When pollutants generated at a worksite are to be controlled by ventilating the entire locale we speak of general ventilation. The use of general ventilation implies accepting the fact that the pollutant will be distributed to some degree through the entire space of the worksite, and could therefore affect workers who are far from the source of contamination. General ventilation is, therefore, a strategy that is the opposite of localized extraction. Localized extraction seeks to eliminate the pollutant by intercepting it as closely as possible to the source (see “Indoor air: methods for control and cleaning”, elsewhere in this chapter).

One of the basic objectives of any general ventilation system is the control of body odours. This can be achieved by supplying no less than 0.45 cubic metres per minute, m³/min, of new air per occupant. When smoking is frequent or the work is physically strenuous, the rate of ventilation required is greater, and may surpass 0.9 m³/min per person.

If the only environmental problems that the ventilation system must overcome are the ones just described, it is a good idea to keep in mind that every space has a certain level of “natural” air renewal by means of so-called “infiltration,” which occurs through doors and windows, even when they are closed, and through other sites of wall penetration. Air-conditioning manuals usually provide ample information in this regard, but it can be said that as a minimum the level of ventilation due to infiltration falls between 0.25 and 0.5 renewals per hour. An industrial site will commonly experience between 0.5 and 3 renewals of air per hour.

When used to control chemical pollutants, general ventilation must be limited to only those situations where the amounts of pollutants generated are not very high, where their toxicity is relatively moderate and where workers do not carry out their tasks in the immediate vicinity of the source of contamination. If these injunctions are not respected, it will be difficult to obtain acceptance for adequate control of the work environment because such high renewal rates must be used that the high air speeds will likely create discomfort, and because high renewal rates are expensive to maintain. It is therefore unusual to recommend the use of general ventilation for the control of chemical substances except in the case of solvents which have admissible concentrations of more than 100 parts per million.

When, on the other hand, the goal of general ventilation is to maintain the thermal characteristics of the work environment with a view to legally acceptable limits or technical recommendations such as the International Organization for Standardization (ISO) guidelines, this method has fewer limitations. General ventilation is therefore used more often to control the thermal environment than to limit chemical contamination, but its usefulness as a complement of localized extraction techniques should be clearly recognized.

While for many years the phrases general ventilation and ventilation by dilution were considered synonymous, today that is no longer the case because of a new general ventilation strategy: ventilation by displacement. Even though ventilation by dilution and ventilation by displacement fit within the definition of general ventilation we have outlined above, they both differ widely in the strategy they employ to control contamination.

Ventilation by dilution has the goal of mixing the air that is introduced mechanically as completely as possible with all the air that is already within the space, so that the concentration of a given pollutant will be as uniform as possible throughout (or so that the temperature will be as uniform as possible, if thermal control is the goal desired). To achieve this uniform mixture air is injected from the ceiling as streams at a relatively high speed, and these streams generate a strong circulation of air. The result is a high degree of mixing of the new air with the air already present inside the space.

Ventilation by displacement, in its ideal conceptualization, consists of injecting air into a space in such a way that new air displaces the air previously there without mixing with it. Ventilation by displacement is achieved by injecting new air into a space at a low speed and close to the floor, and extracting air near the ceiling. Using ventilation by displacement to control the thermal environment has the advantage that it profits from the natural movement of air generated by density variations that are themselves due to temperature differences. Even though ventilation by displacement is already widely used in industrial situations, the scientific literature on the subject is still quite limited, and the evaluation of its effectiveness is therefore still difficult.

Ventilation by Dilution

The design of a system of ventilation by dilution is based on the hypothesis that the concentration of the pollutant is the same throughout the space in question. This is the model that chemical engineers often refer to as a stirred tank.

If you assume that the air that is injected into the space is free of the pollutant and that at the initial time the concentration within the space is zero, you will need to know two facts in order to calculate the required rate of ventilation: the amount of the pollutant that is generated in the space and the level of environmental concentration that is sought (which hypothetically would be the same throughout).

Under these conditions, the corresponding calculations yield the following equation:

where

c(t) = the concentration of the contaminant in the space at time t

a = the amount of the pollutant generated (mass per unit of time)

Q = the rate at which new air is supplied (volume per unit of time)

V = the volume of the space in question.

The above equation shows that the concentration will tend to a steady state at the value a/Q, and that it will do so faster the smaller the value of Q/V, frequently referred to as “the number of renewals per unit of time”. Although occasionally the index of the quality of ventilation is regarded as practically equivalent to that value, the above equation clearly shows that its influence is limited to controlling the speed of stabilization of the environmental conditions, but not the level of concentration at which such a steady state will occur. That will depend only on the amount of the pollutant that is generated (a), and on the rate of ventilation (Q).

When the air of a given space is contaminated but no new amounts of the pollutant are generated, the speed of diminution of the concentration over a period of time is given by the following expression:

where Q and V have the meaning described above, t₁ and t₂ are, respectively, the initial and the final times and c₁ and c₂ are the initial and final concentrations.

Expressions can be found for calculations in instances where the initial concentration is not zero (Constance 1983; ACGIH 1992), where the air injected into the space is not totally devoid of the pollutant (because to reduce heating costs in the winter part of the air is recycled, for example), or where the amounts of the pollutant generated vary as a function of time.

If we disregard the transition stage and assume that the steady state has been achieved, the equation indicates that the rate of ventilation is equivalent to a/c_lim, where c_lim is the value of the concentration that must be maintained in the given space. This value will be established by regulations or, as an ancillary norm, by technical recommendations such as the threshold limit values (TLV) of the American Conference of Governmental Industrial Hygienists (ACGIH), which recommends that the rate of ventilation be calculated by the formula

where a and c_lim have the meaning already described and K is a safety factor. A value of K between 1 and 10 must be selected as a function of the efficacy of the air mixture in the given space, of the toxicity of the solvent (the smaller c_lim is, the greater the value of K will be), and of any other circumstance deemed relevant by the industrial hygienist. The ACGIH, among others, cites the duration of the process, the cycle of operations and the usual location of the workers with respect to the sources of emission of the pollutant, the number of these sources and their location in the given space, the seasonal changes in the amount of natural ventilation and the anticipated reduction in the functional efficacy of the ventilation equipment as other determining criteria.

