AIR POLLUTION CONTROL APPROACHES AND MEASURES
The aim of a health-oriented environmental pollution control programme is to promote a better quality of life by reducing pollution to the lowest level possible. Environmental pollution control programmes and policies, whose implications and priorities vary from country to country, cover all aspects of pollution (air, water, etc.) and involve coordination among areas such as industrial city planning, water resources development and transportation policies. Such aspects are beyond the scope of this article.
As well presented by de Koning (Air pollution and human health), in air pollution basically two approaches can be used for control and prevention of harmful effects: (1) air quality management and (2) best practicable means.
Air quality management aims at the preservation of environmental quality by prescribing the tolerated degree of pollution, leaving it to the local authorities and polluters to devise and implement actions which will ensure that this degree of pollution will not be exceeded. An example of legislation within this approach is the adoption of ambient air quality standards for different pollutants; these are accepted maximum levels of pollutants (or indicators) in the target area (e.g. at ground level at a specified point in a community) and can be either primary or secondary standards. Primary standards, to use the World Health Organization definition (Glossary on air pollution), are the maximum levels consistent with an adequate safety margin and with the preservation of public health, and must be complied with within a specific time limit; secondary standards are those judged to be necessary for protection against known or anticipated adverse effects other than health hazards (mainly on vegetation) and must be complied with "within a reasonable time". Air quality standards are for 24 hours per day, 7 days per week exposure of all living subjects (including children, the elderly and the sick) as well as non-living subjects; this is in contrast to maximum permissible levels for occupational exposure, which are for a partial weekly exposure (e.g. 8 hours per day, 5 days per week) of adult and supposedly healthy workers. Typical measures in air quality management are land-use planning and "shut down' of factories during unfavorable weather conditions.
The best practicable means approach stresses that the air pollutant emissions should be kept to a minimum; this is basically defined through emission standards for single sources of air pollution and could be achieved, for example, through closed systems and high-efficiency collectors. An emission standard is a limit on the amount or concentration of a pollutant emitted from a source. This type of legislation requires a decision, for each industry, on the best means of controlling its emissions. In practice, although the two approaches mentioned differ, it is possible to combine elements from both in one air pollution control programme. The option for one or the other approach, or a combined approach, depends on the country or region concerned.
Among the many factors that must be considered in order to select the most adequate air pollution control strategy for a given situation, the following can be mentioned:
geographical situation and meteorology;
number of sources and their relative location to one another and to communities;
type of source(s) and effluents;
characteristics of the pollutants involved (physicochemical effects, etc.);
degree of control required;
socio-economic aspects and priorities.
Air quality and emission standards can be achieved through one air pollution control measure or, more often, a combination of measures.
Specific air pollution control measures are well described in the specialized literature; however, some of the most widely used are briefly presented below.
Land-use planning
Land-use planning aims basically at reducing the impact of pollution generated; it requires that future situations and problems be foreseen and it involves many aspects such as zoning of industries and other stationary air pollution sources, planning for waste disposal and transportation, and the reservation of parks and open spaces.
Adequate zoning of industries is of fundamental importance in order to reduce future problems. Factors to be considered include topography of the region, water stream characteristics, meteorological factors (particularly prevailing winds and probability of temperature inversion), location of populated areas, already existing pollution sources, etc.
Situations that may favor the accumulation of pollutants and prevent their dispersion should be avoided as much as possible, for example, concentration of industries in a valley where there is the possibility of temperature inversion. Also, the way prevailing winds promote the transportation of pollutants to inhabited areas, farms or plantations should be carefully studied.
Control at the source
The control of industrial air pollution at the source-that is, at the workplace level, can be achieved by one or a combination of the following approaches:
Prevention or reduction of pollutants generation
Whenever facing an industrial air pollution problem, the first question to be asked is the following: "Is it possible to eliminate or reduce the generation of the air pollutant(s) in question?".
Before considering the utilization of air cleaning devices or high stacks, the possibility of eliminating or reducing air pollutants at their source of generation, by controlling operations at the workplace level, should be investigated. This approach, which at the same time protects the health of workers, is discussed under CONTROL TECHNOLOGY FOR OCCUPATIONAL SAFETY AND HEALTH. The fundamental measures involved are the following:
(a) Substitution of materials
Examples: substitution of less toxic solvents for highly toxic ones used in certain industrial processes, use of fuels with lower sulfur content (e.g, washed coal) therefore giving rise to less sulfur compounds, etc.
