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Clean Air World

Air Pollutants

There are many types of air pollutants. The exact composition and concentration of pollutants depend on the source activity or process, the type of fuel and/or chemicals involved, and in some cases the meteorological conditions under which the pollutant is emitted. Air pollutants are pervasive, and are responsible for a range of adverse health and environmental effects. These pollutants include hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), sulfur dioxide (SO2), ozone, volatile organic compounds (VOCs), hydrogen sulfide (H2S), and toxic air contaminants such as lead (Pb). Greenhouse gases such as carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and high-global warming potential gases (e.g., perfluorocarbons, sulfur hexafluoride, hydrofluorocarbons, nitrogen trifluoride, hydrofluoroethers, and ozone depleting substances) have been implicated in global warming effects. Sources of air pollution also emit quantities of other substances which are often referred to collectively as toxic or “hazardous” air pollutants (HAPs). These pollutants can have more serious health impacts than some of the general pollutants, depending on the level of exposure. In many cases, toxic pollutants constitute a small fraction of the total HC or PM emissions.

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Greenhouse Gases

One of the key contributors to global warming is the increased emissions of greenhouse gases (GHGs). When solar radiation passes into the Earth’s atmosphere, most is absorbed by the Earth and some is reradiated back into the atmosphere. GHGs trap the heat, keep it from passing through the atmosphere to space, thus causing the lower atmosphere to warm. Some GHGs occur naturally in the atmosphere, while others are emitting strictly by human activity.

CO2 is emitted by combustion of fossil fuels (oil, natural gas, and coal), solid waste, biomass (e.g., wood products), and by industrial processes (e.g., cement kilns). Also, CO2 can be removed from the atmosphere (or “sequestered”) when it is absorbed by plants as part of the biological carbon cycle. CH4 is emitted during the production and transport of fossil fuels, and can be emitted through livestock and other agricultural practices and by the decay of landfill wastes. N2O is emitted by fossil fuel and solid waste combustion, and during agricultural and industrial activities, Hydrofluorocarbons, sulfur hexafluoride, and perfluorocarbons are emitted from a wide range of industrial processes, including during their production as well as their use in refrigeration and air conditioning, during semiconductor manufacturing, and as substitutes for ozone depleting substances (ODCs). Although these gases are typically emitted in smaller quantities relative to CO2, they have a higher global warming potential (GWP), and are sometimes referred to as “high GWP gases”.

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Mercury and Other Toxic Air Pollutants

Toxic air pollutants are substances that cause or may cause cancer or other serious health effects, such as reproductive or birth defects, and neurological, cardiovascular, and respiratory disease. They can be found in gaseous, aerosol, or particulate forms. Some toxic air pollutants, such as mercury (Hg), are persistent bioaccumulative toxics (i.e., they are stored indefinitely in the body and increase over time). These toxics can deposit onto soils or surface waters, where they are taken up by plants and are ingested by animals, with concentrations increasing as the toxics move up through the food chain to humans. Sources of hazardous air pollutants include stationary sources such as factories, dry cleaners, and hospitals, as well as mobile sources such as cars, buses, and construction equipment.

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Ozone (O3) is created by a chemical reaction between NOx and VOCs that is generated by heat and sunlight. A large share of ozone-generating pollutants are produced by motor vehicles, although any fuel combustion source emits the pollutants that can contribute to ozone formation. Ozone is a major problem in many urban areas around the world where it can reduce lung capacity and increase susceptibility to respiratory illnesses, especially in infants and the elderly.

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Particle Pollution

Particulate matter can be either emitted directly by sources (primary) or formed in the atmosphere from precursors (secondary). Primary particles are generated by combustion such as the burning of diesel fuel, and by mechanical generation such as the churning of road dust, brake wear, and construction activities. Secondary particles form in the air due to complex chemical reactions that convert gaseous precursor pollutants into particles. Most dangerous are the fine particles (PM2.5) which can be absorbed deep in the lungs, causing aggravated asthma, decreased lung function, lung cancer, cardiac problems, and premature death.

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