In any case, the use of the above formula requires a reasonably exact knowledge of the values of a and K that should be used, and we therefore provide some suggestions in this regard.

The amount of pollutant generated may quite frequently be estimated by the amount of certain materials consumed in the process that generates the pollutant. So, in the case of a solvent, the amount used will be a good indication of the maximum amount that can be found in the environment.

As indicated above, the value of K should be determined as a function of the efficacy of the air mixture in the given space. This value will, therefore, be smaller in direct proportion to how good the estimation is of finding the same concentration of the pollutant at any point within the given space. This, in turn, will depend on how air is distributed within the space being ventilated.

According to these criteria, minimum values of K should be used when air is injected into the space in a distributed fashion (by using a plenum, for example), and when the injection and extraction of air are at opposite ends of the given space. On the other hand, higher values for K should be used when air is supplied intermittently and air is extracted at points close to the intake of new air (figure 1).

Figure 1. Schematic of air circulation in room with two supply openings

It should be noted that when air is injected into a given space—especially if it is done at a high speed—the stream of air created will exert a considerable pull on the air surrounding it. This air then mixes with the stream and slows it down, creating measurable turbulence as well. As a consequence, this process results in intense mixing of the air already in the space and the new air that is injected, generating internal air currents. Predicting these currents, even generally, requires a large dose of experience (figure 2).

Figure 2. Suggested K factors for inlet and exhaust locations

In order to avoid problems that result from workers’ being subjected to streams of air at relatively high speeds, air is commonly injected by way of diffusing grates designed in such a way that they facilitate the rapid mixing of new air with the air already present in the space. In this way, the areas where air moves at high speeds are kept as small as possible.

The stream effect just described is not produced near points where air escapes or is extracted through doors, windows, extraction vents or other openings. Air reaches extraction grates from all directions, so even at a relatively short distance from them, air movement is not easily perceived as an air current.

In any case, in dealing with air distribution, it is important to keep in mind the convenience of placing workstations, to the extent possible, in such a way that new air reaches the workers before it reaches the sources of contamination.

When in the given space there are important sources of heat, the movement of air will largely be conditioned by the convection currents that are due to density differences between denser, cold air and lighter, warm air. In spaces of this kind, the designer of air distribution must not fail to keep in mind the existence of these heat sources, or the movement of air may turn out to be very different from the one predicted.

The presence of chemical contamination, on the other hand, does not alter in a measurable way the density of air. While in a pure state the pollutants may have a density that is very different from that of air (usually much greater), given the real, existing concentrations in the workplace, the mix of air and pollutant does not have a density significantly different than the density of pure air.

Furthermore, it should be pointed out that one of the most common mistakes made in applying this type of ventilation is supplying the space only with air extractors, without any forethought given to adequate intakes of air. In these cases, the effectiveness of the extraction ventilators is diminished and, therefore, the actual rates of air extraction are much less than planned. The result is greater ambient concentrations of the pollutant in the given space than those initially calculated.

To avoid this problem some thought should be given to how air will be introduced into the space. The recommended course of action is to use immission ventilators as well as extraction ventilators. Normally, the rate of extraction should be greater than the rate of immission in order to allow for infiltration through windows and other openings. In addition, it is advisable to keep the space under slightly negative pressure to prevent the contamination generated from drifting into areas that are not contaminated.

Ventilation by Displacement

As mentioned above, with ventilation by displacement one seeks to minimize the mixing of new air and the air previously found in the given space, and tries to adjust the system to the model known as plug flow. This is usually accomplished by introducing air at slow speeds and at low elevations in the given space and extracting it near the ceiling; this has two advantages over ventilation by dilution.

In the first place, it makes lower rates of air renewal possible, because pollution concentrates near the ceiling of the space, where there are no workers to breathe it. The average concentration in the given space will then be higher than the c_lim value we have referred to before, but that does not imply a higher risk for the workers because in the occupied zone of the given space the concentration of the pollutant will be the same or lower than a c_lim.

In addition, when the goal of ventilation is the control of the thermal environment, ventilation by displacement makes it possible to introduce warmer air into the given space than would be required by a system of ventilation by dilution. This is because the warm air that is extracted is at a temperature several degrees higher than the temperature in the occupied zone of the space.

The fundamental principles of ventilation by displacement were developed by Sandberg, who in the early 1980s developed a general theory for the analysis of situations where there were nonuniform concentrations of pollutants in enclosed spaces. This allowed us to overcome the theoretical limitations of ventilation by dilution (which presupposes a uniform concentration throughout the given space) and opened the way for practical applications (Sandberg 1981).

Even though ventilation by displacement is widely used in some countries, particularly in Scandinavia, very few studies have been published in which the efficacy of different methods are compared in actual installations. This is no doubt because of the practical difficulties of installing two different ventilation systems in a real factory, and because the experimental analysis of these types of systems require the use of tracers. Tracing is done by adding a tracer gas to the air ventilation current and then measuring the concentrations of the gas at different points within the space and in the extracted air. This sort of examination makes it possible to infer how air is distributed within the space and to then compare the efficacy of different ventilation systems.

The few studies available that have been carried out in actual existing installations are not conclusive, except as regards the fact that systems that employ ventilation by displacement provide better air renewal. In these studies, however, reservations are often expressed about the results in so far as they have not been confirmed by measurements of the ambient level of contamination at the worksites.

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One of the chief functions of a building in which nonindustrial activities are carried out (offices, schools, dwellings, etc.) is to provide the occupants with a healthy and comfortable environment in which to work. The quality of this environment depends, to a large degree, on whether the ventilation and climatization systems of the building are adequately designed and maintained and function properly.

These systems must therefore provide acceptable thermal conditions (temperature and humidity) and an acceptable quality of indoor air. In other words, they should aim for a suitable mix of outside air with indoor air and should employ filtration and cleaning systems capable of eliminating pollutants found in the indoor environment.

