Like light, which is visible, ultraviolet radiation (UVR) is a form of optical radiation with shorter wavelengths and more energetic photons (particles of radiation) than its visible counterpart. Most light sources emit some UVR as well. UVR is present in sunlight and is also emitted from a large number of ultraviolet sources used in industry, science and medicine. Workers may encounter UVR in a wide variety of occupational settings. In some instances, at low ambient light levels, very intense near-ultraviolet (“black light”) sources can be seen, but normally UVR is invisible and must be detected by the glow of materials that fluoresce when illuminated by UVR.

Just as light can be divided into colours which can be seen in a rainbow, UVR is subdivided and its components are commonly denoted as UVA, UVB and UVC. Wavelengths of light and UVR are generally expressed in nanometres (nm); 1 nm is one-billionth (10^–9) of a metre. UVC (very short-wavelength UVR) in sunlight is absorbed by the atmosphere and does not reach the Earth’s surface. UVC is available only from artificial sources, such as germicidal lamps, which emit most of their energy at a single wavelength (254 nm) that is very effective in killing bacteria and viruses on a surface or in the air.

UVB is the most biologically damaging UVR to the skin and eye, and although most of this energy (which is a component of sunlight) is absorbed by the atmosphere, it still produces sunburn and other biological effects. Long-wavelength UVR, UVA, is normally found in most lamp sources, and is also the most intense UVR reaching the Earth. Although UVA can penetrate deeply into tissue, it is not as biologically damaging as UVB because the energies of individual photons are less than for UVB or UVC.

Sources of Ultraviolet Radiation

Sunlight

The greatest occupational exposure to UVR is experienced by outdoor workers under sunlight. The energy of solar radiation is greatly attenuated by the earth’s ozone layer, limiting terrestrial UVR to wavelengths greater than 290-295 nm. The energy of the more dangerous short-wavelength (UVB) rays in sunlight is a strong function of the atmospheric slant path, and varies with the season and the time of day (Sliney 1986 and 1987; WHO 1994).

Artificial sources

The most significant artificial sources of human exposure include the following:

Industrial arc welding. The most significant source of potential UVR exposure is the radiant energy of arc-welding equipment. The levels of UVR around arc-welding equipment are very high, and acute injury to the eye and the skin can occur within three to ten minutes of exposure at close viewing distances of a few metres. Eye and skin protection is mandatory.

Industrial/workplace UVR lamps. Many industrial and commercial processes, such as photochemical curing of inks, paints and plastics, involve the use of lamps which strongly emit in the UV range. While the likelihood of harmful exposure is low due to shielding, in some cases accidental exposure can occur.

“Black lights”. Black lights are specialized lamps that emit predominantly in the UV range, and are generally used for non-destructive testing with fluorescent powders, for the authentication of banknotes and documents, and for special effects in advertising and discotheques. These lamps do not pose any significant exposure hazard to humans (except in certain cases to photosensitized skin).

Medical treatment. UVR lamps are used in medicine for a variety of diagnostic and therapeutic purposes. UVA sources are normally used in diagnostic applications. Exposures to the patient vary considerably according to the type of treatment, and UV lamps used in dermatology require careful use by staff members.

Germicidal UVR lamps. UVR with wavelengths in the range 250–265 nm is the most effective for sterilization and disinfection since it corresponds to a maximum in the DNA absorption spectrum. Low-pressure mercury discharge tubes are often used as the UV source, as more than 90% of the radiated energy lies at the 254 nm line. These lamps are often referred to as “germicidal lamps,” “bactericidal lamps” or simply “UVC lamps”. Germicidal lamps are used in hospitals to combat tuberculosis infection, and are also used inside microbiological safety cabinets to inactivate airborne and surface microorganisms. Proper installation of the lamps and the use of eye protection is essential.

Cosmetic tanning. Sunbeds are found in enterprises where clients may obtain a tan by special sun-tanning lamps, which emit primarily in the UVA range but also some UVB. Regular use of a sunbed may contribute significantly to a person’s annual UV skin exposure; furthermore, the staff working in tanning salons may also be exposed to low levels. The use of eye protection such as goggles or sunglasses should be mandatory for the client, and depending upon the arrangement, even staff members may require eye protectors.

General lighting. Fluorescent lamps are common in the workplace and have been used in the home for a long time now. These lamps emit small amounts of UVR and contribute only a few percent to a person’s annual UV exposure. Tungsten-halogen lamps are increasingly used in the home and in the workplace for a variety of lighting and display purposes. Unshielded halogen lamps can emit UVR levels sufficient to cause acute injury at short distances. The fitting of glass filters over these lamps should eliminate this hazard.

Biological Effects

The skin

Erythema

Erythema, or “sunburn”, is a reddening of the skin that normally appears in four to eight hours after exposure to UVR and gradually fades after a few days. Severe sunburn can involve blistering and peeling of the skin. UVB and UVC are both about 1,000 times more effective in causing erythema than UVA (Parrish, Jaenicke and Anderson 1982), but erythema produced by the longer UVB wavelengths (295 to 315 nm) is more severe and persists longer (Hausser 1928). The increased severity and time-course of the erythema results from deeper penetration of these wavelengths into the epidermis. Maximum sensitivity of the skin apparently occurs at approximately 295 nm (Luckiesh, Holladay and Taylor 1930; Coblentz, Stair and Hogue 1931) with much less (approximately 0.07) sensitivity occurring at 315 nm and longer wavelengths (McKinlay and Diffey 1987).

The minimal erythemal dose (MED) for 295 nm that has been reported in more recent studies for untanned, lightly pigmented skin ranges from 6 to 30 mJ/cm² (Everett, Olsen and Sayer 1965; Freeman, et al. 1966; Berger, Urbach and Davies 1968). The MED at 254 nm varies greatly depending upon the elapsed time after exposure and whether the skin has been exposed much to outdoor sunlight, but is generally of the order of 20 mJ/cm², or as high as 0.1 J/cm². Skin pigmentation and tanning, and, most importantly, thickening of the stratum corneum, can increase this MED by at least one order of magnitude.

Photosensitization

Occupational health specialists frequently encounter adverse effects from occupational exposure to UVR in photosensitized workers. The use of certain medicines may produce a photosensitizing effect on exposure to UVA, as may the topical application of certain products, including some perfumes, body lotions and so on. Reactions to photosensitizing agents involve both photoallergy (allergic reaction of the skin) and phototoxicity (irritation of the skin) after UVR exposure from sunlight or industrial UVR sources. (Photosensitivity reactions during the use of tanning equipment are also common.) This photosensitization of the skin may be caused by creams or ointments applied to the skin, by medications taken orally or by injection, or by the use of prescription inhalers (see figure 1 ). The physician prescribing a potentially photosensitizing medication should always warn the patient to take appropriate measures to ensure against adverse effects, but the patient frequently is told only to avoid sunlight and not UVR sources (since these are uncommon for the general population).

Figure 1. Some phonosensitizing substances

Delayed effects

Chronic exposure to sunlight—especially the UVB component—accelerates the ageing of the skin and increases the risk of developing skin cancer (Fitzpatrick et al. 1974; Forbes and Davies 1982; Urbach 1969; Passchier and Bosnjakovic 1987). Several epidemiological studies have shown that the incidence of skin cancer is strongly correlated with latitude, altitude and sky cover, which correlate with UVR exposure (Scotto, Fears and Gori 1980; WHO 1993).

Exact quantitative dose-response relationships for human skin carcinogenesis have not yet been established, although fair-skinned individuals, particularly those of Celtic origin, are much more prone to develop skin cancer. Nevertheless, it must be noted that the UVR exposures necessary to elicit skin tumours in animal models may be delivered sufficiently slowly that erythema is not produced, and the relative effectiveness (relative to the peak at 302 nm) reported in those studies varies in the same way as sunburn (Cole, Forbes and Davies 1986; Sterenborg and van der Leun 1987).

