on the design of characters, e.g., LEDs or other rastered screens with limited numbers of dots per display. Even the best CRT displays can seldom compete with print (figure 5). In the last years, research has shown that speed and accuracy of reading on screens is about 30% lower than on paper, but whether this is due to features of the display or to other factors is not yet known.

Figure 5. Appearance of a letter at various screen resolutions and on paper (right)

Characteristics with measurable effects.

The effects of some characteristics of alphanumeric representations are measurable, e.g., apparent size of the characters, height/width ratio, stroke width/size ratio, line, word and character spacing.

The apparent size of the characters, measured in minutes of arc, shows an optimum by 20' to 22'; this corresponds to about 3 mm to 3.3 mm in height under normal viewing conditions in offices. Smaller characters may lead to increased errors, visual strain, and also to more postural strain due to restricted viewing distance. Thus, text should not be represented in an apparent size of less than 16'.

However, graphical representations may require text of smaller size to be displayed. To avoid errors, on the one hand, and a high visual load for the user on the other, parts of the text to be edited should be displayed in a separate window to assure good readability. Characters with an apparent size of less than 12' should not be displayed as readable text, but replaced by a rectangular grey block. Good programs allow the user to select the minimum actual size of characters that are to be displayed as alphanumerics.

The optimum height/width ratio of characters is about 1:0.8; legibility is impaired if the ratio is above 1:0.5. For good legible print and also for CRT screens, the ratio of character height to stroke width is about 10:1. However, this is only a rule of thumb; legible characters of high aesthetical value often show different stroke widths (see figure 5).

Optimal line spacing is very important for readability, but also for space saving, if a given amount of information is to be displayed in limited space. The best example for this is the daily newspaper, where an enormous amount of information is displayed within a page, but is still readable. The optimum line spacing is about 20% of character height between the descenders of a line and the ascenders of the next; this is a distance of about 100% of the character height between the baseline of a line of text and the ascenders of the next. If the length of the line is reduced, the space between the lines may be reduced, too, without losing readability.

Character spacing is invariable on character-oriented screens, making them inferior in readability and aesthetic quality to displays with variable space. Proportional spacing depending on the shape and width of the characters is preferable. However, a typographical quality comparable to well-designed printed fonts is achievable only on few displays and when using specific programs.

Ambient Lighting

The specific problems of VDU workstations

During the last 90 years of industrial history, the theories about the lighting of our workplaces have been governed by the notion that more light will improve vision, reduce stress and fatigue, as well as enhance performance. “More light”, correctly speaking “more sunlight”, was the slogan of people in Hamburg, Germany, more than 60 years ago when they took to the streets to fight for better and healthier homes. In some countries like Denmark or Germany, workers today are entitled to have some daylight at their workplaces.

The advent of information technology, with the emergence of the first VDUs in working areas, was presumably the first event ever when workers and scientists began to complain about too much light in working areas. The discussion was fuelled by the easily detectable fact that most VDUs were equipped with CRTs, which have curved glass surfaces prone to veiling reflections. Such devices, sometimes called “active displays”, lose contrast when the level of ambient lighting becomes higher. Redesigning lighting to reduce the visual impairments caused by these effects, however, is complicated by the fact that most users also use paper-based information sources, which generally require increased levels of ambient light for good visibility.

The role of ambient light

Ambient light found in the vicinity of VDU workstations serves two different purposes. First, it illuminates the workspace and working materials like paper, telephones, etc. (primary effect). Secondly, it illuminates the room, giving it its visible shape and giving the users the impression of a light surrounding (secondary effect). Since most lighting installations are planned according to the concept of general lighting, the same lighting sources serve both purposes. The primary effect, illuminating passive visual objects to make them visible or legible, became questionable when people started to use active screens that do not need ambient light to be visible. The remaining benefit of the room lighting was reduced to the secondary effect, if the VDU is the major source of information.

The function of VDUs, both of CRTs (active displays) and of LCDs (passive displays), is impaired by the ambient light in specific ways:

CRTs:

• The curved glass surface reflects bright objects in the environment, and forms a kind of visual “noise”.
• Depending on the intensity of ambient illumination, the contrast of displayed objects is reduced to a degree that readability or legibility of the objects is impaired.
• Images on colour CRTs suffer a twofold degradation: First, the brightness contrast of all displayed objects is reduced, as on monochrome CRTs. Secondly, the colours are changed so that colour contrast is also reduced. In addition, the number of distinguishable colours is reduced.

