- 'Criteria air pollutants' is a term used internationally to describe air pollutants that have been regulated and are used as indicators of air quality.The five primary criteria pollutants include the gases- sulfur dioxide (SO2), nitrogen oxides (NOx) and carbon monoxide (CO), solid or liquid particulates (smaller than 10 µm), and particulate lead.
1. Sulfur dioxide
- Certain fossil fuels, particularly coal, may contain the element sulfur. When these fuels are burned for power or heat, the sulfur is also burned or oxidized. Sulfur dioxide is a colorless gas with a sharp, choking odour. It is a primary pollutant because it is emitted directly in the form of SO2. The sulfuric acid (H2SO4) mist is a secondary pollutant because it is not emitted directly, but is formed subsequently in the atmosphere. It is a constituent of acid rain, an important regional air pollution problem.
2. Nitrogen oxides
- There are many forms of nitrogen oxides (characterized collectively as NOx), but the one that is of greatest importance is nitrogen dioxide (NO2). Most emissions are initially in the form of nitric oxide (NO), which by itself is not harmful at concentrations usually found in the atmosphere. But NO is readily oxidized to NO2, which in the presence of sunlight can further react with hydrocarbons to form photochemical smog. Smog is, of course, harmful. NO2 also reacts with the hydroxyl radical (OH-) to form nitric acid (HNO3), which contributes to the problem of acid rain. Although NO is colorless, NO2 is pungent, irritating gas that tends to give smog a reddish brown color.
3. Carbon Monoxide
- During complete combustion of fossil fuels, carbon atoms in the fuel combine with oxygen molecules to form carbon dioxide (CO2). But the process of combustion is rarely complete. Incomplete combustion of the fuel may occur when the oxygen supply is insufficient, when the combustion temperatures are too low, or when residence time in the combustion chamber is too short. Carbon monoxide (CO), a product of incomplete combustion, is the most abundant of the criteria air pollutants.
- Carbon monoxide is completely invisible; it is colorless, odorless, and tasteless. Almost 70 per cent of the total carbon monoxide emissions come from highway vehicles, and atmospheric concentrations are very much a function of urban traffic patterns. CO levels, which typically range from 5 to 50 ppm in city air, may often reach 100 ppm on congested highways (cigarette smoke contains more than 400 ppm of carbon monoxide).
4. Solid or liquid particulates
- Extremely small fragments of solids or liquid droplets suspended in air are called particulates. Most particulates range in size from 0.1 to 100 µm (one micrometer, or 1 µm, is one millionth of a meter; it may also be called a micron). The particulate materials of most concern with regard to adverse effects on human health are generally less than 10 µm in size and are referred to as PM10.
- Suspended solids roughly 1 to 100 µm in size are called dust particles, while smaller suspended solids (less than 1 µm) may be called either smoke or fumes. Dust is formed from materials handling activities or mechanical operations, including grinding, wood working, and sandblasting. Smoke is a common product of incomplete combustion; smoke particles consist mostly of carbonaceous material. Fumes, usually consisting of very small metallic oxide particles, are typically formed during certain high temperature chemical reactions and vapor condensation.
5. Lead particulates
- This toxic metal, in the form of a fume (less than 0.5 µm in size), is one of the criteria pollutants. In the past, major sources of lead (Pb) fumes were motor vehicles that burned gasoline containing a lead based antiknock additive. Young children are particularly at risk from lead poisoning because even slightly elevated levels of lead in the blood cause learning disabilities, seizures, permanent brain damage, and even death.
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