The eye

Photokeratitis and photoconjunctivitis

These are acute inflammatory reactions resulting from exposure to UVB and UVC radiation which appear within a few hours of excessive exposure and normally resolved after one to two days.

Retinal injury from bright light

Although thermal injury to the retina from light sources is unlikely, photochemical damage can occur from exposure to sources rich in blue light. This can result in temporary or permanent reduction in vision. However the normal aversion response to bright light should prevent this occurrence unless a conscious effort is made to stare at bright light sources. The contribution of UVR to retinal injury is generally very small because absorption by the lens limits retinal exposure.

Chronic effects

Long-term occupational exposure to UVR over several decades may contribute to cataract and such non-eye-related degenerative effects as skin ageing and skin cancer associated with sun exposure. Chronic exposure to infrared radiation also can increase the risk of cataract, but this is very unlikely, given access to eye protection.

Actinic ultraviolet radiation (UVB and UVC) is strongly absorbed by the cornea and conjunctiva. Overexposure of these tissues causes keratoconjunctivitis, commonly referred to as “welder’s flash”, “arc-eye” or “snow-blindness”. Pitts has reported the action spectrum and time course of photokeratitis in the human, rabbit and monkey cornea (Pitts 1974). The latent period varies inversely with the severity of exposure, ranging from 1.5 to 24 hours, but usually occurs within 6 to 12 hours; discomfort usually disappears within 48 hours. Conjunctivitis follows and may be accompanied by erythema of the facial skin surrounding the eyelids. Of course, UVR exposure rarely results in permanent ocular injury. Pitts and Tredici (1971) reported threshold data for photokeratitis in humans for wavebands 10 nm in width from 220 to 310 nm. The maximum sensitivity of the cornea was found to occur at 270 nm—differing markedly from the maximum for the skin. Presumably, 270 nm radiation is biologically more active because of the lack of a stratum corneum to attenuate the dose to the corneal epithelium tissue at shorter UVR wavelengths. The wavelength response, or action spectrum, did not vary as greatly as did the erythema action spectra, with thresholds varying from 4 to 14 mJ/cm² at 270 nm. The threshold reported at 308 nm was approximately 100 mJ/cm².

Repeated exposure of the eye to potentially hazardous levels of UVR does not increase the protective capability of the affected tissue (the cornea) as does skin exposure, which leads to tanning and to thickening of the stratum corneum. Ringvold and associates studied the UVR absorption properties of the cornea (Ringvold 1980a) and aqueous humour (Ringvold 1980b), as well as the effects of UVB radiation upon the corneal epithelium (Ringvold 1983), the corneal stroma (Ringvold and Davanger 1985) and the corneal endothelium (Ringvold, Davanger and Olsen 1982; Olsen and Ringvold 1982). Their electron microscopic studies showed that corneal tissue possessed remarkable repair and recovery properties. Although one could readily detect significant damage to all of these layers apparently appearing initially in cell membranes, morphological recovery was complete after a week. Destruction of keratocytes in the stromal layer was apparent, and endothelial recovery was pronounced despite the normal lack of rapid cell turnover in the endothelium. Cullen et al. (1984) studied endothelial damage that was persistent if the UVR exposure was persistent. Riley et al. (1987) also studied the corneal endothelium following UVB exposure and concluded that severe, single insults were not likely to have delayed effects; however, they also concluded that chronic exposure could accelerate changes in the endothelium related to ageing of the cornea.

Wavelengths above 295 nm can be transmitted through the cornea and are almost totally absorbed by the lens. Pitts, Cullen and Hacker (1977b) showed that cataracts can be produced in rabbits by wavelengths in the 295–320 nm band. Thresholds for transient opacities ranged from 0.15 to 12.6 J/cm², depending on wavelength, with a minimum threshold at 300 nm. Permanent opacities required greater radiant exposures. No lenticular effects were noted in the wavelength range of 325 to 395 nm even with much higher radiant exposures of 28 to 162 J/cm² (Pitts, Cullen and Hacker 1977a; Zuclich and Connolly 1976). These studies clearly illustrate the particular hazard of the 300-315 nm spectral band, as would be expected because photons of these wavelengths penetrate efficiently and have sufficient energy to produce photochemical damage.

Taylor et al. (1988) provided epidemiological evidence that UVB in sunlight was an aetiological factor in senile cataract, but showed no correlation of cataract with UVA exposure. Although once a popular belief because of the strong absorption of UVA by the lens, the hypothesis that UVA can cause cataract has not been supported by either experimental laboratory studies or by epidemiological studies. From the laboratory experimental data which showed that thresholds for photokeratitis were lower than for cataractogenesis, one must conclude that levels lower than those required to produce photokeratitis on a daily basis should be considered hazardous to lens tissue. Even if one were to assume that the cornea is exposed to a level nearly equivalent to the threshold for photokeratitis, one would estimate that the daily UVR dose to the lens at 308 nm would be less than 120 mJ/cm² for 12 hours out of doors (Sliney 1987). Indeed, a more realistic average daily exposure would be less than half that value.

Ham et al. (1982) determined the action spectrum for photoretinitis produced by UVR in the 320–400 nm band. They showed that thresholds in the visible spectral band, which were 20 to 30 J/cm² at 440 nm, were reduced to approximately 5 J/cm² for a 10 nm band centred at 325 nm. The action spectrum was increasing monotonically with decreasing wavelength. We should therefore conclude that levels well below 5 J/cm² at 308 nm should produce retinal lesions, although these lesions would not become apparent for 24 to 48 hours after the exposure. There are no published data for retinal injury thresholds below 325 nm, and one can only expect that the pattern for the action spectrum for photochemical injury to the cornea and lens tissues would apply to the retina as well, leading to an injury threshold of the order of 0.1 J/cm².

Although UVB radiation has been clearly shown to be mutagenic and carcinogenic to the skin, the extreme rarity of carcinogenesis in the cornea and conjunctiva is quite remarkable. There appears to be no scientific evidence to link UVR exposure with any cancers of the cornea or conjunctiva in humans, although the same is not true of cattle. This would suggest a very effective immune system operating in the human eye, since there are certainly outdoor workers who receive a UVR exposure comparable to that which cattle receive. This conclusion is further supported by the fact that individuals suffering from a defective immune response, as in xeroderma pigmentosum, frequently develop neoplasias of the cornea and conjunctiva (Stenson 1982).

Safety Standards

Occupational exposure limits (EL) for UVR have been developed and include an action spectrum curve which envelops the threshold data for acute effects obtained from studies of minimal erythema and keratoconjunctivitis (Sliney 1972; IRPA 1989). This curve does not differ significantly from the collective threshold data, considering measurement errors and variations in individual response, and is well below the UVB cataractogenic thresholds.

The EL for UVR is lowest at 270 nm (0.003 J/cm² at 270 nm), and, for example, at 308 nm is 0.12 J/cm² (ACGIH 1995, IRPA 1988). Regardless of whether the exposure occurs from a few pulsed exposures during the day, a single very brief exposure, or from an 8-hour exposure at a few microwatts per square centimetre, the biological hazard is the same, and the above limits apply to the full workday.

Occupational Protection

Occupational exposure to UVR should be minimized where practical. For artificial sources, wherever possible, priority should be given to engineering measures such as filtration, shielding and enclosure. Administrative controls, such as limitation of access, can reduce the requirements for personal protection.