LCDs (and other passive displays):

• The reflections on LCDs cause less concern than those on CRT surfaces, since these displays have flat surfaces.
• In contrast to active displays, LCDs (without backlight) lose contrast under low levels of ambient illumination.
• Due to poor directional characteristics of some display technologies, visibility or legibility of displayed objects is substantially reduced if the main direction of light incidence is unfavourable.

The extent to which such impairments exert a stress on users or lead to a substantial reduction of visibility/readability/legibility of visual objects in real working environments varies greatly. For example, the contrast of alphanumeric characters on monochrome (CRT) displays is reduced in principle, but, if the illuminance on the screen is ten times higher than in normal working environments, many screens will still have a contrast sufficient to read alphanumeric characters. On the other hand, colour displays of computer-aided design (CAD) systems decrease substantially in visibility so that most users prefer to dim the artificial lighting or even to switch it off, and, in addition, to keep the daylight out of their working area.

Possible remedies

Changing illuminance levels.

Since 1974, numerous studies have been performed which led to recommendations for reducing illuminance at the workplace. However, these recommendations were mostly based on studies with unsatisfactory screens. The recommended levels were between 100 lux and 1,000 lx, and generally, levels well below the recommendations of the existing standards for office lighting (e.g., 200 lx or 300 to 500 lx) have been discussed.

When positive screens with a luminance of approximately 100 cd/m² brightness and some kind of efficient anti-glare treatment are used, the utilization of a VDU does not limit the acceptable illuminance level, since users find illuminance levels up to 1,500 lx acceptable, a value which is very rare in working areas.

If the relevant characteristics of the VDUs do not allow comfortable working under normal office lighting, as can occur when working with storage tubes, microimage readers, colour screens etc., the visual conditions can be improved substantially by introducing two-component lighting. Two-component lighting is a combination of indirect room lighting (secondary effect) and direct task lighting. Both components should be controllable by the users.

Controlling glare on screens.

Controlling glare on screens is a difficult task since almost all remedies that improve the visual conditions are likely to impair some other important characteristic of the display. Some remedies, proposed for many years, such as mesh filters, remove reflections from the displays but they also impair the legibility of the display. Low luminance luminaires cause less reflected glare on screens, but the quality of such lighting generally is judged by users to be worse than that of any other type of lighting.

For this reason, any measures (see figure 6) should be applied cautiously, and only after analysing the real cause of the annoyance or disturbance. Three possible ways of controlling glare on screens are: selection of the correct location of the screen with respect to glare sources; selection of suitable equipment or addition of elements to it; and use of lighting. The costs of the measures to be taken are of the same order: it costs almost nothing to place screens in such a way as to eliminate reflected glare. However, this may not be possible in all cases; thus, equipment-related measures will be more expensive but may be necessary in various working environments. Glare control by lighting is often recommended by lighting specialists; however, this method is the most expensive but not the most successful way of controlling glare.

Figure 6. Strategies for controlling glare on screens

The most promising measure at present is the introduction of positive screens (displays with bright background) with an additional anti-glare treatment for the glass surface. Even more successful than this will be the introduction of flat screens with a nearly matt surface and bright background; such screens, however, are not available for general use today.

Adding hoods to displays is the ultima ratio of the ergonomists for difficult work environments like production areas, towers of airports or operator cabins of cranes, etc. If hoods are really needed, it is likely that there will be more severe problems with lighting than just reflected glare on visual displays.

Changing luminaire design is mainly accomplished in two ways: first, by reducing the luminance (corresponds to apparent brightness) of parts of the light fittings (so called “VDU lighting”), and secondly, by introducing indirect light instead of direct light. The results of current research show that introducing indirect light yields substantial improvements for users, reduces visual load, and is well accepted by users.

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There have been a comparatively large number of studies devoted to visual discomfort in visual display unit (VDU) workers, many of which have yielded contradictory results. From one survey to another, there are discrepancies in reported prevalence of disorders ranging from practically 0 per cent to 80 per cent or more (Dainoff 1982). Such differences should not be considered too surprising because they reflect the large number of variables which can influence complaints of eye discomfort or disability.

Correct epidemiological studies of visual discomfort must take into account several population variables, such as sex, age, eye deficiencies, or use of lenses, as well as socio-economic status. The nature of the job being carried out with the VDU and the characteristics of the workstation layout and of the work organization are also important and many of these variables are interrelated.