(b) Modification or change of the industrial process or equipment
Examples: in the steel industry, a change from raw ore to pelleted sintered ore (to reduce the dust released during ore handling). use of closed systems instead of open ones: change of fuel heating systems to steam, hot water or electrical systems; use of catalysers at the exhaust air outlets (combustion processes), etc.
Modifications in processes, as well as in plant layout, may also facilitate and/or improve the conditions for dispersion and collection of pollutants. For example, a different plant layout may facilitate the installation of a local exhaust system; the performance of a process at a lower rate may allow the use of a certain collector (with volume limitations but otherwise adequate). Process controls that concentrate pollutants in smaller air volumes offer an advantage since the cost of control equipment is closely related to the volume of effluent handled and the efficiency of some air cleaning equipment increases with the concentration of pollutants in the effluent.
Both the substitution of materials and the modification of processes may have technical and/or economic limitations, and these should be considered.
(c) Adequate housekeeping and storage
Examples: strict sanitation in food and animal product processing; avoidance of open storage of chemicals (e.g. sulfur piles) or dusty materials (e.g. sand), or, failing this, spraying of the piles of loose particulates with water (if possible) or application of surface coatings (e.g. wetting agents, plastic) to piles of materials likely to give off pollutants.
d) Adequate disposal of wastes
Examples: avoidance of simply piling up chemical wastes (such as scraps from polymerisation reactors), as well as of dumping pollutant materials (solid or liquid) in water streams. This latter practice not only causes water pollution but can also create a secondary source of air pollution as in the case of liquid wastes from sulphite process pulp mills, which release offensive odors and gaseous pollutants.
e) Maintenance
Example: well maintained and well tuned internal combustion engines produce less carbon monoxide and hydrocarbons.
f) Work practices
Example: taking into account meteorological conditions, particularly winds, when spraying pesticides.
By analogy with adequate practices at the workplace, good practices at the community level can contribute to air pollution control, e.g. changes in the use of motor vehicles (more collective transportation, small cars, etc.), control of heating facilities (better insulation of buildings in order to require less heating, better fuels, etc.).
Control of emissions
Once the pollutants have been generated and are likely to disperse throughout the workplace and, beyond its boundaries, throughout the surrounding community. their control falls basically into two categories:
Polluting operations and processes can be enclosed, or the pollutants can be captured as they are generated by means of suction hoods. In both cases the pollutants would be removed by means of local exhaust ventilation. However, the pollutants removed from the working environment should not be simply discharged into the general environment. This would mean transferring the problem from the workplace to the community without solving it, and the health hazards involved would continue to exist.
The exhaust air (effluent) can be controlled either by the utilization of air cleaning equipment (collectors), by being discharged through sufficiently high stacks (dispersion), or by a combination of both methods. Closed systems (as in chemical plants and petroleum refineries) have vent stacks which should be fitted with adequate air cleaning devices (e.g. vent scrubbers) or whose height and location should be such as to promote adequate dispersion of the chemicals vented out.
The selection of the most suitable method to control emissions will depend on several factors which include the nature and concentration of pollutants in the effluent, the discharge rate, location of the industrial source in relation to other sources and urban areas, the degree of control required, economical feasibility, etc.
Air cleaning devices (collectors). The most efficient way to control emissions is by the use of adequate. well designed, well installed, efficiently operated and main tained air cleaning devices, also called separators or collectors.
According to the WHO Glossary on air pollution, a separator or collector can be defined as an "apparatus for separating any one or more of the following from a gaseous medium in which they are suspended or mixed: solid particles (filter and dust separators). liquid particles (filter and droplet separator), and gases (gas purifier)". The basic types of air pollution control equipment are the following:
(a) For particulate matter:
inertial separators (e.g. cyclones);
fabric filters (baghouses);
electrostatic precipitators;
wet collectors (scrubbers).
(b) For gaseous pollutants:
wet collectors (scrubbers);
adsorption units (e.g. adsorption beds);
afterburners, which can be direct-fired (thermal incineration) or catalytic (catalytic combustion).
Wet collectors (scrubbers) can be used to collect, at the same time, gaseous pollutants and particulate matter. Also, certain types of combustion devices can burn combustible gases and vapors as well as certain combustible aerosols. Depending on the type of effluent. one or a combination of more than one collector can be used (see AIR POLLUTION CONTROL EQUIPMENT in the Annex).