The idea that clean outdoor air is necessary for well-being in indoor spaces has been expressed since the eighteenth century. Benjamin Franklin recognized that air in a room is healthier if it is provided with natural ventilation by opening the windows. The idea that providing great quantities of outside air could help reduce the risk of contagion for illnesses like tuberculosis gained currency in the nineteenth century.

Studies carried out during the 1930s showed that, in order to dilute human biological effluvia to concentrations that would not cause discomfort due to odours, the volume of new outside air required for a room is between 17 and 30 cubic metres per hour per occupant.

In standard No. 62 set in 1973, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) recommends a minimum flow of 34 cubic metres of outside air per hour per occupant to control odours. An absolute minimum of 8.5 m³/hr/occupant is recommended to prevent carbon dioxide from surpassing 2,500 ppm, which is half of the exposure limit set for industrial settings.

This same organization, in standard No. 90, set in 1975—in the middle of an energy crisis—adopted the aforementioned absolute minimum leaving aside, temporarily, the need for greater ventilation flows to dilute pollutants such as tobacco smoke, biological effluvia and so forth.

In its standard No. 62 (1981) ASHRAE rectified this omission and established its recommendation as 34 m³/hr/occupant for areas where smoking is permitted and 8.5 m³/hr/occupant in areas where smoking is forbidden.

The last standard published by ASHRAE, also No. 62 (1989), established a minimum of 25.5 m³/hr/occupant for occupied indoor spaces independently of whether smoking is permitted or not. It also recommends increasing this value when the air brought into the building is not mixed adequately in the breathing zone or if there are unusual sources of pollution present in the building.

In 1992, the Commission of European Communities published its Guidelines for Ventilation Requirements in Buildings. In contrast with existing recommendations for ventilation standards, this guide does not specify volumes of ventilation flow that should be provided for a given space; instead, it provides recommendations that are calculated as a function of the desired quality of indoor air.

Existing ventilation standards prescribe set volumes of ventilation flow that should be supplied per occupant. The tendencies evidenced in the new guidelines show that volume calculations alone do not guarantee a good quality of indoor air for every setting. This is the case for three fundamental reasons.

First, they assume that occupants are the only sources of contamination. Recent studies show that other sources of pollution, in addition to the occupants, should be taken into consideration as possible sources of pollution. Examples include furniture, upholstery and the ventilation system itself. The second reason is that these standards recommend the same amount of outside air regardless of the quality of air that is being conveyed into the building. And the third reason is that they do not clearly define the quality of indoor air required for the given space. Therefore, it is proposed that future ventilation standards should be based on the following three premises: the selection of a defined category of air quality for the space to be ventilated, the total load of pollutants in the occupied space and the quality of outside air available.

The Perceived Quality of Air

The quality of indoor air can be defined as the degree to which the demands and requirements of the human being are met. Basically, the occupants of a space demand two things of the air they breathe: to perceive the air they breathe as fresh and not foul, stale or irritating; and to know that the adverse health effects that may result from breathing that air are negligible.

It is common to think that the degree of quality of the air in a space depends more on the components of that air than on the impact of that air on the occupants. It may thus seem easy to evaluate the quality of the air, assuming that by knowing its composition its quality can be ascertained. This method of evaluating air quality works well in industrial settings, where we find chemical compounds that are implicated in or derived from the production process and where measuring devices and reference criteria to assess the concentrations exist. This method does not, however, work in nonindustrial settings. Nonindustrial settings are places where thousands of chemical substances can be found, but at very low concentrations, sometimes a thousand times lower than the recommended exposure limits; evaluating these substances one by one would result in a false assessment of the quality of that air, and the air would likely be judged to be of a high quality. But there is a missing aspect that remains to be considered, and that is the lack of knowledge that exists about the combined effect of those thousands of substances on human beings, and that may be the reason why that air is perceived as being foul, stale or irritating.

The conclusion that has been reached is that traditional methods used for industrial hygiene are not well-adapted to define the degree of quality that will be perceived by the human beings that breathe the air being evaluated. The alternative to chemical analysis is to use people as measuring devices to quantify air pollution, employing panels of judges to make the evaluations.

Human beings perceive the quality of air by two senses: the olfactory sense, situated in the nasal cavity and sensitive to hundreds of thousands of odorous substances, and the chemical sense, situated in the mucous membranes of the nose and eyes, and sensitive to a similar number of irritating substances present in air. It is the combined response of these two senses that determines how air is perceived and that allows the subject to judge whether its quality is acceptable.

The olf unit

One olf (from Latin = olfactus) is the emission rate of air pollutants (bioeffluents) from a standard person. One standard person is an average adult who works in an office or in a similar nonindustrial workplace, sedentary and in thermal comfort with a hygienic standard equipment to 0.7 bath/day. Pollution from a human being was chosen to define the term olf for two reasons: the first is that biological effluvia emitted by a person are well-known, and the second is that there was much data on the dissatisfaction caused by such biological effluvia.

Any other source of contamination can be expressed as the number of standard persons (olfs) needed to cause the same amount of dissatisfaction as the source of contamination that is being evaluated.

Figure 1 depicts a curve that defines an olf. This curve shows how contamination produced by a standard person (1 olf) is perceived at different rates of ventilation, and allows the calculation of the rate of dissatisfied individuals—in other words, those that will perceive the quality of air to be unacceptable just after they have entered the room. The curve is based on different European studies in which 168 people judged the quality of air polluted by over a thousand people, both men and women, considered to be standard. Similar studies conducted in North America and Japan show a high degree of correlation with the European data.

Figure 1. Olf definition curve

The decipol unit

The concentration of pollution in air depends on the source of contamination and its dilution as a result of ventilation. Perceived air pollution is defined as the concentration of human biological effluvia that would cause the same discomfort or dissatisfaction as the concentration of polluted air that is being evaluated. One decipol (from the Latin pollutio) is the contamination caused by a standard person (1 olf) when the rate of ventilation is 10 litres per second of noncontaminated air, so that we may write

1 decipol = 0.1 olf/(litre/second)

Figure 2, derived from the same data as the previous figure, shows the relation between the perceived quality of air, expressed as a percentage of dissatisfied individuals and in decipols.