Most often, questionnaires have been used to assess the eye discomfort of VDU operators. The prevalence of visual discomfort differs thus with the content of questionnaires and their statistical analysis. Appropriate questions for surveys concern the extent of symptoms of distress asthenopia suffered by VDU operators. Symptoms of this condition are well known and can include itching, redness, burning and tearing of the eyes. These symptoms are related to the fatigue of the accommodative function in the eye. Sometimes this eye symptoms are accompanied by a headache, with the pain located in the front portion of the head. There may also be disturbances in eye function, with symptoms such as double vision and reduced accommodative power. Visual acuity, itself, however, is rarely depressed, provided the conditions of measurement are carried out with a constant pupil size.

If a survey includes general questions, such as “Do you feel well at the end of the working day?” or “Have you ever had visual problems when working with VDUs?” the prevalence of positive responses may be higher than when single symptoms related to asthenopia are evaluated.

Other symptoms may also be strongly associated to asthenopia. Pains in the neck, shoulders and arms are frequently found. There are two main reasons that these symptoms may occur together with eye symptoms. The muscles of the neck participate in keeping a steady distance between eye and screen in VDU work and VDU work has two main components: screen and keyboard, which means that the shoulders and arms and the eyes are all working at the same time and thus may be subject to similar work-related strains.

User Variables Related to Visual Comfort

Sex and Age

In the majority of surveys, women report more eye discomfort than men. In one French study, for example, 35.6% of women complained of eye discomfort, against 21.8% of men (p J 05 significance level) (Dorard 1988). In another study (Sjödren and Elfstrom 1990) it was observed that while the difference in the degree of discomfort between women (41%) and men (24%) was great, it “was more pronounced for those working 5-8 hours a day than for those working 1-4 hours a day”. Such differences are not necessarily sex-related, however, since women and men seldom share similar tasks. For example, in one computer plant studied, when women and men were both occupied in a traditional “woman’s job”, both sexes displayed the same amount of visual discomfort. Furthermore when women worked in traditional “men’s jobs”, they did not report more discomfort than men. In general, regardless of sex, the number of visual complaints among skilled workers who use VDUs on their jobs is much lower than the number of complaints from workers in unskilled, hectic jobs, such as data entry or word processing (Rey and Bousquet 1989). Some of these data are given in table 1.

Table 1. Prevalence of ocular symptoms in 196 VDU operators according to 4 categories

Source: From Dorard 1988 and Rey and Bousquet 1989.

The highest number of visual complaints usually arise in the 40–50-year-old group, probably because this is the time when changes in accommodation ability of the eye are occurring rapidly. However, although older operators are perceived as having more visual complaints than younger workers, and, as a consequence, presbyopia (vision impairment due to ageing) is often cited as the main visual defect associated with visual discomfort at VDU workstations, it is important to consider that there is also a strong association between having acquired advanced skills in VDU work and age. There is usually a higher proportion of older women among unskilled female VDU operators, and younger male workers tend to more often be employed in skilled jobs. Thus before broad generalizations about age and visual problems associated with VDU can be made, the figures should be adjusted to take into account the comparative nature and skill level of the work being done at the VDU.

Eye defects and corrective lenses

In general, about half of all VDU operators display some kind of eye deficiency and most of these people use prescriptive lenses of one type or another. Often VDU user populations do not differ from the working population as far as eye defects and eye correction are concerned. For example, one survey (Rubino 1990) conducted among Italian VDU operators revealed that roughly 46% had normal vision and 38% were nearsighted (myopic), which is consistent with figures observed among Swiss and French VDU operators (Meyer and Bousquet 1990). Estimates of the prevalence of eye defects will vary according to the assessment technique used (Çakir 1981).

Most experts believe that presbyopia itself does not appear to have a significant influence on the incidence of asthenopia (persistent tiredness of the eyes). Rather, the use of unsuitable lenses appears to be likely to induce eye fatigue and discomfort. There is some disagreement about the effects in shortsighted young persons. Rubino has observed no effect while, according to Meyer and Bousquet (1990), myopic operators readily complain of undercorrection for the distance between eye and screen (usually 70 cm). Rubino also has proposed that people who suffer from a deficiency in eye coordination may be more likely to suffer from visual complaints in VDU work.