The control of odors that are chemically identifiable relies on the control of the chemical agent(s) from which they emanate (e.g. by absorption, by incineration).
However, when an odor is not defined chemically or the producing agent is found at extremely low levels, other techniques may be used such as masking (by a stronger, more agreeable and harmless agent) or counteraction (by an additive which counteracts or partially neutralizes the offensive odor).
It should be kept in mind that adequate operation and maintenance are indispensable to ensure the expected efficiency from a collector. This should be ensured both from the know-how and financial points of view, at the planning stage. Energy requirements must not be overlooked. Whenever selecting an air cleaning device, not only the initial cost but also operational and maintenance costs should be considered.
Whenever dealing with radioactive or high-toxicity pollutants, high efficiency should be ensured, as well as special procedures for maintenance and disposal of waste materials. Workers involved in these activities (e.g. change of filters) should be adequately trained and protected.
One aspect to consider is that, while some emissions contain only undesirable wastes, others contain materials of economical value (fuels, metals, etc.) whose recovery can be financially interesting. The possibility of product recovery may be a consideration when selecting one type of air pollution control equipment rather than another. As an example could be mentioned the collection of fluorine and alumina pollutants, discharged from reduction cells in the aluminum industry, and their chemical conversion to cryolite (which can be recharged to the cells), which is accomplished in wet scrubbers.
Atmospheric dispersion
Dispersion involves the utilization of meteorological factors such as winds, horizontal and vertical diffusion parameters, temperatures, etc., as well as topographical factors and characteristics of the pollutant carrier gases (temperature, velocity, etc.).
Dispersion and dilution of air pollutants in the atmosphere reduce their concentration and may be used as a control measure or as a complement to other measures.
There are, in the atmosphere, mechanisms which operate in the sense of removing pollutants, such as chemical reactions, physical mechanisms (e.g. rain) and biological mechanisms.
However, the environmental capacity of dealing with pollutants is limited and this becomes more critical as industrialisation takes place and pollution from all sources increases. Therefore, dispersion and dilution, as means of achieving acceptable ground level concentrations of pollutants, have limitations, particularly in heavily industrialized and/or heavily populated areas.
Tall stacks. Since air quality standards refer to ground pollution levels, it may be helpful if pollutants are emitted at increased heights and are dispersed over very large areas. A way of improving the degree of dispersion is the use of tall discharge stacks which may be installed at the outlet of a ventilation system, a closed system or a particular operation (e.g. a furnace).
Adequate stack heights for specific situations can be determined taking into account characteristics of the effluent (temperature, discharge rate, type and con centration of pollutants, etc.), topography and prevailing meteorological conditions, as well as air quality standards.
The effective stack height (equal to stack height plus the height that the effluent plume initially rises above the stack) increases with the effluent temperature and velocity of discharge. The most convenient are tall, wide stacks.
If the total amount of pollutants to be dispersed is very large and there are many sources in the area, effects such as reduction of sunlight may occur. Besides, in such a situation the ground pollution levels obtained through dispersion and dilution might still be unacceptable. Atmospheric dispersion, particularly tall stacks, may be used as a complement to air cleaning devices should these not be efficient enough to achieve the required degree of air pollution control.
It should be realized that tall stacks may reduce the pollution problem temporarily: in the long run, as the pollution sources increase, they will no more provide a solution since the pollutants, although dispersed, are not removed and remain in the atmosphere where they will gradually accumulate. The best and most effective method of controlling air pollutants is either to eliminate or reduce their generation at the source or to contain them and use collectors before emission.
Prevention of air pollution episodes
The combination of fluctuations in emissions and meteorological conditions may lead to dangerous build ups in the concentration of air pollutants. With adequate air monitoring, changes in pollutant levels can be detected and upward trends observed: these data, combined with adequate weather forecasts, can predict such episodes, which can then be prevented by reduction in pollutant generation or, in extreme cases, partial or complete shut-down of the sources.
In order to prevent air pollution episodes, alert levels can be used. Alert levels are concentrations of pollutants indicative of imminent or actual danger to health. Several different alert levels may be defined, ranging from a concentration at which a preliminary warning is issued to one that necessitates emergency action. As an example can be mentioned a three alert level scheme as follows: a first level, which constitutes an initial warning indicating that conditions have started to deteriorate; a second level, which triggers instructions for the curtailment of certain significant air pollution sources; and, a third level, which requires that emergency action be taken, e.g. shut-down of factories.
