Control of Air Pollutants at Workplace

Emissions of air pollutants are highly regulated in many parts of the world. This guideline sets out an approach to control emissions of air pollutants to ensure regulatory compliance and minimize environmental impacts.

Control of air pollutants is a significant challenge for businesses.

Emissions of air pollutants may harm human health, damage the environment or cause a nuisance to neighbors. It is, therefore, essential to know what air pollutants are, how they are made, and how dangerous they can be before devising any control of air pollutants.

Common Air Pollutants

Carbon monoxide (CO)

Carbon monoxide (CO) is a colorless, odorless, and tasteless air pollutant gas that is harmful to humans even at low concentrations. At high concentrations, it can cause death instantly. Carbon monoxide (CO) is produced as a result of the combustion of fossil fuels and as a byproduct of the incomplete combustion of organic matter. Common sources of CO pollution are power plants, biomass burning, forest fires, and the wood industry. Indoors, carbon monoxide is produced by boilers, fireplaces, ovens, cooker hoods, tobacco smoke, and propane heaters.

Sulfur dioxide (SO₂)

Sulfur dioxide (SO₂) mainly arises from the combustion and refining of coal, oil, metal-containing ores, and shipping pollutants. Sulfur dioxide is a respiratory irritant for humans and animals. Its main danger is its subsequent transformation into sulfuric acid (H₂SO₄), which causes acid rain that destroys plants, buildings, and materials and acidifies marine and terrestrial environments.

Nitrogen Oxides (NOx)

Nitrogen oxides (NOx) include two pollutants named nitric oxide (NO) and nitrogen dioxide (NO₂). Both of these pollutants are formed through the combustion processes such as diesel engines and coal, oil, gas, wood, and waste plants. These pollutants dissolve in atmospheric water vapor to form an acid that damages vegetation, buildings, and materials, contributing to the acidification of terrestrial and aquatic ecosystems.

Particulate matter (PM)

Particulate matter comprises liquid and solid particles in the atmosphere. Primary particulate matter is emitted from a direct source, including power plants, vehicle traffic, construction sites, and indoor stoves and heaters. Secondary particulate matter, on the other hand, is produced by chemical and physical reactions with other air pollutants, including sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and ammonia (NH₃).
Particulate matter has been linked to cardiovascular and respiratory diseases such as asthma, bronchitis, and emphysema. . The size of particulate matter particles determines the magnitude of the health damage they inflict. PM 10 refers to particles with a mass median diameter of less than 10 microns, whereas PM 2.5 refers to particles with a mass median diameter of less than 2.5 microns. PM 2.5 are referred to as fine particles. PM 0.1, often known as ultrafine particles, may be included in more recent classifications. The smaller the particle, the higher the health risk due to their ability to penetrate deep into the respiratory and circulatory systems, causing damage to the lungs, heart, and brain.

Ammonia (NH₃)

Ammonia is a colorless gas with a pungent odor. Ammonia irritates the eyes, nose, throat, and respiratory tract if inhaled in small amounts due to its corrosive nature and is poisonous in large quantities. It pollutes and contributes to the eutrophication and acidification of terrestrial and aquatic ecosystems. Furthermore, ammonia forms secondary particulate matter (PM 2.5) when combined with other air pollutants. Its primary source is agricultural processes, particularly fertilizer production and livestock waste management. 

Ground-level Ozone (O₃)

Ground-level ozone is a pale blue gas with a pungent smell. It is mainly produced by the photochemical reactions of various pollutants, including nitrogen oxides, carbon monoxide, and volatile organic compounds, under the influence of intense sunlight and ultraviolet radiation. Ozone is suspected of having carcinogenic effects. It leads to reduced lung function and respiratory diseases, with exposure linked to premature mortality. Besides its impact on the human body, ozone also damages vegetation, contributing to decreased crop productivity and forest decline. Ozone is a significant component of air pollution and accelerates the deterioration of rubbers, dyes, paints, varnishes, and textiles.

Volatile Organic Compounds (VOCs)

Volatile Organic Compounds (VOCs) include pure hydrocarbons, partially oxidized hydrocarbons, and organic compounds containing chlorine, sulfur, or nitrogen.  These compounds are poisonous air pollutants and generate photochemical oxidants such as ozone. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. VOCs can cause damage to the liver, kidney, and central nervous system.

Some organics can cause cancer in animals, and some are suspected or known to cause cancer in humans.

In addition to the air pollutants mentioned above, heavy metals, such as lead (Pb) and mercury (Hg), also pose a significant threat due to their toxicity and lack of decomposition in nature. They are generated mainly by combustion plants, cement or glass manufacturing, and waste incineration facilities.