Figure 2. Relation between the perceived quality of air expressed as a percentage of dissatisfied individuals and in decipols

To determine the rate of ventilation required from the point of view of comfort, selecting the degree of air quality desired in the given space is essential. Three categories or levels of quality are proposed in Table 1, and they are derived from Figures 1 and 2. Each level corresponds to a certain percentage of dissatisfied people. The selection of one or another level will depend, most of all, on what the space will be used for and on economic considerations.

Table 1. Levels of quality of indoor air

¹ Assuming that outside air is clean and the efficiency of the ventilation system is equal to one.

Source: CEC 1992.

As noted above, the data are the result of experiments carried out with panels of judges, but it is important to keep in mind that some of the substances found in air that can be dangerous (carcinogenic compounds, micro-organisms and radioactive substances, for example) are not recognized by the senses, and that the sensory effects of other contaminants bear no quantitative relationship to their toxicity.

Sources of Contamination

As was indicated earlier, one of the shortcomings of today’s ventilation standards is that they take into account only the occupants as the sources of contamination, whereas it is recognized that future standards should take all the possible sources of pollution into account. Aside from the occupants and their activities, including the possibility that they might smoke, there are other sources of pollution that contribute significantly to air pollution. Examples include furniture, upholstery and carpeting, construction materials, products used for decoration, cleaning products and the ventilation system itself.

What determines the load of pollution of air in a given space is the combination of all these sources of contamination. This load can be expressed as chemical contamination or as sensory contamination expressed in olfs. The latter integrates the effect of several chemical substances as they are perceived by human beings.

The chemical load

Contamination that emanates from a given material can be expressed as the rate of emission of each chemical substance. The total load of chemical pollution is calculated by adding all the sources, and is expressed in micrograms per second (μg/s).

In reality, it may be difficult to calculate the load of pollution because often little data are available on the rates of emission for many commonly used materials.

Sensory load

The load of pollution perceived by the senses is caused by those sources of contamination that have an impact on the perceived quality of air. The given value of this sensory load can be calculated by adding all the olfs of different sources of contamination that exist in a given space. As in the previous case, there is still not much information available on the olfs per square metre (olfs/m²) of many materials. For that reason it turns out to be more practical to estimate the sensory load of the entire building, including the occupants, the furnishings and the ventilation system.

Table 2 shows the pollution load in olfs by the occupants of the building as they carry out different types of activities, as a proportion of those who smoke and don’t smoke, and the production of various compounds like carbon dioxide (CO₂), carbon monoxide (CO) and water vapour. Table 3 shows some examples of the typical occupancy rates in different kinds of spaces. And last, table 4 reflects the results of the sensory load—measured in olfs per square metre—found in different buildings.

Table 2. Contamination due to the occupants of a building

¹ 1 met is the metabolic rate of a sedentary person at rest (1 met = 58 W/m² of skin surface).
² Average consumption of 1.2 cigarettes/hour per smoker. Average rate of emission, 44 ml of CO per cigarette.
³ From tobacco smoke.
⁴ Applicable to people close to thermal neutrality.

Source: CEC 1992.

Table 3. Examples of the degree of occupancy of  different buildings

Source: CEC 1992.

Table 4. Contamination due to the building

¹ Data obtained in 24 mechanically ventilated offices.
² Data obtained in 6 mechanically ventilated schools.
³ Data obtained in 9 mechanically ventilated child-care centres.
⁴ Data obtained in 5 mechanically ventilated theatres.
⁵ Goal that should be reached by new buildings.

Source: CEC 1992.

Quality of Outside Air

Another premise, one that rounds out the inputs needed for creation of ventilation standards for the future, is the quality of available outside air. Recommended exposure values for certain substances, both from inside and outside spaces, appear in the publication Air Quality Guidelines for Europe by the WHO (1987).

Table 5 shows the levels of perceived outside air quality, as well as the concentrations of several typical chemical pollutants found out of doors.

Table 5. Quality levels of outside air

¹ The values of perceived air quality are daily average values.
² The values of pollutants correspond to average yearly concentrations.

Source: CEC 1992.

It should be kept in mind that in many cases the quality of outside air can be worse than the levels indicated in the table or in the guidelines of the WHO. In such cases air needs to be cleaned before it is conveyed into occupied spaces.

Efficiency of Ventilation Systems

Another important factor that will affect the calculation of the ventilation requirements for a given space is the efficiency of ventilation (E_v), which is defined as the relation between the concentration of pollutants in extracted air (C_e) and the concentration in the breathing zone (C_b).

E_v = C_e/C_b

The efficiency of ventilation depends on the distribution of air and the location of the sources of pollution in the given space. If air and the contaminants are mixed completely, the efficiency of ventilation is equal to one; if the quality of air in the breathing zone is better than that of extracted air, then the efficiency is greater than one and the desired quality of air can be attained with lower rates of ventilation. On the other hand, greater rates of ventilation will be needed if the efficiency of ventilation is less than one, or to put it differently, if the quality of air in the breathing zone is inferior to the quality of extracted air.

In calculating the efficiency of ventilation it is useful to divide spaces into two zones, one into which the air is delivered, the other comprising the rest of the room. For ventilation systems that work by the mixing principle, the zone where air is delivered is generally found above the breathing zone, and the best conditions are reached when mixing is so thorough that both zones become one. For ventilation systems that work by the displacement principle, air is supplied in the zone occupied by people and the extraction zone is usually found overhead; here the best conditions are reached when mixing between both zones is minimal.

The efficiency of ventilation, therefore, is a function of the location and characteristics of the elements that supply and extract air and the location and characteristics of the sources of contamination. In addition, it is also a function of the temperature and of the volumes of air supplied. It is possible to calculate the efficiency of a ventilation system by numerical simulation or by taking measurements. When data are not available the values in figure 3 can be used for different ventilation systems. These reference values take into consideration the impact of air distribution but not the location of sources of pollution, assuming instead that they are uniformly distributed throughout the ventilated space.