What in occupational health are usually called "administrative controls" also apply to air pollution. "Limitation of exposure" would be equivalent to "period of air pollution sources shut-down".
Monitoring data, emission records, pollution transport and meteorological data must be collected, statistically analysed, stored and, when needed, retrieved and disseminated. Surveillance networks with adequate and efficient lines of communication are needed to collect the data and to transmit warnings and shut-down orders whenever necessary.
Monitoring in air pollution
Monitoring programmes in air pollution are concerned with pollutant sources, pollutant levels in environmental health media and effects of pollutants. Such programmes are of fundamental importance and are related to air pollution control programmes.
The basic components of a complete monitoring and surveillance programme are: continuous and systematic collection of data on pollutant sources and levels, as well as on resulting health and other effects: analysis and evaluation of data, use and dissemination of data with a view to taking the necessary control measures.
Evaluation of an industrial emission source requires, for example, stack sampling: air quality monitoring requires the measurement of ground-level concentrations at representative sites around the source(s) and in the community. Both types of sampling are important for air pollution monitoring and control programmes; reliance on one or another depends on the legislative approach adopted.
Whenever emissions are to be controlled, the nature of pollutants present and their concentration should be determined, through adequate air sampling, in order to establish the need for control, the degree of control required and the most suitable techniques to achieve it. Once the control measures have been introduced, monitoring is necessary to check on their efficiency, which should not only be initially adequate but should be maintained in the long run.
Air quality monitoring is closely related to air pollution management actions and is performed with objectives which include:
to determine the nature and ground-level concentrations of pollutants (at certain selected points) in order to assess compliance with adopted air quality standards and, in this way, to establish the need for control measures and the degree of control required;
to observe long-term air pollution trends, particularly with a view to detecting any deterioration in air quality (resulting, for example, from increased number of pollution sources, relaxing of controls. etc.);
The strategy for environmental sampling and monitoring, that is, when monitoring programmes should be started, which pollutants should be monitored, where and how monitoring should be carried out, as well as the objectives of monitoring, should be carefully considered before launching a programme, since these are determinant factors for its design and implementation. These aspects, as well as methods of measuring specific pollutants in the atmosphere are described in the specialized literature, including the WHO Offset Publications, mentioned in the select bibliography at the end of this article.
Whenever considering factors such as pollutants to be measured, number of stations, sampling instruments and procedures, etc., the specific conditions of the place or region concerned must be kept in mind.
The importance of data storage, analysis and retrieval facilities, as well as efficient communication lines among all involved in a monitoring and surveillance network, should not be overlooked.
To conclude, it is obvious that in order to apply and coordinate air pollution control measures appropriate legislation (involving air quality standards and/or emission standards), as well as adequate management, are essential. Appropriate legislation provides a basis for effective pollution control; however, it should be realistic in terms of achievements and time. In addition to legislation, administrative provisions to ensure its enforcement are also needed.
Not only is it necessary to determine the approach to be followed, the degree of pollution control to be achieved and which are the most suitable techniques to achieve it, but it is also important to establish strategies for the implementation of control measures, and to decide at which stage this should be done, keeping in mind the priorities to be considered and how the responsibility for their design and implementation should be shared among all concerned.
Only a well coordinated, continuous and efficient multidisciplinary effort by industry, community and government can avoid the great damage which air pollution, if uncontrolled, can cause to the environment and mankind.
GOELZER FERRARI, B. I.
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Health aspects of environmental pollution control: planning and implementation of national programmes. Report of a WHO expert committee, Technical Report Series No. 554 (Geneva, World Health Organisation, 1974), 57 p.
Glossary on air pollution. WHO regional publications, European series No. 9 (Copenhagen, World Health Organisation. 1980). 114 p.
Air pollution engineering manual. Danielson, J. A. (ed.). Public Health Service Publication No. 999-AP-40 (Washington. DC, US Department of Health, Education and Welfare, Public Health Service, National Center for Air Pollution Control. 1967), 892 p. Illus. 334 ref.
Industrial pollution control handbook. Lund, H. F. (ed.). (New York, McGraw-Hill, 1971), 792 p. Illus. Ref.
Selected methods of measuring air pollutants. WHO offset publication No. 24 (Geneva, World Health Organization. 1976), 112 p.
Air monitoring programme design for urban and industrial areas. WHO offset publication No. 33 (Geneva, World Health Organization, 1977), 46 p.