Control of Air Pollutants

1. Identify Locations & Sources of Emissions for Air Pollutants

Emissions of air pollutants are often classified as either point-source emissions or fugitive emissions. 

Point-source emissions 

Point-source emissions are those that are collected and deliberately conveyed via a chimney, stack, or ductwork to the point of release of air pollutants into the atmosphere. The location of point sources can be identified by:

• Observing chimneys, stacks, ventilators, and other mechanisms for discharging to the atmosphere. These are usually, but not always, situated at elevated locations, such as on a roof, to disperse the air pollutants.

• Generating an inventory of emission-producing equipment and then physically tracing pipes or ductwork from that equipment to the release point. This exercise has the additional benefit of identifying the origin of the air pollutant emissions;

• By reviewing engineering drawings and equipment specifications.

Fugitive emissions 

Fugitive emissions are not collected but inadvertently escape to the atmosphere; examples include solvent vapors from leaks in piping and fittings or equipment or dust emissions from conveying, transferring, or discharging powders or granules in an unsealed manner. 

Fugitive emissions are typically more challenging to identify.  Although some can be seen (e.g., dark smoke) or identified by odor, many fugitive emissions can be detected only by instrumentation.  For example, Volatile Organic Compounds (VOCs) emissions can be identified via a portable instrument such as an Organic Vapor Analyzer (OVA). Because volatile chemicals can leak from any fitting or transfer point in piping systems, a complete survey of fugitive sources would require a laborious process of applying an OVA to all flanges, valves, and other fittings, recording the readings, and tagging each location where the emission of an air pollutant is detected. 

For large workplaces that potentially have a large number of fugitive sources, this process should be prioritized. It should begin with highly volatile solvents and plant equipment with a risk of significant leakage (e.g., pumps). Piping systems should be checked for leaks when the pipes are full and at peak flow. Discharge or transfer points should be surveyed when the same conditions apply.

It would be a good practice to also identify carbon dioxide emissions. While not strictly defined as an air pollutant, carbon dioxide is considered a greenhouse gas implicated in climate change. There may also be local incentives for reducing carbon dioxide emissions or penalties for not doing so.

2. Assess the Environmental Impacts of Air Pollutants

The location of any potential environmental impact will vary depending on the nature of the pollutants.  For example, emissions of ozone-depleting substances do not have a local impact, whereas emissions of Particulate matter do.  Assessments of the significance of these different types of impact require using different methods.  For non-local impacts, the assessment should be based on the total mass emitted over a defined period, e.g., over one year.  In the case of local impacts, the assessment should follow a three-step process:

1. The mass emission rates (e.g., kg/h) should be quantified.
2. Ground-level concentrations resulting from those air pollutant emissions should be determined, either in general or at specific downwind locations.
3. The ground-level concentrations should be compared to relevant air quality standards to determine if the emissions have a measurable impact on surrounding communities.

Quantification of Emissions

Mass emission rates of air pollutants may be measured, calculated, or estimated depending on the circumstances.  Measurements fall into two distinct types:

• Discrete measurements involve inserting a probe into a stack or duct through purpose-built ports and then sampling and measuring at multiple locations across the cross-section. Through a combination of laboratory analysis of field samples and calculations, the mass emission rate of a specific air pollutant can be determined;
• Continuous measurement uses a direct-reading instrument mounted in the stack to provide a continuous or periodic measurement.

Mass emission rates of air pollutants can often be determined without the need for measurement. For example, they can be calculated from the:

• material balances, where these are available and reliable;
• potential emissions derived from process information.  Where air-pollution control equipment is in place, actual emission rates can then be calculated by applying a known removal efficiency.

Fugitive emissions are, by their nature, difficult to measure.  However, the use of process information and material balances provides one option.  Other methods for VOCs include:

• using emission factors, i.e., average VOC leakage rates for different fittings and equipment items as published by international bodies such as the US EPA. The number of items is counted and then multiplied by the appropriate emission factors. 
• using a combination of measurement and estimation.  For example, an OVA can be used to measure VOC emission rates from a representative sample of leakage points and the total mass emission rate estimated by multiplying the average result by the number of such emission points.

Determining Ground-level Concentrations

Usually, air pollutants are dispersed upward some distance above the stack exit (owing to the flow velocity and thermal buoyancy) and then carried downwind.  As buoyancy is lost, the plume eventually reaches ground level (touches down).  Of particular interest will be the following:

• location of touchdown;
• ground-level concentration at the point of touchdown;
• ground-level concentrations at susceptible environmental receptors (e.g., local houses, schools, or other public buildings).