Figure 3. Effectiveness of ventilation in breathing zone according to different ventilation principles

Calculating Ventilation Requirements

Figure 4 shows the equations used to calculate ventilation requirements from the point of view of comfort as well as that of protecting health.

Figure 4. Equations for calculating ventilation requirements

Ventilation requirements for comfort

The first steps in the calculation of comfort requirements is to decide the level of quality of indoor air that one wishes to obtain for the ventilated space (see Table 1), and to estimate the quality of outside air available (see Table 5).

The next step consists in estimating the sensory load, using Tables 8, 9, and 10 to select the loads according to the occupants and their activities, the type of building, and the level of occupancy by square metre of surface. The total value is obtained by adding all the data.

Depending on the operating principle of the ventilation system and using Figure 9, it is possible to estimate the efficiency of ventilation. Applying equation (1) in Figure 9 will yield a value for the required amount of ventilation.

Ventilation requirements for health protection

A procedure similar to the one described above, but using equation (2) in Figure 3, will provide a value for the stream of ventilation required to prevent health problems. To calculate this value it is necessary to identify a substance or group of critical chemical substances which one proposes to control and to estimate their concentrations in air; it is also necessary to allow for different evaluation criteria, taking into account the effects of the contaminant and the sensitivity of the occupants that you wish to protect—children or the elderly, for example.

Unfortunately, it is still difficult to estimate the ventilation requirements for health protection owing to the lack of information on some of the variables that enter into the calculations, such as the rates of emission of the contaminants (G), the evaluation criteria for indoor spaces (C_v) and others.

Studies carried out in the field show that spaces where ventilation is required to achieve comfortable conditions the concentrations of chemical substances is low. Nevertheless, those spaces may contain sources of pollution that are dangerous. The best policy in these cases is to eliminate, to substitute or to control the sources of pollution instead of diluting the contaminants by general ventilation.

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With regard to heating, a given person’s needs will depend on many factors. They can be classified into two main groups, those related to the surroundings and those related to human factors. Among those related to the surroundings one might count geography (latitude and altitude), climate, the type of exposure of the space the person is in, or the barriers that protect the space against the external environment, etc. Among the human factors are the worker’s energy consumption, the pace of work or the amount of exertion needed for the job, the clothing or garments used against the cold and personal preferences or tastes.

The need for heating is seasonal in many regions, but this does not mean that heating is dispensable during the cold season. Cold environmental conditions affect health, mental and physical efficiency, precision and occasionally may increase the risk of accidents. The goal of a heating system is to maintain pleasant thermal conditions that will prevent or minimize adverse health effects.

The physiological characteristics of the human body allow it to withstand great variations in thermal conditions. Human beings maintain their thermal balance through the hypothalamus, by means of thermal receptors in the skin; body temperature is kept between 36 and 38°C as shown in figure 1.

Figure 1. Thermoregulatory mechanisms in human beings

Heating systems need to have very precise control mechanisms, especially in cases where workers carry out their tasks in a sitting or a fixed position that does not stimulate blood circulation to their extremities. Where the work performed allows a certain mobility, the control of the system may be somewhat less precise. Finally, where the work performed takes place in abnormally adverse conditions, as in refrigerated chambers or in very cold climatic conditions, support measures may be undertaken to protect special tissues, to regulate the time spent under those conditions or to supply heat by electrical systems incorporated into the worker’s garments.

Definition and Description of the Thermal Environment

A requirement that can be demanded of any properly functioning heating or air conditioning system is that it should allow for control of the variables that define the thermal environment, within specified limits, for each season of the year. These variables are

1. air temperature
2. average temperature of the inside surfaces that define the space
3. air humidity
4. speeds and uniformity of speeds of air flow within the space

It has been shown that there is a very simple relation between the temperature of the air and of the wall surfaces of a given space, and the temperatures that provide the same perceived thermal sensation in a different room. This relation can be expressed as

where

T_eat = equivalent air temperature for a given thermal sensation

T_dbt = air temperature measured with a dry bulb thermometer

T_ast = measured average surface temperature of the walls.

For example, if in a given space the air and the walls are at 20° C, the equivalent temperature will be 20°C, and the perceived sensation of heat will be the same as in a room where the average temperature of the walls is 15°C and the air temperature is 25°C, because that room would have the same equivalent temperature. From the standpoint of temperature, the perceived sensation of thermal comfort would be the same.

Properties of humid air

In implementing an air-conditioning plan, three things that must be taken into consideration are the thermodynamic state of the air in the given space, of the air outside, and of the air that will be supplied to the room. The selection of a system capable of transforming the thermodynamic properties of the air supplied to the room will then be based on the existing thermal loads of each component. We therefore need to know the thermodynamic properties of humid air. They are as follows:

T_dbt = the dry bulb temperature reading, measured with a thermometer insulated from radiated heat

T_dpt = the dew point temperature reading. This is the temperature at which nonsaturated dry air reaches the saturation point

W = a humidity relation that ranges from zero for dry air to W_s for saturated air. It is expressed as kg of water vapour by kg of dry air

RH = relative humidity

t* = thermodynamic temperature with moist bulb

v = specific volume of air and water vapour (expressed in units of m³/kg). It is the inverse of density

H = enthalpy, kcal/kg of dry air and associated water vapour.

Of the above variables, only three are directly measurable. They are the dry bulb temperature reading, the dew point temperature reading and relative humidity. There is a fourth variable that is experimentally measurable, defined as the wet bulb temperature. The wet bulb temperature is measured with a thermometer whose bulb has been moistened and which is moved, typically with the aid of a sling, through nonsaturated moist air at a moderate speed. This variable differs by an insignificant amount from the thermodynamic temperature with a dry bulb (3 per cent), so they can both be used for calculations without erring too much.

Psychrometric diagram

The properties defined in the previous section are functionally related and can be portrayed in graphic form. This graphic representation is called a psychrometric diagram. It is a simplified graph derived from tables of the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). Enthalpy and the degree of humidity are shown on the coordinates of the diagram; the lines drawn show dry and humid temperatures, relative humidity and specific volume. With the psychrometric diagram, knowing any two of the aforementioned variables enables you to derive all the properties of humid air.