Outdoor workers such as agricultural workers, labourers, construction workers, fishermen and so on can minimize their risk from solar UV exposure by wearing appropriate tightly woven clothing, and most important, a brimmed hat to reduce face and neck exposure. Sunscreens can be applied to exposed skin to reduce further exposure. Outdoor workers should have access to shade and be provided with all the necessary protective measures mentioned above.

In industry, there are many sources capable of causing acute eye injury within a short exposure time. A variety of eye protection is available with various degrees of protection appropriate to the intended use. Those intended for industrial use include welding helmets (additionally providing protection both from intense visible and infrared radiation as well as face protection), face shields, goggles and UV-absorbing spectacles. In general, protective eyewear provided for industrial use should fit snugly on the face, thus ensuring that there are no gaps through which UVR can directly reach the eye, and they should be well-constructed to prevent physical injury.

The appropriateness and selection of protective eyewear is dependent on the following points:

• the intensity and spectral emission characteristics of the UVR source

• the behavioural patterns of people near UVR sources (distance and exposure time are important)

• the transmission properties of the protective eyewear material

• the design of the frame of the eyewear to prevent peripheral exposure of the eye from direct unabsorbed UVR.

In industrial exposure situations, the degree of ocular hazard can be assessed by measurement and comparison with recommended limits for exposure (Duchene, Lakey and Repacholi 1991).

Measurement

Because of the strong dependence of biological effects on wavelength, the principal measurement of any UVR source is its spectral power or spectral irradiance distribution. This must be measured with a spectroradiometer which consists of suitable input optics, a monochromator and a UVR detector and readout. Such an instrument is not normally used in occupational hygiene.

In many practical situations, a broad-band UVR meter is used to determine safe exposure durations. For safety purposes, the spectral response can be tailored to follow the spectral function used for the exposure guidelines of the ACGIH and the IRPA. If appropriate instruments are not used, serious errors of hazard assessment will result. Personal UVR dosimeters are also available (e.g., polysulphone film), but their application has been largely confined to occupational safety research rather than in hazard evaluation surveys.

Conclusions

Molecular damage of key cellular components arising from UVR exposure occurs constantly, and repair mechanisms exist to deal with the exposure of skin and ocular tissues to ultraviolet radiation. Only when these repair mechanisms are overwhelmed does acute biological injury become apparent (Smith 1988). For these reasons, minimizing occupational UVR exposure continues to remain an important object of concern among occupational health and safety workers.

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Infrared radiation is that part of the non-ionizing radiation spectrum located between microwaves and visible light. It is a natural part of the human environment and thus people are exposed to it in small amounts in all areas of daily life—for example, at home or during recreational activities in the sun. Very intense exposure, however, may result from certain technical processes at the workplace.

Many industrial processes involve thermal curing of various kinds of materials. The heat sources used or the heated material itself will usually emit such high levels of infrared radiation that a large number of workers are potentially at risk of being exposed.

Concepts and Quantities

Infrared radiation (IR) has wavelengths ranging from 780 nm to 1 mm. Following the classification by the International Commission on Illumination (CIE), this band is subdivided into IRA (from 780 nm to 1.4 μm), IRB (from 1.4 μm to 3 μm) and IRC (from 3 μm to 1 mm). This subdivision approximately follows the wavelength-dependent absorption characteristics of IR in tissue and the resulting different biological effects.

The amount and the temporal and spatial distribution of infrared radiation are described by different radiometric quantities and units. Due to optical and physiological properties, especially of the eye, a distinction is usually made between small “point” sources and “extended” sources. The criterion for this distinction is the value in radians of the angle (α) measured at the eye that is subtended by the source. This angle can be calculated as a quotient, the light source dimension D_L divided by the viewing distance r. Extended sources are those which subtend a viewing angle at the eye greater than α_min, which normally is 11 milliradians. For all extended sources there is a viewing distance where α equals α_min; at greater viewing distances, the source can be treated like a point source. In optical radiation protection the most important quantities concerning extended sources are the radiance (L, expressed in Wm^–2sr^–1) and the time-integrated radiance (L_p in Jm^–2sr^–1), which describe the “brightness” of the source. For health risk assessment, the most relevant quantities concerning point sources or exposures at such distances from the source where α< α_min, are the irradiance (E, expressed in Wm^–2), which is equivalent to the concept of exposure dose rate, and the radiant exposure (H, in Jm^–2), equivalent to the exposure dose concept.

In some bands of the spectrum, the biological effects due to exposure are strongly dependent on wavelength. Therefore, additional spectroradiometric quantities must be used (e.g., the spectral radiance, L_l, expressed in Wm^–2 sr^–1 nm^–1) to weigh the physical emission values of the source against the applicable action spectrum related to the biological effect.

Sources and Occupational Exposure

Exposure to IR results from various natural and artificial sources. The spectral emission from these sources may be limited to a single wavelength (laser) or may be distributed over a broad wavelength band.

The different mechanisms for the generation of optical radiation in general are:

• thermal excitation (black-body radiation)

• gas discharge

• light amplification by stimulated emission of radiation (laser), with the mechanism of gas discharge being of lesser importance in the IR band.

The emission from the most important sources used in many industrial processes results from thermal excitation, and can be approximated using the physical laws of black-body radiation if the absolute temperature of the source is known. The total emission (M, in Wm^–2) of a black-body radiator (figure 1) is described by the Stefan-Boltzmann law:

M(T) = 5.67 x 10^-8T⁴

and depends on the 4th power of the temperature (T, in K) of the radiating body. The spectral distribution of the radiance is described by Planck’s radiation law:

and the wavelength of maximum emission (λ_max) is described according to Wien’s law by:

λ_max = (2.898 x 10^-8) / T

Figure 1. Spectral radiance λ_maxof a black body radiator at the absolute temperature shown in degrees Kelvin on each curve

Many lasers used in industrial and medical processes will emit very high levels of IR. In general, compared with other radiation sources, laser radiation has some unusual features that may influence the risk following an exposure, such as very short pulse duration or extremely high irradiance. Therefore, laser radiation is discussed in detail elsewhere in this chapter.

Many industrial processes require the use of sources emitting high levels of visible and infrared radiation, and thus a large number of workers like bakers, glass blowers, kiln workers, foundry workers, blacksmiths, smelters and fire-fighters are potentially at risk of exposure. In addition to lamps, such sources as flames, gas torches, acetylene torches, pools of molten metal and incandescent metal bars must be considered. These are encountered in foundries, steel mills and in many other heavy industrial plants. Table 1 summarizes some examples of IR sources and their applications.

Table 1. Different sources of IR, population exposed and approximate exposure levels

Biological Effects

Optical radiation in general does not penetrate very deeply into biological tissue. Therefore, the primary targets of an IR exposure are the skin and the eye. Under most exposure conditions the main interaction mechanism of IR is thermal. Only the very short pulses that lasers may produce, but which are not considered here, can also lead to mechanothermal effects. Effects from ionization or from the breakage of chemical bonds are not expected to appear with IR radiation because the particle energy, being less than approximately 1.6 eV, is too low to cause such effects. For the same reason, photochemical reactions become significant only at shorter wavelengths in the visual and in the ultraviolet region. The different wavelength-dependent health effects of IR arise mainly from the wavelength-dependent optical properties of tissue—for example, the spectral absorption of the ocular media (figure 2).

Figure 2. Spectral absorption of the ocular media

Effects on the eye

In general, the eye is well adapted to protect itself against optical radiation from the natural environment. In addition, the eye is physiologically protected against injury from bright light sources, such as the sun or high intensity lamps, by an aversion response that limits the duration of exposure to a fraction of a second (approximately 0.25 seconds).