This information can be obtained by applying mathematical models. However, air-emission modeling is complex and requires sophisticated tools and computer programs. For these reasons, it is usually carried out by specialist consultants.  To model air pollutant emissions accurately, it is necessary to gather or estimate all the various inputs, including:

• mass emission rate of the air pollutant;
• temperature, pressure, and humidity of the emission;
• physical parameters such as the height of the discharge point, dimensions of nearby buildings, and terrain features;
• meteorological data for the area and region, e.g., on the prevailing wind.

A number of different scenarios may then be modeled on the computer to predict ground-level concentrations as a function of direction, distance, and time. Typically, the focus is on the maximum ground-level concentration that is predicted under worst-case scenarios.

Comparing Ground-level Concentrations to Relevant Air Quality Standards

The last step in the process is to compare the predicted ground-level concentrations with appropriate air quality standards, i.e., limits for airborne pollutants established by a regulatory authority that is not to be exceeded.

For widespread air pollutants (e.g., particulate matter), there will likely be national or regional air quality standards that can be used.  If no such standards are available, the predicted concentrations can be compared to a surrogate standard, such as national standards from another country. Air quality standards are usually based on human health effects because humans are often the most sensitive environmental receptors.  In most cases, they are derived from Permissible Exposure Limits (PELs) by dividing the relevant PEL by an appropriate safety factor (e.g., by 100).  

In some cases, however, it may be necessary to supplement such a comparison with an assessment of the potential impact on plants and animals (i.e., an ecological risk assessment), particularly if the air pollutant source is located near a wildlife reserve or an area that is of special scientific interest because of its flora and fauna.  In addition, where the emission is odorous, compliance with a health-based air quality standard may not be sufficient to prevent a nuisance to local residents.  In such cases, a more detailed analysis should be undertaken. 

Whatever the anticipated location of the impact, the results of the assessment should determine whether measurable impacts are possible and identify the control levels required to avoid adverse impacts.

3. Select Controls to Eliminate or Manage Air Pollutant Emissions 

The assessment results described above should identify air pollutant emissions that may harm the environment. Whenever feasible, these should be eliminated by redesigning the process or substituting the materials involved (e.g., changing from an organic solvent-based process to an aqueous-based process). Avoiding the generation of an air pollutant in the first instance is the most effective and permanent method of reducing emissions.

Where such an approach is not feasible, emissions of air pollutants should be controlled to below the levels identified by the impact assessment. This should be achieved at the source of the emission or by the application of suitable emission-abatement technologies. Controls should be applied in priority order, i.e., addressing the highest impact first. 

Care should be taken to ensure that the controls selected represent the Best Feasible Environmental Option (BFEO). The various available technologies should be identified and then compared based on the following:

• removal efficiency;
• cost-effectiveness, i.e., to ensure that operation and maintenance costs (as well as capital costs) are favorable;
• downstream effects, i.e., complications from using the technology, e.g., generation of waste that is difficult to treat, unusual ductwork requirements, or the need for large quantities of water or other liquid.

Once the BFEO has been identified, appropriate action should be taken to ensure that it is implemented.
When selecting the BFEO, it is important to consider the impacts on land and water and the impacts on air. Examples of land impacts include solids that may be generated by the controls (e.g., scrubber sludge) and that require treatment or disposal. Examples of water impacts include liquid streams (e.g., scrubber blow-down) that may represent a water resource requirement or an issue for treatment as wastewater.

4. Monitor the Composition and Volume of Air Pollutants 

Monitoring should be carried out to demonstrate that the air pollutants are being effectively managed and to identify improvement opportunities. The emission rates to air and environmental impacts described above should be maintained in an inventory of air emissions, which is essentially a catalog emission by point and pollutant. The inventory should be kept up to date through periodic reviews and by updating whenever significant changes are made to site activities. Where necessary, emissions should be measured regularly to ensure that:

• assessments of environmental impacts remain accurate;
• emissions are adequately controlled;
• regulatory limits are complied with.

The periodic reviews should also include assessing how well the site is managing its air emissions and whether there are any opportunities for improvement.

Depending on the nature of a site’s emissions, and their potential impacts, it may be appropriate to consider a continuous improvement program in accordance with site objectives and budget. The details of such a program can be determined only on a site-specific basis but should include an effort to reduce year-by-year total air emissions.  In devising a program for emission reductions, it is important to consider elimination or substitution as the first option in all cases.