Conditions for thermal comfort

Thermal comfort is defined as a state of mind that expresses satisfaction with the thermal environment. It is influenced by physical and physiological factors.

It is difficult to prescribe general conditions that should be met for thermal comfort because conditions differ in various work situations; different conditions could even be required for the same work post when it is occupied by different people. A technical norm for thermal conditions required for comfort cannot be applied to all countries because of the different climatic conditions and their different customs governing dress.

Studies have been carried out with workers that do light manual labour, establishing a series of criteria for temperature, speed and humidity that are shown in table 1 (Bedford and Chrenko 1974).

Table 1. Proposed norms for environmental factors

The above factors are interrelated, requiring a lower air temperature in cases where there is high thermal radiation and requiring a higher air temperature when the speed of the air flow is also higher.

Generally, the corrections that should be carried out are the following:

The air temperature should be increased:

• if the speed of the air flow is high

• for sedentary work situations

• if clothing used is light

• when people must be acclimatized to high indoor temperatures.

The air temperature should be decreased:

• if the work involves heavy manual labour

• when warm clothing is used.

For a good sensation of thermal comfort the most desirable situation is one where the temperature of the environment is slightly higher than the temperature of the air, and where the flow of radiating thermal energy is the same in all directions and is not excessive overhead. The increase in temperature by height should be minimized, keeping feet warm without creating too much of a thermal load overhead. An important factor that has a bearing on the sensation of thermal comfort is the speed of the air flow. There are diagrams that give recommended air speeds as a function of the activity that is being carried out and the kind of clothing used (figure 2).

Figure 2. Comfort zones based on readings of overall temperatures and speed of air currents

In some countries there are norms for minimal environmental temperatures, but optimal values have not yet been established. Typically, the maximum value for air temperature is given as 20°C. With recent technical improvements, the complexity of measuring thermal comfort has increased. Many indexes have appeared, including the index of effective temperature (ET) and the index of effective temperature, corrected (CET); the index of caloric overload; the Heat Stress Index (HSI); the wet bulb globe temperature (WBGT); and the Fanger index of median values (IMV), among others. The WBGT index allows us to determine the intervals of rest required as a function of the intensity of the work performed so as to preclude thermal stress under working conditions. This is discussed more fully in the chapter Heat and Cold.

Thermal comfort zone in a psychrometric diagram

The range on the psychrometric diagram corresponding to conditions under which an adult perceives thermal comfort has been carefully studied and has been defined in the ASHRAE norm based on the effective temperature, defined as the temperature measured with a dry bulb thermometer in a uniform room with 50 per cent relative humidity, where people would have the same interchange of heat by radiant energy, convection and evaporation as they would with the level of humidity in the given local environment. The scale of effective temperature is defined by ASHRAE for a level of clothing of 0.6 clo—clo is a unit of insulation; 1 clo corresponds to the insulation provided by a normal set of clothes—that assumes a level of thermal insulation of 0.155 K m²W^–1, where K is the exchange of heat by conduction measured in Watts per square metre (W m^–2) for a movement of air of 0.2 m s^–1 (at rest), for an exposure of one hour at a chosen sedentary activity of 1 met (unit of metabolic rate=50 Kcal/m²h). This comfort zone is seen in figure 2 and can be used for thermal environments where the measured temperature from radiant heat is approximately the same as the temperature measured by a dry bulb thermometer, and where the speed of air flow is below 0.2 m s^–1 for people dressed in light clothing and carrying out sedentary activities.

Comfort formula: The Fanger method

The method developed by PO Fanger is based on a formula that relates variables of ambient temperature, average radiant temperature, relative speed of air flow, pressure of water vapour in ambient air, level of activity and thermal resistance of the clothing worn. An example derived from the comfort formula is shown in table 2, which can be used in practical applications for obtaining a comfortable temperature as a function of the clothing worn, the metabolic rate of the activity carried out and the speed of the air flow.

Table 2. Temperatures of thermal comfort (°C), at 50% relative humidity (based on the formula by PO Fanger)

Heating Systems

The design of any heating system should be directly related to the work to be performed and the characteristics of the building where it will be installed. It is hard to find, in the case of industrial buildings, projects where the heating needs of the workers are considered, often because the processes and workstations have yet to be defined. Normally systems are designed with a very free range, considering only the thermal loads that will exist in the building and the amount of heat that needs to be supplied to maintain a given temperature within the building, without regard to heat distribution, the situation of workstations and other similarly less general factors. This leads to deficiencies in the design of certain buildings that translate into shortcomings like cold spots, draughts, an insufficient number of heating elements and other problems.

To end up with a good heating system in planning a building, the following are some of the considerations that should be addressed:

• Consider the proper placement of insulation to save energy and to minimize temperature gradients within the building.

• Reduce as much as possible the infiltration of cold air into the building to minimize temperature variations in the work areas.

• Control air pollution through localized extraction of air and ventilation by displacement or diffusion.

• Control the emissions of heat due to the processes used in the building and their distribution in occupied areas of the building.

When heating is provided by burners without exhaust chimneys, special consideration should be given to the inhalation of the products of combustion. Normally, when the combustible materials are heating oil, gas or coke, they produce sulphur dioxide, nitrogen oxides, carbon monoxide and other combustion products. There exist human exposure limits for these compounds and they should be controlled, especially in closed spaces where the concentration of these gases can increase rapidly and the efficiency of the combustion reaction can decrease.

Planning a heating system always entails balancing various considerations, such as a low initial cost, flexibility of the service, energy efficiency and applicability. Therefore, the use of electricity during off-peak hours when it might be cheaper, for example, could make electric heaters cost-effective. The use of chemical systems for heat storage that can then be put to use during peak demand (using sodium sulphide, for example) is another option. It is also possible to study the placement of several different systems together, making them work in such a way that costs can be optimized.