IRA affects primarily the retina, because of the transparency of the ocular media. When directly viewing a point source or laser beam, the focusing properties in the IRA region additionally render the retina much more susceptible to damage than any other part of the body. For short exposure periods, heating of the iris from the absorption of visible or near IR is considered to play a role in the development of opacities in the lens.

With increasing wavelength, above approximately 1 μm, the absorption by ocular media increases. Therefore, absorption of IRA radiation by both the lens and the pigmented iris is considered to play a role in the formation of lenticular opacities. Damage to the lens is attributed to wavelengths below 3 μm (IRA and IRB). For infrared radiation of wavelengths longer than 1.4 μm, the aqueous humour and the lens are particularly strongly absorbent.

In the IRB and IRC region of the spectrum, the ocular media become opaque as a result of the strong absorption by their constituent water. Absorption in this region is primarily in the cornea and in the aqueous humour. Beyond 1.9 μm, the cornea is effectively the sole absorber. The absorption of long wavelength infrared radiation by the cornea may lead to increased temperatures in the eye due to thermal conduction. Because of a quick turnover rate of the surface corneal cells, any damage limited to the outer corneal layer can be expected to be temporary. In the IRC band the exposure can cause a burn on the cornea similar to that on the skin. Corneal burns are not very likely to occur, however, because of the aversion reaction triggered by the painful sensation caused by strong exposure.

Effects on the skin

Infrared radiation will not penetrate the skin very deeply. Therefore, exposure of the skin to very strong IR may lead to local thermal effects of different severity, and even serious burns. The effects on the skin depend on the optical properties of the skin, such as wavelength-dependent depth of penetration (figure 3 ). Especially at longer wavelengths, an extensive exposure may cause a high local temperature rise and burns. The threshold values for these effects are time dependent, because of the physical properties of the thermal transport processes in the skin. An irradiation of 10 kWm^–2, for example, may cause a painful sensation within 5 seconds, whereas an exposure of 2 kWm^–2 will not cause the same reaction within periods shorter than approximately 50 seconds.

Figure 3. Depth of penetration into the skin for different wavelengths

If the exposure is extended over very long periods, even at values well below the pain threshold, the burden of heat to the human body may be great. Especially if the exposure covers the whole body as, for example, in front of a steel melt. The result may be an imbalance of the otherwise physiologically well balanced thermoregulation system. The threshold for tolerating such an exposure will depend on different individual and environmental conditions, such as the individual capacity of the thermoregulation system, the actual body metabolism during exposure or the environmental temperature, humidity and air movement (wind speed). Without any physical work, a maximum exposure of 300 Wm^–2 may be tolerated over eight hours under certain environmental conditions, but this value decreases to approximately 140 Wm^–2 during heavy physical work.

Exposure Standards

The biological effects of IR exposure which are dependent on wavelength and on the duration of exposure, are intolerable only if certain threshold intensity or dose values are exceeded. To protect against such intolerable exposure conditions, international organizations such as the World Health Organization (WHO), the International Labour Office (ILO), the International Committee for Non-Ionizing Radiation of the International Radiation Protection Association (INIRC/IRPA), and its successor, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the American Conference of Governmental Industrial Hygienists (ACGIH) have suggested exposure limits for infrared radiation from both coherent and incoherent optical sources. Most of the national and international suggestions on guidelines for limiting human exposure to infrared radiation are either based on or even identical with the suggested threshold limit values (TLVs) published by the ACGIH (1993/1994). These limits are widely recognized and are frequently used in occupational situations. They are based on current scientific knowledge and are intended to prevent thermal injury of the retina and cornea and to avoid possible delayed effects on the lens of the eye.

The 1994 revision of the ACGIH exposure limits is as follows:

1. For the protection of the retina from thermal injury in case of exposure to visible light, (for example, in the case of powerful light sources), the spectral radiance L_λ in W/(m² sr nm) weighted against the retinal thermal hazard function R_λ (see table 2) over the wavelength interval Δ_λ and summed over the range of wavelength 400 to 1400 nm, should not exceed:

where t is the viewing duration limited to intervals from 10^-3 to 10 seconds (that is, for accidental viewing conditions, not fixated viewing), and α is the angular subtense of the source in radians calculated by α = maximum extension of the source/distance to the source R_λ  (table 2 ).

2. To protect the retina from the exposure hazards of infrared heat lamps or any near IR source where a strong visual stimulus is absent, the infrared radiance over the wavelength range 770 to 1400 nm as viewed by the eye (based on a 7 mm pupil diameter) for extended duration of viewing conditions should be limited to:

This limit is based on a pupil diameter of 7 mm since, in this case, the aversion response (closing the eye, for example) may not exist due to the absence of visible light.

3. To avoid possible delayed effects on the lens of the eye, such as delayed cataract, and to protect the cornea from overexposure, the infrared radiation at wavelengths greater than 770 nm should be limited to 100 W/m² for periods greater than 1,000 s and to:

or for shorter periods.

4. For aphakic patients, separate weighting functions and resulting TLVs are given for the wavelength range of ultraviolet and visible light (305–700 nm).

Table 2. Retinal thermal hazard function

Source: ACGIH 1996.

Measurement

Reliable radiometric techniques and instruments are available that make it possible to analyse the risk to the skin and the eye from exposure to sources of optical radiation. For characterizing a conventional light source, it is generally very useful to measure the radiance. For defining hazardous exposure conditions from optical sources, the irradiance and the radiant exposure are of greater importance. The evaluation of broad-band sources is more complex than the evaluation of sources that emit at single wavelengths or very narrow bands, since spectral characteristics and source size must be considered. The spectrum of certain lamps consists of both a continuum emission over a wide wavelength band and emission on certain single wavelengths (lines). Significant errors may be introduced into the representation of those spectra if the fraction of energy in each line is not properly added to the continuum.

For health-hazard assessment the exposure values must be measured over a limiting aperture for which the exposure standards are specified. Typically a 1 mm aperture has been considered to be the smallest practical aperture size. Wavelengths greater than 0.1 mm present difficulties because of significant diffraction effects created by a 1 mm aperture. For this wavelength band an aperture of 1 cm² (11 mm diameter) was accepted, because hot spots in this band are larger than at shorter wavelengths. For the evaluation of retinal hazards, the size of the aperture was determined by an average pupil size and therefore an aperture of 7 mm was chosen.

In general, measurements in the optical region are very complex. Measurements taken by untrained personnel may lead to invalid conclusions. A detailed summary of measurement procedures is to be found in Sliney and Wolbarsht (1980).

Protective Measures

The most effective standard protection from exposure to optical radiation is the total enclosure of the source and all of the radiation pathways that may exit from the source. By such measures, compliance with the exposure limits should be easy to achieve in the majority of cases. Where this is not the case, personal protection is applicable. For example, available eye protection in the form of suitable goggles or visors or protective clothing should be used. If the work conditions will not allow for such measures to be applied, administrative control and restricted access to very intense sources may be necessary. In some cases a reduction of either the power of the source or the working time (work pauses to recover from heat stress), or both, might be a possible measure to protect the worker.

Conclusion

In general, infrared radiation from the most common sources such as lamps, or from most industrial applications, will not cause any risk to workers. At some workplaces, however, IR can cause a health risk for the worker. In addition, there is a rapid increase in the application and use of special-purpose lamps and in high temperature processes in industry, science and medicine. If the exposure from those applications is sufficiently high, detrimental effects (mainly in the eye but also on the skin) cannot be excluded. The importance of internationally recognized optical radiation exposure standards is expected to increase. To protect the worker from excessive exposure, protective measures like shielding (eye shields) or protective clothing should be mandatory.

The principal adverse biological effects attributed to infrared radiation are cataracts, known as glass blower’s or furnaceman’s cataracts. Long-term exposure even at relatively low levels causes heat stress to the human body. At such exposure conditions additional factors such as body temperature and evaporative heat loss as well as environmental factors must be considered.