One interesting observation that resulted from a French study involving a thorough eye examination by ophthalmologists of 275 VDU operators and 65 controls was that 32% of those examined could have their vision improved by good correction. In this study 68% had normal vision, 24% were shortsighted and 8% farsighted (Boissin et al., 1991). Thus, although industrialized countries are, in general, well equipped to provide excellent eye care, eye correction is probably either completely neglected or inappropriate for those working at a VDU. An interesting finding in this study was that more cases of conjunctivitis were found in the VDU operators (48%) than in the controls. Since conjunctivitis and poor eyesight are correlated, this implies that better eye correction is needed.

Physical and Organizational Factors Affecting Visual Comfort

It is clear that in order to assess, correct and prevent visual discomfort in VDU work an approach which takes into account the many different factors described here and elsewhere in this chapter is essential. Fatigue and eye discomfort can be the result of individual physiological difficulties in normal accommodation and convergence in the eye, from conjunctivitis, or from wearing glasses that are poorly corrected for distance. Visual discomfort can be related to the workstation itself and can also be linked to work organization factors such as monotony and time spent on the job with and without a break. Inadequate lighting, reflections on screen, flicker and too much luminance of characters can also increase the risk of eye discomfort. Figure 1 illustrates some of these points.

Figure 1. Factors that increase the risk of eye fatigue among VDU workers

Many of the appropriate characteristics of workstation layout are described more fully earlier in the chapter.

The best viewing distance for visual comfort which still leaves enough space for the keyboard appears to be about 65 cm. However, according to many experts, such as Akabri and Konz (1991), ideally, “it would be best to determine an individual’s dark focus so workstations could be adjusted to specific individuals rather than population means”. As far as the characters themselves go, in general, a good rule of thumb is “bigger is better”. Usually, letter size increases with the size of the screen, and a compromise is always struck between the readability of letters and the number of words and sentences that can be displayed on the screen at one time. The VDU itself should be selected according to the task requirements and should try to maximize user comfort.

In addition to the design of the workstation and the VDU itself is the need to allow the eyes to rest. This is particularly important in unskilled jobs, in which the freedom of “moving around” is generally much lower than in skilled jobs. Data entry work or other activities of the same type are usually performed under time pressure, sometimes even accompanied by electronic supervision, which times operator output very precisely. In other interactive VDU jobs which involve using databases, operators are obliged to wait for a response from the computer and thus must remain at their posts.

Flicker and eye discomfort

Flicker is the change in brightness of the characters on the screen over time and is more fully described above. When characters do not refresh themselves frequently enough, some operators are able to perceive flicker. Younger workers may be more affected since their flicker fusion frequency is higher than that of older people (Grandjean 1987). The rate of flicker increases with increase in brightness, which is one reason why many VDU operators do not commonly make use of the whole range of brightness of the screen that are available. In general a VDU with a refresh rate of at least 70 Hz should “fit” the visual needs of a large proportion of VDU operators.

The sensitivity of the eyes to flicker is enhanced by increased brightness and contrast between the fluctuating area and the surrounding area. The size of the fluctuating area also affects sensitivity because the larger the area to be viewed, the larger the area of the retina that is stimulated. The angle at which the light from the fluctuating area strikes the eye and the amplitude of modulation of the fluctuating area are other important variables.

The older the VDU user, the less sensitive the eye because older eyes are less transparent and the retina is less excitable. This is also true in sick people. Laboratory findings such as these help to explain the observations made in the field. For example, it has been found that operators are disturbed by flicker from the screen when reading paper documents (Isensee and Bennett as quoted in Grandjean 1987), and the combination of fluctuation from the screen and fluctuation of fluorescent light has been found to be particularly disturbing.

Lighting

The eye functions best when the contrast between the visual target and its background is maximum, as for example, with a black letter on white paper. Efficiency is further enhanced when the outer edge of the visual field is exposed to slightly lower levels of brightness. Unfortunately, with a VDU the situation is just the reverse of this, which is one reason that so many VDU operators try to protect their eyes against excess light.

Inappropriate contrasts in brightness and unpleasant reflections produced by fluorescent light, for example, can lead to visual complaints among VDU operators. In one study, 40% of 409 VDU workers made such complaints (Läubli et al., 1989).

In order to minimize problems with lighting, just as with viewing distances, flexibility is important. One should be able to adapt light sources to the visual sensitivity of individuals. Workplaces should be provided to offer individuals the opportunity to adjust their lighting.