The installation of heaters that are capable of using gas or heating oil is especially interesting. The direct use of electricity means consuming first-class energy that may turn out to be costly in many cases, but that may afford the needed flexibility under certain circumstances. Heat pumps and other cogeneration systems that take advantage of residual heat can afford solutions that may be very advantageous from the financial point of view. The problem with these systems is their high initial cost.

Today the tendency of heating and air conditioning systems is to aim to deliver optimal functioning and energy savings. New systems therefore include sensors and controls distributed throughout the spaces to be heated, obtaining a supply of heat only during the times necessary to obtain thermal comfort. These systems can save up to 30% of the energy costs of heating. Figure 3 shows some of the heating systems available, indicating their positive characteristics and their drawbacks.

Figure 3. Characteristics of the most common heating systems employed in worksites

Air-conditioning systems

Experience shows that industrial environments that are close to the comfort zone during summer months increase productivity, tend to register fewer accidents, have lower absenteeism and, in general, contribute to improved human relations. In the case of retail establishments, hospitals and buildings with large surfaces, air conditioning usually needs to be directed to be able to provide thermal comfort when outside conditions require it.

In certain industrial environments where external conditions are very severe, the goal of heating systems is geared more to providing enough heat to prevent possible adverse health effects than to providing enough heat for a comfortable thermal environment. Factors that should be carefully monitored are the maintenance and proper use of the air-conditioning equipment, especially when equipped with humidifiers, because they can become sources of microbial contamination with the risks that these contaminants may pose to human health.

Today ventilation and climate-control systems tend to cover, jointly and often using the same installation, the needs for heating, refrigerating and conditioning the air of a building. Multiple classifications may be used for refrigerating systems.

Depending on the configuration of the system they may be classified in the following way:

• Hermetically sealed units, with refrigerating fluid installed at the factory, that can be opened and recharged in a repair shop. These are air-conditioning units normally used in offices, dwellings and the like.

• Semi-hermetic units of medium size, factory made, that are of larger size than home units and that can be repaired through openings designed for that purpose.

• Segmented systems for warehouses and large surfaces, which consist of parts and components that are clearly differentiated and physically separate (the compressor and the condenser are physically separate from the evaporator and the expansion valve). They are used for large office buildings, hotels, hospitals, large factories and industrial buildings.

Depending on the coverage they provide, they can be classified in the following way:

• Systems for a single zone: one air treatment unit serves various rooms in the same building and at the same time. The rooms served have similar heating, refrigeration and ventilation needs and they are regulated by a common control (a thermostat or similar device). Systems of this type can end up being unable to supply an adequate level of comfort to each room if the design plan did not take into consideration the different thermal loads between rooms in the same zone. This may happen when there is an increase in the occupancy of a room or when lighting or other heat sources are added, like computers or copying machines, that were unforeseen during the original design of the system. Discomfort may also occur because of seasonal changes in the amount of solar radiation a room receives, or even because of the changes from one room to the next during the day.

• Systems for multiple zones: systems of this type can provide different zones with air at different temperatures and humidities by heating, cooling, humidifying or dehumidifying air in each zone and by varying the flow of air. These systems, even if they generally have a common and centralized air cooling unit (compressor, evaporator, etc.), are equipped with a variety of elements, such as devices that control the flow of air, heating coils and humidifiers. These systems are capable of adjusting the conditions of a room based on specific thermal loads, which they detect by means of sensors distributed in the rooms throughout the area they serve.

• Depending on the flow of air that these systems pump into the building they are classified in the following way:

• Constant volume (CV): these systems pump a constant flow of air into each room. Temperature changes are effected by heating or cooling the air. These systems frequently mix a percentage of outside air with recycled indoor air.

• Variable volume (VAV): these systems maintain thermal comfort by varying the amount of heated or cooled air supplied to each space. Even though they function primarily based on this mixing principle, they can also be combined with systems that change the temperature of the air they introduce into the room.

The problems that most frequently plague these types of systems are excess heating or cooling if the system is not adjusted to respond to variations in thermal loads, or a lack of ventilation if the system does not introduce a minimal amount of outside air to renew the circulating indoor air. This creates stale indoor environments in which the quality of air deteriorates.

The basic elements of all air-conditioning systems are (see also figure 4):

• Units to retain solid matter, usually bag filters or electrostatic precipitators.

• Air heating or cooling units: heat is exchanged in these units by thermal exchange with cold water or refrigerating liquids, by forced ventilation in the summer and by heating with electrical coils or by combustion in the winter.

• Units to control humidity: in winter humidity can be added by directly injecting water vapour or by direct water evaporation; in the summer it can be removed by refrigerated coils that condense excess humidity in the air, or by a refrigerated water system in which moist air flows through a curtain of drops of water that is colder than the dew point of the moist air.

Figure 4. Simplified schematic of air-conditioning system

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Ionization is one of the techniques used to eliminate particulate matter from air. Ions act as condensation nuclei for small particles which, as they stick together, grow and precipitate.

The concentration of ions in closed indoor spaces is, as a general rule and if there are no additional sources of ions, inferior to that of open spaces. Hence the belief that increasing the concentration of negative ions in indoor air improves air quality.

Some studies based on epidemiological data and on planned experimental research assert that increasing the concentration of negative ions in work environments leads to improved worker efficiency and enhances the mood of employees, while positive ions have an adverse affect. However, parallel studies show that existing data on the effects of negative ionization on workers’ productivity are inconsistent and contradictory. Therefore, it seems that it is still not possible to assert unequivocally that the generation of negative ions is really beneficial.

Natural Ionization

Individual gas molecules in the atmosphere can ionize negatively by gaining, or positively by losing, an electron. For this to occur a given molecule must first gain enough energy—usually called the ionization energy of that particular molecule. Many sources of energy, both of cosmic and terrestrial origin, occur in nature that are capable of producing this phenomenon: background radiation in the atmosphere; electromagnetic solar waves (especially ultraviolet ones), cosmic rays, atomization of liquids such as the spray caused by waterfalls, the movement of great masses of air over the earth’s surface, electrical phenomena such as lightning and storms, the process of combustion and radioactive substances.