In order to inform and instruct workers some practical guides were developed in industrial countries. A comprehensive summary can be found in Sliney and Wolbarsht (1980).

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Light and infrared (IR) radiant energy are two forms of optical radiation, and together with ultraviolet radiation, they form the optical spectrum. Within the optical spectrum, different wavelengths have considerably different potentials for causing biological effects, and for this reason the optical spectrum may be further subdivided.

The term light should be reserved for wavelengths of radiant energy between 400 and 760 nm, which evoke a visual response at the retina (CIE 1987). Light is the essential component of the output of illuminating lamps, visual displays and a wide variety of illuminators. Aside from the importance of illumination for seeing, some light sources may, however, pose unwanted physiological reactions such as disability and discomfort glare, flicker and other forms of eye stress due to poor ergonomic design of workplace tasks. The emission of intense light is also a potentially hazardous side-effect of some industrial processes, such as arc welding.

Infrared radiation (IRR, wavelengths 760 nm to 1 mm) may also be referred to quite commonly as thermal radiation (or radiant heat), and is emitted from any warm object (hot engines, molten metals and other foundry sources, heat-treated surfaces, incandescent electric lamps, radiant heating systems, etc.). Infrared radiation is also emitted from a large variety of electrical equipment such as electric motors, generators, transformers and various electronic equipment.

Infrared radiation is a contributory factor in heat stress. High ambient air temperature and humidity and a low degree of air circulation can combine with radiant heat to produce heat stress with the potential for heat injuries. In cooler environments, unwelcome or poorly designed sources of radiant heat can also produce discomfort—an ergonomic consideration.

Biological Effects

Occupational hazards presented to the eye and skin by visible and infrared forms of radiation are limited by the eye’s aversion to bright light and the pain sensation in the skin resulting from intense radiant heating. The eye is well-adapted to protect itself against acute optical radiation injury (due to ultraviolet, visible or infrared radiant energy) from ambient sunlight. It is protected by a natural aversion response to viewing bright light sources that normally protects it against injury arising from exposure to sources such as the sun, arc lamps and welding arcs, since this aversion limits the duration of exposure to a fraction (about two-tenths) of a second. However, sources rich in IRR without a strong visual stimulus can be hazardous to the lens of the eye in the case of chronic exposure. One can also force oneself to stare at the sun, a welding arc or a snow field and thereby suffer a temporary (and sometimes a permanent) loss of vision. In an industrial setting in which bright lights appear low in the field of view, the eye’s protective mechanisms are less effective, and hazard precautions are particularly important.

There are at least five separate types of hazards to the eye and skin from intense light and IRR sources, and protective measures must be chosen with an understanding of each. In addition to the potential hazards presented by ultraviolet radiation (UVR) from some intense light sources, one should consider the following hazards (Sliney and Wolbarsht 1980; WHO 1982):

1. Thermal injury to the retina, which can occur at wavelengths from 400 nm to 1,400 nm. Normally the danger of this type of injury is posed only by lasers, a very intense xenon-arc source or a nuclear fireball. The local burning of the retina results in a blind spot (scotoma).
2. Blue-light photochemical injury to the retina (a hazard principally associated with blue light of wavelengths from 400 nm to 550 nm) (Ham 1989). The injury is commonly termed “blue light” photoretinitis; a particular form of this injury is named, according to its source, solar retinitis. Solar retinitis was once referred to as “eclipse blindness” and associated “retinal burn”. Only in recent years has it become clear that photoretinitis results from a photochemical injury mechanism following exposure of the retina to shorter wavelengths in the visible spectrum, namely, violet and blue light. Until the 1970s, it was thought to be the result of a thermal injury mechanism. In contrast to blue light, IRA radiation is very ineffective in producing retinal injuries. (Ham 1989; Sliney and Wolbarsht 1980).
3. Near-infrared thermal hazards to the lens (associated with wavelengths of approximately 800 nm to 3,000 nm) with potential for industrial heat cataract. The average corneal exposure to infrared radiation in sunlight is of the order of 10 W/m². By comparison, glass and steel workers exposed to infrared irradiances of the order of 0.8 to 4 kW/m² daily for 10 to 15 years have reportedly developed lenticular opacities (Sliney and Wolbarsht 1980). These spectral bands include IRA and IRB (see figure 1). The American Conference of Governmental Industrial Hygienists (ACGIH) guideline for IRA exposure of the anterior of the eye is a time-weighted total irradiance of 100 W/m² for exposure durations exceeding 1,000 s (16.7 min) (ACGIH 1992 and 1995).
4. Thermal injury of the cornea and conjunctiva (at wavelengths of approximately 1,400 nm to 1 mm). This type of injury is almost exclusively limited to exposure to laser radiation.
5. Thermal injury of the skin. This is rare from conventional sources but can occur across the entire optical spectrum.

The importance of wavelength and time of exposure

Thermal injuries (1) and (4) above are generally limited to very brief exposure durations, and eye protection is designed to prevent these acute injuries. However, photochemical injuries, such as are mentioned in (2) above, can result from low dose rates spread over the entire workday. The product of the dose rate and the exposure duration always results in the dose (it is the dose that governs the degree of photochemical hazard). As with any photochemical injury mechanism, one must consider the action spectrum which describes the relative effectiveness of different wavelengths in causing a photobiological effect. For example, the action spectrum for photochemical retinal injury peaks at approximately 440 nm (Ham 1989). Most photochemical effects are limited to a very narrow range of wavelengths; whereas a thermal effect can occur at any wavelength in the spectrum. Hence, eye protection for these specific effects need block only a relatively narrow spectral band in order to be effective. Normally, more than one spectral band must be filtered in eye protection for a broad-band source.

Sources of Optical Radiation

Sunlight

The greatest occupational exposure to optical radiation results from exposure of outdoor workers to the sun’s rays. The solar spectrum extends from the stratospheric ozone-layer cut-off of about of 290-295 nm in the ultraviolet band to at least 5,000 nm (5 μm) in the infrared band. Solar radiation can attain a level as high as 1 kW/m² during the summer months. It can result in heat stress, depending upon ambient air temperature and humidity.

Artificial sources

The most significant artificial sources of human exposure to optical radiation include the following:

1. Welding and cutting. Welders and their co-workers are typically exposed not only to intense UV radiation, but also to intense visible and IR radiation emitted from the arc. Under rare instances, these sources have produced acute injury to the retina of the eye. Eye protection is mandatory for these environments.
2. Metals industries and foundries. The most significant source of visible and infrared exposure are from molten and hot metal surfaces in the steel and aluminium industries and in foundries. Worker exposure typically ranges from 0.5 to 1.2 kW/m².
3. Arc lamps. Many industrial and commercial processes, such as those involving photochemical curing lamps, emit intense, short-wave visible (blue) light as well as UV and IR radiation. While the likelihood of harmful exposure is low due to shielding, in some cases accidental exposure can occur.
4. Infrared lamps. These lamps emit predominantly in the IRA range and are generally used for heat treatment, paint drying and related applications. These lamps do not pose any significant exposure hazard to humans since the discomfort produced upon exposure will limit exposure to a safe level.
5. Medical treatment. Infrared lamps are used in physical medicine for a variety of diagnostic and therapeutic purposes. Exposures to the patient vary considerably according to the type of treatment, and IR lamps require careful use by staff members.
6. General lighting. Fluorescent lamps emit very little infrared and are generally not bright enough to pose a potential hazard to the eye. Tungsten and tungsten-halogen incandescent lamps emit a large fraction of their radiant energy in the infrared. Additionally, the blue light emitted by tungsten-halogen lamps can pose a retinal hazard if a person stares at the filament. Fortunately, the eye’s aversion response to bright light prevents acute injury even at short distances. Placing glass “heat” filters over these lamps should minimize/eliminate this hazard.
7. Optical projectors and other devices. Intense light sources are used in searchlights, film projectors and other light-beam collimating devices. These may pose a retinal hazard with the direct beam at very close distances.

