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Pollutants in the Ambient Air
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The composition of "unpolluted" air is unknown to us. Humans have lived on the planet thousands of years and influenced the composition of the air through their many activities before it was possible to measure the constituents of the air. Air is a complex mixture made up of many chemical components. The primary components of air are nitrogen (N2), oxygen (O2), and water vapor (H2O). About 99 percent of air is nitrogen (78%) and oxygen (21%). The remaining percent includes trace quantities of substances such as carbon dioxide (CO2), methane (CH4), hydrogen (H2), argon (Ar) and helium (He).
In theory, the air has always been polluted to some degree. Natural phenomena such as volcanoes, wind storms, the decomposition of plants and animals, and even the aerosols emitted by the ocean "pollute" the air. However, the pollutants we usually refer to when we talk about air pollution are those generated as a result of human activity. An air pollutant can be considered as a substance in the air that, in high enough concentrations, produces a detrimental environmental effect. These effects can be either health effects or welfare effects. A pollutant can affect the health of humans, as well as the health of plants and animals. Pollutants can also affect non-living materials such as paints, metals, and fabrics. An environmental effect is defined as a measurable or perceivable detrimental change resulting from contact with an air pollutant.
Human activities have had a detrimental effect on the makeup of air. Activities such as driving cars and trucks, burning of coal, oil and other fossil fuels, and manufacturing chemicals have changed the composition of air by introducing many pollutants. There are hundreds of pollutants in the ambient air. Ambient air is the air to which the general public has access, i.e. any unconfined portion of the atmosphere. The two basic physical forms of air pollutants are particulate matter and gases. Particulate matter includes small solid and liquid particles such as dust, smoke, sand, pollen, mist, and fly ash. Gases include substances such as carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NO2), and volatile organic compounds (VOCs).
Pollutants can also be classified as either primary pollutants or secondary pollutants. A primary pollutant is one that is emitted into the atmosphere directly from the source of the pollutant and retains the same chemical form. An example of a primary pollutant is the ash produced by the burning of solid waste. A secondary pollutant is one that is formed by atmospheric reactions of precursor or primary emissions. Secondary pollutants undergo a chemical change once they reach the atmosphere. An example of a secondary pollutant is ozone created from organic vapors given off at a gasoline station. The organic vapors react with sunlight in the atmosphere to produce the ozone, the primary component of smog. Control of secondary pollutants is generally more problematic than that of primary pollutants, because mitigation of secondary pollutants requires the identification of the precursor compounds and their sources as well as an understanding of the specific chemical reactions that result in the formation of the secondary pollutants.
The Environmental Protection Agency (EPA) has further classified ambient air pollutants for regulatory purposes as hazardous air pollutants (HAPs) and criteria pollutants. Criteria pollutants are pollutants that have been identified as being both common and detrimental to human welfare and are found over all the United States (ubiquitous pollutants). EPA currently designates six pollutants as criteria pollutants. These criteria pollutants are: carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), ozone (O3), lead (Pb), and particulate matter (PM). On the other hand, EPA refers to chemicals that cause serious health and environmental hazards as hazardous air pollutants (HAPs) or air toxics. Hazardous air pollutants are those pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental effects. Units 5 and 6 of this module discuss in more details the criteria pollutants and the hazardous air pollutants.
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