The electrical configurations of the ions that are formed this way, while not completely known yet, seems to include the ions of carbonation and H₊, H₃O₊, O₊, N₊, OH^–, H₂O^– and O₂^–. These ionized molecules can aggregate through adsorption on suspended particles (fog, silica and other contaminants). Ions are classified according to their size and their mobility. The latter is defined as a velocity in an electrical field expressed as a unit such as centimetres per second by voltage per centimetre (cm/s/V/cm), or, more compactly,

Atmospheric ions tend to disappear by recombination. Their half-life depends on their size and is inversely proportional to their mobility. Negative ions are statistically smaller and their half-life is of several minutes, while positive ions are larger and their half-life is about one half hour. The spatial charge is the quotient of the concentration of positive ions and the concentration of negative ions. The value of this relation is greater than one and depends on factors such as climate, location and season of the year. In living spaces this coefficient can have values that are lower than one. Characteristics are given in table 1.

Table 1. Characteristics of ions of given mobilities and diameter

Artificial Ionization

Human activity modifies the natural ionization of air. Artificial ionization can be caused by industrial and nuclear processes and fires. Particulate matter suspended in air favours the formation of Langevin ions (ions aggregated on particulate matter). Electrical radiators increase the concentration of positive ions considerably. Air-conditioners also increase the spatial charge of indoor air.

Workplaces have machinery that produces positive and negative ions simultaneously, as in the case of machines that are important local sources of mechanical energy (presses, spinning and weaving machines), electrical energy (motors, electronic printers, copiers, high-voltage lines and installations), electromagnetic energy (cathode-ray screens, televisions, computer monitors) or radioactive energy (cobalt-42 therapy). These kinds of equipment create environments with higher concentrations of positive ions due to the latter’s higher half-life as compared to negative ions.

Environmental Concentrations of Ions

Concentrations of ions vary with environmental and meteorological conditions. In areas with little pollution, such as in forests and mountains, or at great altitudes, the concentration of small ions grows; in areas close to radioactive sources, waterfalls, or river rapids the concentrations can reach thousands of small ions per cubic centimetre. In the proximity of the sea and when the levels of humidity are high, on the other hand, there is an excess of large ions. In general, the average concentration of negative and positive ions in clean air is 500 and 600 ions per cubic centimetre respectively.

Some winds can carry great concentrations of positive ions—the Föhn in Switzerland, the Santa Ana in the United States, the Sirocco in North Africa, the Chinook in the Rocky Mountains and the Sharav in the Middle East.

In workplaces where there are no significant ionizing factors there is often an accumulation of large ions. This is especially true, for example, in places that are hermetically sealed and in mines. The concentration of negative ions decreases significantly in indoor spaces and in contaminated areas or areas that are dusty. There are many reasons why the concentration of negative ions also decreases in indoor spaces that have air-conditioning systems. One reason is that negative ions remain trapped in air ducts and air filters or are attracted to surfaces that are positively charged. Cathode-ray screens and computer monitors, for example, are positively charged, creating in their immediate vicinity a microclimate deficient in negative ions. Air filtration systems designed for “clean rooms” that require that levels of contamination with particulate matter be kept at a very low minimum seem also to eliminate negative ions.

On the other hand, an excess of humidity condenses ions, while a lack of it creates dry environments with large amounts of electrostatic charges. These electrostatic charges accumulate in plastic and synthetic fibres, both in the room and on people.

Ion Generators

Generators ionize air by delivering a large amount of energy. This energy may come from a source of alpha radiation (such as tritium) or from a source of electricity by the application of a high voltage to a sharply pointed electrode. Radioactive sources are forbidden in most countries because of the secondary problems of radioactivity.

Electric generators are made of a pointed electrode surrounded by a crown; the electrode is supplied with a negative voltage of thousands of volts, and the crown is grounded. Negative ions are expelled while positive ions are attracted to the generator. The amount of negative ions generated increases in proportion to the voltage applied and to the number of electrodes that it contains. Generators that have a greater number of electrodes and use a lower voltage are safer, because when voltage exceeds 8,000 to 10,000 volts the generator will produce not only ions, but also ozone and some nitrous oxides. The dissemination of ions is achieved by electrostatic repulsion.

The migration of ions will depend on the alignment of the magnetic field generated between the emission point and the objects that surround it. The concentration of ions surrounding the generators is not homogeneous and diminishes significantly as the distance from them increases. Fans installed in this equipment will increase the ionic dispersion zone. It is important to remember that the active elements of the generators need to be cleaned periodically to insure proper functioning.

The generators may also be based on atomizing water, on thermoelectric effects or on ultraviolet rays. There are many different types and sizes of generators. They may be installed on ceilings and walls or may be placed anywhere if they are the small, portable type.

Measuring Ions

Ion measuring devices are made by placing two conductive plates 0.75 cm apart and applying a variable voltage. Collected ions are measured by a picoamperemeter and the intensity of the current is registered. Variable voltages permit the measurement of concentrations of ions with different mobilities. The concentration of ions (N) is calculated from the intensity of the electrical current generated using the following formula:

where I is the current in amperes, V is the speed of the air flow, q is the charge of a univalent ion (1.6x10^–19) in Coulombs and A is the effective area of the collector plates. It is assumed that all ions have a single charge and that they are all retained in the collector. It should be kept in mind that this method has its limitations due to background current and the influence of other factors such as humidity and fields of static electricity.

The Effects of Ions on the Body

Small negative ions are the ones which are supposed to have the greatest biological effect because of their greater mobility. High concentrations of negative ions can kill or block the growth of microscopic pathogens, but no adverse effects on humans have been described.

Some studies suggest that exposure to high concentrations of negative ions produces biochemical and physiological changes in some people that have a relaxing effect, reduce tension and headaches, improve alertness and cut reaction time. These effects could be due to the suppression of the neural hormone serotonin (5-HT) and of histamine in environments loaded with negative ions; these factors could affect a hypersensitive segment of the population. However, other studies reach different conclusions on the effects of negative ions on the body. Therefore, the benefits of negative ionization are still open to debate and further study is needed before the matter is decided.

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