Job characteristics

Jobs which are carried out under time pressure, especially if they are unskilled and monotonous, are often accompanied by sensations of general fatigue, which, in turn, can give rise to complaints of visual discomfort. In the authors’ laboratory, it was found that visual discomfort increased with the number of accommodative changes the eyes needed to make to carry out the task. This occurred more often in data entry or word processing than in tasks which involved dialogues with the computer. Jobs which are sedentary and provide little opportunity for moving around also provide less opportunity for muscular recovery and hence enhance the likelihood of visual discomfort.

Job organization

Eye discomfort is just one aspect of the physical and mental problems that can be associated with many jobs, as described more fully elsewhere in this chapter. It is not surprising, therefore, to find a high correlation between the level of eye discomfort and job satisfaction. Although night work is still not widely practised in office work, its effects on eye discomfort in VDU work may well be unexpected. This is because, although there are few data as yet available to confirm this, on the one hand, eye capacity during the night shift may be somehow depressed and thus more vulnerable to VDU effects, while on the other hand, the lighting environment is easier to adjust without disturbance from natural lighting, provided that the reflections from fluorescent lamps on dark windows are eliminated.

Individuals who use VDUs to work at home should ensure that they provide themselves with the appropriate equipment and lighting conditions to avoid the adverse environmental factors found in many formal workplaces.

Medical Surveillance

No single, particular hazardous agent has been identified as a visual risk. Asthenopia among VDU operators appears rather to be an acute phenomenon, although there is some belief that sustained strain of accommodation may occur. Unlike many other chronic diseases, misadjustment to VDU work is usually noticed very soon by the “patient”, who may be more likely to seek medical care than will workers in other workplace situations. After such visits, spectacles are often prescribed, but unfortunately they are sometimes ill adapted to needs of the workplace which have been described here. It is essential that practitioners be specially trained to care for patients who work with VDUs. A special course, for example, has been created at the Swiss Federal Institute of Technology in Zurich just for this purpose.

The following factors must be taken into consideration in caring for VDU workers. In comparison to traditional office work, the distance between the eye and the visual target, the screen, is usually of 50 to 70 cm and cannot be changed. Therefore, lenses should be prescribed which take this steady viewing distance into account. Bifocal lenses are inappropriate because they will require a painful extension of the neck in order for the user to read the screen. Multifocal lenses are better, but as they limit rapid eye movements, their use can lead to more head movements, producing additional strain.

Eye correction should be as precise as possible, taking into account the slightest visual defects (e.g., astigmatism) and also the viewing distance of the VDU. Tinted glasses which reduce the illumination level in the centre of the visual field should not be prescribed. Partially tinted spectacles are not useful, since eyes at the workplace are always moving in all directions. Offering special spectacles to employees, however, should not mean that further complaints of visual discomfort from workers may be ignored since the complaints could be justified by poor ergonomic design of the workstation and equipment.

It should be said, finally, that the operators who suffer the most discomfort are those who need raised illumination levels for detail work and who, at the same time, have a higher glare sensitivity. Operators with undercorrected eyes will thus display a tendency to get closer to the screen for more light and will be in this way more exposed to flicker.

Screening and secondary prevention

The usual principles of secondary prevention in public health are applicable to the working environment. Screening therefore should be targeted towards known hazards and is most useful for diseases with long latency periods. Screening should take place prior to any evidence of preventable disease and only tests with high sensitivity, high specificity and high predictive power are useful. The results of screening examinations can be used to assess the extent of exposure both of individuals and of groups.

Since no severe adverse effects on the eye have ever been identified in VDU work, and since no hazardous level of radiations associated with visual problems have been detected, it has been agreed that there is no indication that work with VDUs “will cause disease or damage to the eye” (WHO 1987). The ocular fatigue and eye discomfort that have been reported to occur in VDU operators are not the kinds of health effect which generally form the basis for medical surveillance in a secondary prevention programme.

However, pre-employment visual medical examinations of VDU operators are widespread in most member countries of the International Labour Organization, a requirement supported by trade unions and employers (ILO 1986). In many European countries (including France, the Netherlands and the United Kingdom), medical surveillance for VDU operators, including ocular tests, has also been instituted subsequent to the issuing of Directive 90/270/EEC on work with display screen equipment.