Measurement of Source Properties

The most important characteristic of any optical source is its spectral power distribution. This is measured using a spectroradiometer, which consists of suitable input optics, a monochromator and a photodetector.

In many practical situations, a broad-band optical radiometer is used to select a given spectral region. For both visible illumination and safety purposes, the spectral response of the instrument will be tailored to follow a biological spectral response; for example, lux-meters are geared to the photopic (visual) response of the eye. Normally, aside from UVR hazard meters, the measurement and hazard analysis of intense light sources and infrared sources is too complex for routine occupational health and safety specialists. Progress is being made in standardizations of safety categories of lamps, so that measurements by the user will not be required in order to determine potential hazards.

Human Exposure Limits

From knowledge of the optical parameters of the human eye and the radiance of a light source, it is possible to calculate irradiances (dose rates) at the retina. Exposure of the anterior structures of the human eye to infrared radiation may also be of interest, and it should be further borne in mind that the relative position of the light source and the degree of lid closure can greatly affect the proper calculation of an ocular exposure dose. For ultraviolet and short-wavelength light exposures, the spectral distribution of the light source is also important.

A number of national and international groups have recommended occupational exposure limits (ELs) for optical radiation (ACGIH 1992 and 1994; Sliney 1992). Although most such groups have recommended ELs for UV and laser radiation, only one group has recommended ELs for visible radiation (i.e., light), namely, the ACGIH, an agency well-known in the field of occupational health. The ACGIH refers to its ELs as threshold limit values, or TLVs, and as these are issued yearly, there is an opportunity for a yearly revision (ACGIH 1992 and 1995). They are based in large part on ocular injury data from animal studies and from data from human retinal injuries resulting from viewing the sun and welding arcs. TLVs are furthermore based on the underlying assumption that outdoor environmental exposures to visible radiant energy are normally not hazardous to the eye except in very unusual environments, such as snow fields and deserts, or when one actually fixes the eyes on the sun.

Optical Radiation Safety Evaluation

Since a comprehensive hazard evaluation requires complex measurements of spectral irradiance and radiance of the source, and sometimes very specialized instruments and calculations as well, it is rarely carried out onsite by industrial hygienists and safety engineers. Instead, the eye protective equipment to be deployed is mandated by safety regulations in hazardous environments. Research studies evaluated a wide range of arcs, lasers and thermal sources in order to develop broad recommendations for practical, easier-to-apply safety standards.

Protective Measures

Occupational exposure to visible and IR radiation is seldom hazardous and is usually beneficial. However, some sources emit a considerable amount of visible radiation, and in this case, the natural aversion response is evoked, so there is little chance of accidental overexposure of the eyes. On the other hand, accidental exposure is quite likely in the case of artificial sources emitting only near-IR radiation. Measures which can be taken to minimize the unnecessary exposure of staff to IR radiation include proper engineering design of the optical system in use, wearing appropriate goggles or face visors, limiting access to persons directly concerned with the work, and ensuring that workers are aware of the potential hazards associated with exposure to intense visible and IR radiation sources. Maintainance staff who replace arc lamps must have adequate training so as to preclude hazardous exposure. It is unacceptable for workers to experience either skin erythema or photokeratitis. If these conditions do occur, working practices should be examined and steps taken to ensure that overexposure is made unlikely in the future. Pregnant operators are at no specific risk to optical radiation as regards the integrity of their pregnancy.

Eye protector design and standards

The design of eye protectors for welding and other operations presenting sources of industrial optical radiation (e.g., foundry work, steel and glass manufacture) started at the beginning of this century with the development of Crooke’s glass. Eye protector standards which evolved later followed the general principle that since infrared and ultraviolet radiation are not needed for vision, those spectral bands should be blocked as best as possible by currently available glass materials.

The empirical standards for eye protective equipment were tested in the 1970s and were shown to have included large safety factors for infrared and ultraviolet radiation when the transmission factors were tested against current occupational exposure limits, whereas the protection factors for blue light were just sufficient. Some standards’ requirements were therefore adjusted.

Ultraviolet and infrared radiation protection

A number of specialized UV lamps are used in industry for fluorescence detection and for photocuring of inks, plastic resins, dental polymers and so on. Although UVA sources normally pose little risk, these sources may either contain trace amounts of hazardous UVB or pose a disability glare problem (from fluorescence of the eye’s crystalline lens). UV filter lenses, glass or plastic, with very high attenuation factors are widely available to protect against the entire UV spectrum. A slight yellowish tint may be detectable if protection is afforded to 400 nm. It is of paramount importance for this type of eyewear (and for industrial sunglasses) to provide protection for the peripheral field of vision. Side shields or wraparound designs are important to protect against the focusing of temporal, oblique rays into the nasal equatorial area of the lens, where cortical cataract frequently originates.

Almost all glass and plastic lens materials block ultraviolet radiation below 300 nm and infrared radiation at wavelengths greater than 3,000 nm (3 μm), and for a few lasers and optical sources, ordinary impact-resistant clear safety eyewear will provide good protection (e.g., clear polycarbonate lenses effectively block wavelengths greater than 3 μm). However, absorbers such as metal oxides in glass or organic dyes in plastics must be added to eliminate UV up to about 380–400 nm, and infrared beyond 780 nm to 3 μm. Depending upon the material, this may be either easy or very difficult or expensive, and the stability of the absorber may vary somewhat. Filters that meet the American National Standards Institute’s ANSI Z87.1 standard must have the appropriate attenuation factors in each critical spectral band.

Protection in various industries

Fire-fighting

Fire-fighters may be exposed to intense near-infrared radiation, and aside from the crucially important head and face protection, IRR attenuating filters are frequently prescribed. Here, impact protection is also important.

Foundry and glass industry eyewear

Spectacles and goggles designed for ocular protection against infrared radiation generally have a light greenish tint, although the tint may be darker if some comfort against visible radiation is desired. Such eye protectors should not be confused with the blue lenses used with steel and foundry operations, where the objective is to check the temperature of the melt visually; these blue spectacles do not provide protection, and should be worn only briefly.

Welding

Infrared and ultraviolet filtration properties can be readily imparted to glass filters by means of additives such as iron oxide, but the degree of strictly visible attenuation determines the shade number, which is a logarithmic expression of attenuation. Normally a shade number of 3 to 4 is used for gas welding (which calls for goggles), and a shade number of 10 to 14 for arc welding and plasma arc operations (here, helmet protection is required). The rule of thumb is that if the welder finds the arc comfortable to view, adequate attenuation is provided against ocular hazards. Supervisors, welder’s helpers and other persons in the work area may require filters with a relatively low shade number (e.g., 3 to 4) to protect against photokeratitis (“arc eye” or “welder’s flash”). In recent years a new type of welding filter, the autodarkening filter has appeared on the scene. Regardless of the type of filter, it should meet ANSI Z87.1 and Z49.1 standards for fixed welding filters specified for dark shade (Buhr and Sutter 1989; CIE 1987).