If a programme for the medical surveillance of VDU operators is to be set up, the following issues must be addressed in addition to deciding on the contents of the screening programme and the appropriate testing procedures:

• What is the meaning of the surveillance and how should its results be interpreted?
• Are all VDU operators in need of the surveillance?
• Are any ocular effects which are observed appropriate for a secondary prevention programme?

Most routine visual screening tests available to the occupational physician have poor sensitivity and predictive power for eye discomfort associated with VDU work (Rey and Bousquet 1990). Snellen visual testing charts are particularly inappropriate for the measurement of visual acuity of VDU operators and for predicting their eye discomfort. In Snellen charts the visual targets are dark, precise letters on a clear, well illuminated background, not at all like typical VDU viewing conditions. Indeed, because of the inapplicability of other methods, a testing procedure has been developed by the authors (the C45 device) which simulates the reading and lighting conditions of a VDU workplace. Unfortunately, this remains for the time being a laboratory set-up. It is important to realise, however, that screening examinations are not a substitute for a well-designed workplace and good work organization.

Ergonomic Strategies to Reduce Visual Discomfort

Although systematic ocular screening and systematic visits to the eye specialist have not been shown to be effective in reducing visual symptomatology, they have been widely incorporated into occupational health programmes for VDU workers. A more cost-effective strategy could include an intensive ergonomic analysis of both the job and the workplace. Workers with known ocular diseases should try to avoid intensive VDU work as much as possible. Poorly corrected vision is another potential cause of operator complaints and should be investigated if such complaints occur. The improvement of the ergonomics of the workplace, which could include providing for a low reading angle to avoid a decreased blinking rate and neck extension, and providing the opportunity to rest and to move about on the job, are other effective strategies. New devices, with separate keyboards, allow distances to be adjusted. The VDU may also be made to be moveable, such as by placing it on a mobile arm. Eye strain will thus be reduced by permitting changes in viewing distance which match the corrections to the eye. Often the steps taken to reduce muscular pain in the arms, shoulders and back will at the same time also allow the ergonomist to reduce visual strain. In addition to the design of equipment, the quality of the air can affect the eye. Dry air leads to dry eyes, so that appropriate humidification is needed.

In general the following physical variables should be addressed:

• the distance between the screen and the eye
• the reading angle, which determines the position of the head and the neck
• the distance to walls and windows
• the quality of paper documents (often very poor)
• luminances of screen and surroundings (for artificial and natural lighting)
• flicker effects
• glare sources and reflections
• the humidity level.

Among the organizational variables that should be addressed in improving visual working conditions are:

• content of the task, responsibility level
• time schedules, night work, duration of work
• freedom to “move around”
• full time or part time jobs, etc.

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The purpose of the experimental studies described here, using animal models is, in part, to answer the question as to whether extremely low frequency (ELF) magnetic field exposures at levels similar to those around VDU workstations can be shown to affect reproductive functions in animals in a manner that can be equated to a human health risk.

The studies considered here are limited to in vivo studies (those performed on live animals) of reproduction in mammals exposed to very low frequency (VLF) magnetic fields with appropriate frequencies, excluding, therefore, studies on the biological effects in general of VLF or ELF magnetic fields. These studies on experimental animals fail to demonstrate unequivocally that magnetic fields, such as are found around VDUs, affect reproduction. Moreover, as can be seen from considering the experimental studies described in some detail below, the animal data do not shed a clear light on possible mechanisms for human reproductive effects of VDU use. These data complement the relative absence of indications of a measurable effect of VDU use on reproductive outcomes from human population studies.

Studies of Reproductive Effects of VLF Magnetic Fields in Rodents

VLF magnetic fields similar to those around VDUs have been used in five teratological studies, three with mice and two with rats. The results of these studies are summarized in table 1. Only one study (Tribukait and Cekan 1987), found an increased number of foetuses with external malformations. Stuchly et al. (1988) and Huuskonen, Juutilainen and Komulainen (1993) both reported a significant increase in the number of foetuses with skeletal abnormalities, but only when the analysis was based on the foetus as a unit. The study by Wiley and Corey (1992) did not demonstrate any effect of magnetic field exposures on placental resorption, or other pregnancy outcomes. Placental resorptions roughly correspond to spontaneous abortions in humans. Finally, Frölén and Svedenstål (1993) performed a series of five experiments. In each experiment, the exposure occurred on a different day. Among the first four experimental subgroups (start day 1–start day 5), there were significant increases in the number of placental resorptions among exposed females. No such effects were seen in the experiment where exposure started on day 7 and which is illustrated in figure 1.

Table 1. Teratological studies with rats or mice exposed to 18-20 kHz saw-tooth formed magnetic fields

¹ Total number of litters in the maximum exposure category.

² Peak-to-peak amplitude.

³ Exposure varied from 7 to 24 hours/day in different experiments.

⁴ “Difference” refers to statistical comparisons between exposed and unexposed animals, “increase”  refers to a comparison of the highest exposed group vs. the unexposed group.

Figure 1. The percentage of female mice with placental resorptions in relation to exposure

The interpretations given by the researchers to their findings include the following. Stuchly and co-workers reported that the abnormalities they observed were not unusual and ascribed the result to “common noise that appears in every teratological evaluation”. Huuskonen et al., whose findings were similar to Stuchly et al., were less negative in their appraisal and considered their result to be more indicative of a real effect, but they too remarked in their report that the abnormalities were “subtle and would probably not impair the later development of the foetuses”. In discussing their findings in which effects were observed in the early onset exposures but not the later ones, Frölén and Svedenstål suggest that the effects observed could be related to early effects on reproduction, before the fertilized egg is implanted in the uterus.

In addition to the reproductive outcomes, a decrease in white and red blood cells were noted in the highest exposure group in the study by Stuchly and co-workers. (Blood cell counts were not analysed in the other studies.) The authors, while suggesting that this could indicate a mild effect of the fields, also noted that the variations in blood cell counts were “within the normal range”. The absence of histological data and the absence of any effects on bone marrow cells made it difficult to evaluate these latter findings.

Interpretation and comparison of studies 

Few of the results described here are consistent with one another. As stated by Frölén and Svedenstål, “qualitative conclusions with regard to corresponding effects in human beings and test animals may not be drawn”. Let us examine some of the reasoning that could lead to such a conclusion.

The Tribukait findings are generally not considered to be conclusive for two reasons. First, the experiment only yielded positive effects when the foetus was used as the unit of observation for statistical analysis, whereas the data themselves actually indicated a litter-specific effect. Second, there is a discrepancy in the study between the findings in the first and the second part, which implies that the positive findings may be the result of random variations and/or uncontrolled factors in the experiment.

Epidemiological studies investigating specific malformations have not observed an increase in skeletal malformations among children born of mothers working with VDUs—and thus exposed to VLF magnetic fields. For these reasons (foetus-based statistical analysis, abnormalities probably not health-related, and lack of concordance with epidemiological findings), the results—on minor skeletal malformations—are not such as to provide a firm indication of a health risk for humans.

Technical Background

Units of observation

When statistically evaluating studies on mammals, consideration must be given to at least one aspect of the (often unknown) mechanism. If the exposure affects the mother—which in turn affects the foetuses in the litter, it is the status of the litter as a whole which should be used as the unit of observation (the effect which is being observed and measured), since the individual outcomes among litter-mates are not independent. If, on the other hand, it is hypothesized that the exposure acts directly and independently on the individual foetuses within the litter, then one can appropriately use the foetus as a unit for statistical evaluation. The usual practice is to count the litter as the unit of observation, unless evidence is available that the effect of the exposure on one foetus is independent of the effect on the other foetuses in the litter.

Wiley and Corey (1992) did not observe a placental resorption effect similar to that seen by Frölén and Svedenstål. One reason put forward for this discrepancy is that different strains of mice were used, and the effect could be specific for the strain used by Frölén and Svedenstål. Apart from such a speculated species effect, it is also noteworthy that both females exposed to 17 μT fields and controls in the Wiley study had resorption frequencies similar to those in exposed females in the corresponding Frölén series, whereas most non-exposed groups in the Frölén study had much lower frequencies (see figure 1). One hypothetical explanation could be that a higher stress level among the mice in the Wiley study resulted from the handling of animals during the three hour period without exposure. If this is the case, an effect of the magnetic field could perhaps have been “drowned” by a stress effect. While it is difficult to definitely dismiss such a theory from the data provided, it does appear somewhat far-fetched. Furthermore, a “real” effect attributable to the magnetic field would be expected to be observable above such a constant stress effect as the magnetic field exposure increased. No such trend was observed in the Wiley study data.