Autodarkening welding filters

The autodarkening welding filter, whose shade number increases with the intensity of the optical radiation impinging upon it, represents an important advance in the ability of welders to produce consistently high-quality welds more efficiently and ergonomically. Formerly, the welder had to lower and raise the helmet or filter each time an arc was started and quenched. The welder had to work “blind” just prior to striking the arc. Furthermore, the helmet is commonly lowered and raised with a sharp snap of the neck and head, which can lead to neck strain or more serious injuries. Faced with this uncomfortable and cumbersome procedure, some welders frequently initiate the arc with a conventional helmet in the raised position—leading to photokeratitis. Under normal ambient lighting conditions, a welder wearing a helmet fitted with an autodarkening filter can see well enough with the eye protection in place to perform tasks such as aligning the parts to be welded, precisely positioning the welding equipment and striking the arc. In the most typical helmet designs, light sensors then detect the arc flash virtually as soon as it appears and direct an electronic drive unit to switch a liquid crystal filter from a light shade to a preselected dark shade, eliminating the need for the clumsy and hazardous manoeuvres practised with fixed-shade filters.

The question has frequently been raised whether hidden safety problems may develop with autodarkening filters. For example, can afterimages (“flash blindness”) experienced in the workplace result in permanently impaired vision? Do the new types of filter really offer a degree of protection that is equivalent or better than that which conventional fixed filters can provide? Although one can answer the second question in the affirmative, it must be understood that not all autodarkening filters are equivalent. Filter reaction speeds, the values of the light and dark shades achieved under a given intensity of illumination, and the weight of each unit may vary from one pattern of equipment to another. The temperature dependence of the unit’s performance, the variation in the degree of shade with electrical battery degradation, the “resting state shade” and other technical factors vary depending upon each manufacturer’s design. These considerations are being addressed in new standards.

Since adequate filter attenuation is afforded by all systems, the single most important attribute specified by the manufacturers of autodarkening filters is the speed of filter switching. Current autodarkening filters vary in switching speed from one tenth of a second to faster than 1/10,000th of a second. Buhr and Sutter (1989) have indicated a means of specifying the maximum switching time, but their formulation varies relative to the time-course of switching. Switching speed is crucial, since it gives the best clue to the all-important (but unspecified) measure of how much light will enter the eye when the arc is struck as compared with the light admitted by a fixed filter of the same working shade number. If too much light enters the eye for each switching during the day, the accumulated light-energy dose produces “transient adaptation” and complaints about “eye strain” and other problems. (Transient adaptation is the visual experience caused by sudden changes in one’s light environment, which may be characterized by discomfort, a sensation of having been exposed to glare and temporary loss of detailed vision.) Current products with switching speeds of the order of ten milliseconds will better provide adequate protection against photoretinitis. However, the shortest switching time—of the order of 0.1 ms—has the advantage of reducing transient adaptation effects (Eriksen 1985; Sliney 1992).

Simple check tests are available to the welder short of extensive laboratory testing. One might suggest to the welder that he or she simply look at a page of detailed print through a number of autodarkening filters. This will give an indication of each filter’s optical quality. Next, the welder may be asked to try striking an arc while observing it through each filter being considered for purchase. Fortunately, one can rely on the fact that light levels which are comfortable for viewing purposes will not be hazardous. The effectiveness of UV and IR filtration should be checked in the manufacturer’s specification sheet to make sure that unnecessary bands are filtered out. A few repeated arc strikings should give the welder a sense of whether discomfort will be experienced from transient adaptation, although a one-day trial would be best.

The resting or failure state shade number of an autodarkening filter (a failure state occurs when the battery fails) should provide 100% protection for the welder’s eyes for at least one to several seconds. Some manufacturers use a dark state as the “off” position and others use an intermediate shade between the dark and the light shade states. In either case, the resting state transmittance for the filter should be appreciably lower than the light shade transmittance in order to preclude a retinal hazard. In any case, the device should provide a clear and obvious indicator to the user as to when the filter is switched off or when a system failure occurs. This will ensure that the welder is warned in advance in case the filter is not switched on or is not operating properly before welding is begun. Other features, such as battery life or performance under extreme temperature conditions may be of importance to certain users.

Conclusions

Although technical specifications can appear to be somewhat complex for devices that protect the eye from optical radiation sources, safety standards exist which specify shade numbers, and these standards provide a conservative safety factor for the wearer.

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A laser is a device which produces coherent electromagnetic radiant energy within the optical spectrum from the extreme ultraviolet to the far infrared (submillimetre). The term laser is actually an acronym for light amplification by stimulated emission of radiation. Although the laser process was theoretically predicted by Albert Einstein in 1916, the first successful laser was not demonstrated until 1960. In recent years lasers have found their way from the research laboratory to the industrial, medical and office setting as well as construction sites and even households. In many applications, such as videodisk players and optical fibre communication systems, the laser’s radiant energy output is enclosed, the user faces no health risk, and the presence of a laser embedded in the product may not be obvious to the user. However, in some medical, industrial or research applications, the laser’s emitted radiant energy is accessible and may pose a potential hazard to the eye and skin.

Because the laser process (sometimes referred to as “lasing”) can produce a highly collimated beam of optical radiation (i.e., ultraviolet, visible or infrared radiant energy), a laser can pose a hazard at a considerable distance—quite unlike most hazards encountered in the workplace. Perhaps it is this characteristic more than anything else that has led to special concerns expressed by workers and by occupational health and safety experts. Nevertheless, lasers can be used safely when appropriate hazard controls are applied. Standards for the safe use of lasers exist worldwide, and most are “harmonized” with each other (ANSI 1993; IEC 1993). All of the standards make use of a hazard classification system, which groups laser products into one of four broad hazard classes according to the laser’s output power or energy and its ability to cause harm. Safety measures are then applied commensurate to the hazard classification (Cleuet and Mayer 1980; Duchene, Lakey and Repacholi 1991).

Lasers operate at discrete wavelengths, and although most lasers are monochromatic (emitting one wavelength, or single colour), it is not uncommon for a laser to emit several discrete wavelengths. For example, the argon laser emits several different lines within the near ultraviolet and visible spectrum, but is generally designed to emit only one green line (wavelength) at 514.5 nm and/or a blue line at 488 nm. When considering potential health hazards, it is always crucial to establish the output wavelength(s).

All lasers have three fundamental building blocks:

1. an active medium (a solid, liquid or gas) that defines the possible emission wavelengths
2. an energy source (e.g., electric current, pump lamp or chemical reaction)
3. a resonant cavity with output coupler (generally two mirrors).

Most practical laser systems outside of the research laboratory also have a beam delivery system, such as an optical fibre or articulated arm with mirrors to direct the beam to a work station, and focusing lenses to concentrate the beam on a material to be welded, etc. In a laser, identical atoms or molecules are brought to an excited state by energy delivered from the pump lamp. When the atoms or molecules are in an excited state, a photon (“particle” of light energy) can stimulate an excited atom or molecule to emit a second photon of the same energy (wavelength) travelling in phase (coherent) and in the same direction as the stimulating photon. Thus light amplification by a factor of two has taken place. This same process repeated in a cascade causes a light beam to develop that reflects back and forth between the mirrors of the resonant cavity. Since one of the mirrors is partially transparent, some light energy leaves the resonant cavity forming the emitted laser beam. Although in practice, the two parallel mirrors are often curved to produce a more stable resonant condition, the basic principle holds for all lasers.

Although several thousand different laser lines (i.e., discrete laser wavelengths characteristic of different active media) have been demonstrated in the physics laboratory, only 20 or so have been developed commercially to the point where they are routinely applied in everyday technology. Laser safety guidelines and standards have been developed and published which basically cover all wavelengths of the optical spectrum in order to allow for currently known laser lines and future lasers.