The Wiley study reports on environmental monitoring and on rotation of cages to eliminate the effects of uncontrolled factors which might vary within the room environment itself, as magnetic fields can, while the Frölén study does not. Thus, control of “other factors” is at least better documented in the Wiley study. Hypothetically, uncontrolled factors that were not randomized could conceivably offer some explanations. It is also interesting to note that the lack of effect observed in the day 7 series of the Frölén study appears to be due not to a decrease in the exposed groups, but to an increase in the control group. Thus variations in the control group are probably important to consider while comparing the disparate results of the two studies.

Studies of Reproductive Effects of ELF Magnetic Fields in Rodents

Several studies have been performed, mostly on rodents, with 50–80 Hz fields. Details on six of these studies are shown in table 2. While other studies of ELF have been carried out, their results have not appeared in the published scientific literature and are generally available only as abstracts from conferences. In general the findings are of “random effects”, “no differences observed” and so on. One study, however, found a reduced number of external abnormalities in CD–1 mice exposed to a 20 mT, 50 Hz field but the authors suggested that this might reflect a selection problem. A few studies have been reported on species other than rodents (rhesus monkeys and cows), again apparently without observations of adverse exposure effects.

Table 2. Teratological studies with rats or mice exposed to 15-60 Hz sinusoidal or square pulsed magnetic fields

¹ Number of animals (mothers) in the highest exposure category given unless otherwise noted.

As can be seen from table 2, a wide range of results were obtained. These studies are more difficult to summarize because there are so many variations in exposure regimens, the endpoints under study as well as other factors. The foetus (or the surviving, “culled” pup) was the unit used in most studies. Overall, it is clear that these studies do not show any gross teratogenic effect of magnetic field exposure during pregnancy. As remarked above, “minor skeletal anomalies” do not appear to be of importance when evaluating human risks. The behavioural study results of Salzinger and Freimark (1990) and McGivern and Sokol (1990) are intriguing, but they do not form a basis for indications of human health risks at a VDU workstation, either from the standpoint of procedures (use of the foetus, and, for McGivern, a different frequency) or of effects.

Summary of specific studies

Behavioural retardation 3–4 months after birth was observed in the offspring of exposed females by Salzinger and McGivern. These studies appear to have used individual offspring as the statistical unit, which may be questionable if the stipulated effect is due to an effect on the mother. The Salzinger study also exposed the pups during the first 8 days after birth, so that this study involved more than reproductive hazards. A limited number of litters was used in both studies. Furthermore, these studies cannot be considered to confirm each other’s findings since the exposures varied greatly between them, as can be seen in table 2.

Apart from a behavioural change in the exposed animals, the McGivern study noted an increased weight of some male sex organs: the prostate, the seminal vesicles and the epididymis (all parts of the male reproductive system). The authors speculate as to whether this could be linked to stimulation of some enzyme levels in the prostate since magnetic field effects on some enzymes present in the prostate have been observed for 60 Hz.

Huuskonen and co-workers (1993) noted an increase in the number of foetuses per litter (10.4 foetuses/litter in the 50 Hz exposed group vs. 9 foetuses/litter in the control group). The authors, who had not observed similar trends in other studies, downplayed the importance of this finding by noting that it “may be incidental rather than an actual effect of the magnetic field”. In 1985 Rivas and Rius reported a different finding with a slightly lower number of live births per litter among exposed versus nonexposed groups. The difference was not statistically significant. They carried out the other aspects of their analyses on both a “per foetus” and “per litter” basis. The noted increase in minor skeletal malformations was only seen with the analysis using the foetus as the unit of observation.

Recommendations and Summary

Despite the relative lack of positive, consistent data demonstrating either human or animal reproductive effects, attempts at replications of the results of some studies are still warranted. These studies should attempt to reduce the variations in exposures, methods of analysis and strains of animals used.

In general, the experimental studies performed with 20 kHz magnetic fields have provided somewhat varied results. If adhering strictly to the litter analysis procedure and statistical hypothesis testing, no effects have been shown in rats (although similar nonsignificant findings were made in both studies). In mice, the results have been varied, and no single coherent interpretation of them appears possible at present. For 50 Hz magnetic fields, the situation is somewhat different. Epidemiological studies which are relevant to this frequency are scarce, and one study did indicate a possible risk of miscarriage. By contrast, the experimental animal studies have not produced results with similar outcomes. Overall, the results do not establish an effect of extremely low frequency magnetic fields from VDUs on the outcome of pregnancies. The totality of results fails thus to suggest an effect of VLF or ELF magnetic fields from VDUs on reproduction.

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