Laser Hazard Classification

Current laser safety standards throughout the world follow the practice of categorizing all laser products into hazard classes. Generally, the scheme follows a grouping of four broad hazard classes, 1 through 4. Class 1 lasers cannot emit potentially hazardous laser radiation and pose no health hazard. Classes 2 through 4 pose an increasing hazard to the eye and skin. The classification system is useful since safety measures are prescribed for each class of laser. More stringent safety measures are required for the highest classes.

Class 1 is considered an “eye-safe”, no-risk grouping. Most lasers that are totally enclosed (for example, laser compact disc recorders) are Class 1. No safety measures are required for a Class 1 laser.

Class 2 refers to visible lasers that emit a very low power that would not be hazardous even if the entire beam power entered the human eye and was focused on the retina. The eye’s natural aversion response to viewing very bright light sources protects the eye against retinal injury if the energy entering the eye is insufficient to damage the retina within the aversion response. The aversion response is composed of the blink reflex (approximately 0.16–0.18 second) and a rotation of the eye and movement of the head when exposed to such bright light. Current safety standards conservatively define the aversion response as lasting 0.25 second. Thus, Class 2 lasers have an output power of 1 milliwatt (mW) or less that corresponds to the permissible exposure limit for 0.25 second. Examples of Class 2 lasers are laser pointers and some alignment lasers.

Some safety standards also incorporate a subcategory of Class 2, referred to as “Class 2A”. Class 2A lasers are not hazardous to stare into for up to 1,000 s (16.7 min). Most laser scanners used in point-of-sales (super-market checkout) and inventory scanners are Class 2A.

Class 3 lasers pose a hazard to the eye, since the aversion response is insufficiently fast to limit retinal exposure to a momentarily safe level, and damage to other structures of the eye (e.g., cornea and lens) could also take place. Skin hazards normally do not exist for incidental exposure. Examples of Class 3 lasers are many research lasers and military laser rangefinders.

A special subcategory of Class 3 is termed “Class 3A” (with the remaining Class 3 lasers termed “Class 3B”). Class 3A lasers are those with an output power between one and five times the accessible emission limits (AEL) for the Class 1 or Class 2, but with an output irradiance not exceeding the relevant occupational exposure limit for the lower class. Examples are many laser alignment and surveying instruments.

Class 4 lasers may pose a potential fire hazard, a significant skin hazard or a diffuse-reflection hazard. Virtually all surgical lasers and material processing lasers used for welding and cutting are Class 4 if not enclosed. All lasers with an average power output exceeding 0.5 W are Class 4. If a higher power Class 3 or Class 4 is totally enclosed so that hazardous radiant energy is not accessible, the total laser system could be Class 1. The more hazardous laser inside the enclosure is termed an embedded laser.

Occupational Exposure Limits

The International Commission on Non-Ionizing Radiation Protection (ICNIRP 1995) has published guidelines for human exposure limits for laser radiation that are periodically updated. Representative exposure limits (ELs) are provided in table 1 for several typical lasers. Virtually all laser beams exceed permissible exposure limits. Thus, in actual practice, the exposure limits are not routinely used to determine safety measures. Instead, the laser classification scheme—which is based upon the ELs applied under realistic conditions—is really applied to this end.

Table 1. Exposure limits for typical lasers

All standards/guidelines have MPE’s at other wavelengths and exposure durations.

Note: To convert MPE’s in mW/cm² to mJ/cm², multiply by exposure time t in seconds. For example, the He-Ne or Argon MPE at 0.1 s is 0.32 mJ/cm².

Source: ANSI Standard Z-136.1(1993); ACGIH TLVs (1995) and Duchene, Lakey and Repacholi (1991).

Laser Safety Standards

Many nations have published laser safety standards, and most are harmonized with the international standard of the International Electrotechnical Commission (IEC). IEC Standard 825-1 (1993) applies to manufacturers; however, it also provides some limited safety guidance for users. The laser hazard classification described above must be labelled on all commercial laser products. A warning label appropriate to the class should appear on all products of Classes 2 through 4.

Safety Measures

The laser safety classification system greatly facilitates the determination of appropriate safety measures. Laser safety standards and codes of practice routinely require the use of increasingly more restrictive control measures for each higher classification.

In practice, it is always more desirable to totally enclose the laser and beam path so that no potentially hazardous laser radiation is accessible. In other words, if only Class 1 laser products are employed in the workplace, safe use is assured. However, in many situations, this is simply not practical, and worker training in safe use and hazard control measures is required.

Other than the obvious rule—not to point a laser at a person’s eyes—there are no control measures required for a Class 2 laser product. For lasers of higher classes, safety measures are clearly required.

If total enclosure of a Class 3 or 4 laser is not feasible, the use of beam enclosures (e.g., tubes), baffles and optical covers can virtually eliminate the risk of hazardous ocular exposure in most cases.

When enclosures are not feasible for Class 3 and 4 lasers, a laser controlled area with controlled entry should be established, and the use of laser eye protectors is generally mandated within the nominal hazard zone (NHZ) of the laser beam. Although in most research laboratories where collimated laser beams are used, the NHZ encompasses the entire controlled laboratory area, for focused beam applications, the NHZ may be surprisingly limited and not encompass the entire room.

To assure against misuse and possible dangerous actions on the part of unauthorized laser users, the key control found on all commercially manufactured laser products should be utilized.

The key should be secured when the laser is not in use, if people can gain access to the laser.

Special precautions are required during laser alignment and initial set-up, since the potential for serious eye injury is very great then. Laser workers must be trained in safe practices prior to laser set-up and alignment.

Laser-protective eyewear was developed after occupational exposure limits had been established, and specifications were drawn up to provide the optical densities (or ODs, a logarithmic measure of the attenuation factor) that would be needed as a function of wavelength and exposure duration for specific lasers. Although specific standards for laser eye protection exist in Europe, further guidelines are provided in the United States by the American National Standards Institute under the designations ANSI Z136.1 and ANSI Z136.3.

Training

When investigating laser accidents in both laboratory and industrial situations, a common element emerges: lack of adequate training. Laser safety training should be both appropriate and sufficient for the laser operations around which each employee will work. Training should be specific to the type of laser and the task to which the worker is assigned.

Medical Surveillance

Requirements for medical surveillance of laser workers vary from country to country in accordance with local occupational medicine regulations. At one time, when lasers were confined to the research laboratory and little was known about their biological effects, it was quite typical that each laser worker was periodically given a thorough general ophthalmological examination with fundus (retinal) photography to monitor the status of the eye. However, by the early 1970s, this practice was questioned, since the clinical findings were almost always negative, and it became clear that such exams could identify only acute injury which was subjectively detectable. This led the WHO task group on lasers, meeting in Don Leaghreigh, Ireland, in 1975, to recommend against such involved surveillance programmes and to emphasize testing of visual function. Since that time, most national occupational health groups have continuously reduced medical examination requirements. Today, complete ophthalmological examinations are universally required only in the event of a laser eye injury or suspected overexposure, and pre-placement visual screening is generally required. Additional examinations may be required in some countries.

Laser Measurements

Unlike some workplace hazards, there is generally no need to perform measurements for workplace monitoring of hazardous levels of laser radiation. Because of the highly confined beam dimensions of most laser beams, the likelihood of changing beam paths and the difficulty and expense of laser radiometers, current safety standards emphasize control measures based upon hazard class and not workplace measurement (monitoring). Measurements must be performed by the manufacturer to assure compliance with laser safety standards and proper hazard classification. Indeed, one of the original justifications for laser hazard classification related to the great difficulty of performing proper measurements for hazard evaluation.

Conclusions

Although the laser is relatively new to the workplace, it is rapidly becoming ubiquitous, as are programmes concerned with laser safety. The keys to the safe use of lasers are first to enclose the laser radiant energy if at all possible, but if not possible, to set up adequate control measures and to train all personnel working with lasers.

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