Criteria air pollutants are common throughout the United States. These pollutants can injure health, harm the environment and cause property damage. U.S. EPA has identified six criteria pollutants:

Carbon Monoxide
Lead
Nitrogen Dioxide (one of several Nitrogen Oxides)
Ozone (formed from precursor Volatile Organic Compounds)
Particulate Matter
Sulfur Dioxide

EPA has established National Ambient Air Quality Standards (NAAQS) for each criteria pollutant, which define the maximum legally allowable concentration. If the NAAQS for a pollutant is exceeded, adverse effects on human health may occur. EPA and state agencies monitor area quality to assess compliance with these standards. Areas of the country where air pollution levels persistently exceed the standards may be designated by the U.S. EPA as nonattainment areas.

CARBON MONOXIDE (CO)
Carbon monoxide (CO) is a colorless, odorless and poisonous gas produced by incomplete burning of carbon in fuels. When CO enters the bloodstream, it reduces the delivery of oxygen to the body's organs and tissues. Health threats are most serious for those who suffer from cardiovascular disease. Exposure to elevated CO levels can cause impairment of visual perception, manual dexterity, learning ability and performance of complex tasks.

77% of the nationwide CO emissions are from transportation sources. The largest emissions contribution comes from highway motor vehicles. Thus, the focus of CO monitoring has been at traffic-oriented sites in urban areas where the main source of CO is motor vehicle exhaust. Other major CO sources are wood-burning stoves, incinerators and industrial sources.

NAAQS: 9 ppm (8-hr average) and 35 ppm (1-hr average) More in Scorecard's chemical profile of carbon monoxide.

LEAD (Pb)
Lead (Pb) is a widely used metal that, once released to the environment, can contaminate air, food, water, or soil. Exposures to even small amounts of lead over a long time can accumulate to reach harmful levels. Harmful effects may therefore develop gradually without warning. Short-term exposure to high levels of lead may also cause harm. Lead can adversely affect the nervous, reproductive, digestive, cardiovascular blood-forming systems, and the kidney. In men, adverse reproductive effects include reduced sperm count and abnormal sperm. In women, adverse reproductive effects include reduced fertility, still-birth, or miscarriage. Children are a sensitive population as they absorb lead more readily and their developing nervous system puts them at increased risk for lead-related harm, including learning disabilities.

Lead gasoline additives, non-ferrous smelters, and battery plants are the most significant contributors to Pb emissions into the atmosphere. In 1993 transportation sources contributed 33% of the annual emissions, down substantially from 81% in 1985. Total Pb emissions from all sources dropped from 20,100 tons in 1985 to 4,900 tons in 1993. The decrease in Pb emissions from cars and trucks shifting to lead-free gasoline accounts for essentially all of this decline.

NAAQS: 1.5 ug/m3 (quarterly average)

More in Scorecard's chemical profile of lead.

NITROGEN DIOXIDE (NO2)
Nitrogen dioxide (NO2) is a brownish, highly reactive gas that is present in all urban atmospheres. NO2 can irritate the lungs, cause bronchitis and pneumonia, and lower resistance to respiratory infections.

The major mechanism for the formation of NO2 in the atmosphere is the oxidation of nitric oxide (NO), which is produced by most combustion processes.

NAAQS: 0.053 ppm (annual mean)

More in Scorecard's chemical profile of nitrogen dioxide.

NITROGEN OXIDES (NOx)
Nitrogen oxides (NOx) include various nitrogen compounds like nitrogen dioxide (NO2) and nitric oxide (NO). These compounds play an important role in the atmospheric reactions that create ozone (O3) and acid rain. Individually, they may affect ecosystems, both on land and in water.

NOx forms when fuels are burned at high temperatures. The two major emissions sources are transportation vehicles and stationary combustion sources such as electric utility and industrial boilers.

More in Scorecard's chemical profile of nitrogen oxides.

OZONE (O3)
Ozone (O3) is the major component of smog. Although O3 in the upper atmosphere is beneficial because it shields the earth from the sun's harmful ultraviolet radiation, high concentrations of O3 at ground level are a major health and environmental concern. The reactivity of O3 causes health problems because it damages lung tissue, reduces lung function and sensitizes the lungs to other irritants. Scientific evidence indicates that ambient levels of O3 not only affect people with impaired respiratory systems, such as asthmatics, but healthy adults and children as well. Exposure to O3 for several hours at relatively low concentrations has been found to significantly reduce lung function and induce respiratory inflammation in normal, healthy people during exercise.

O3 is not usually emitted directly but is formed through complex chemical reactions in the atmosphere. Precursor compounds like volatile organic compounds (VOC) and oxides of nitrogen (NOx) react to form O3 in the presence of sunlight. These reactions are stimulated by ultraviolet radiation and temperature, so peak O3 levels typically occur during the warmer times of the day and year.

NAAQS: 0.12 ppm (1-hr average) and 0.08 ppm (8-hr average)

More in Scorecard's chemical profile of ozone.

PARTICULATE MATTER (PM)
Particulate matter (PM) is a mixture of particles that can adversely effect human health, damage materials and form atmospheric haze that degrades visibility. PM is usually divided up into different classes based on size, ranging from total suspended matter (TSP) to PM-10 (particles less than 10 microns in aerodynamic diameter) to PM-2.5 (particles less than 2.5 microns). In general, the smallest particles pose the highest human health risks. PM exposure can affect breathing, aggravate existing respiratory and cardiovascular disease, alter the body's defense systems against foreign materials, and damage lung tissue, contributing to cancer and premature death. Individuals with chronic obstructive pulmonary or cardiovascular disease, asthmatics, the elderly and children are most sensitive to the effects of PM.

Particulate matter (PM) includes dust, dirt, soot, smoke and liquid droplets directly emitted into the air by sources such as factories, power plants, cars, construction activity, fires and natural windblown dust. Particles formed in the atmosphere by condensation or the transformation of emitted gases such as SO2 and VOCs are also considered particulate matter.

PM10 NAAQS: 50 ug/m3 (annual mean) and 150 ug/m3 (24-hr average)

PM2.5 NAAQS: 15 ug/m3 (annual mean) and 65 ug/m3 (24-hr average)

More in Scorecard's chemical profiles of PM-10 and PM-2.5.

SULFUR DIOXIDE (SO2)
High concentrations of sulfur dioxide (SO2) affect breathing and may aggravate existing respiratory and cardiovascular disease. Sensitive populations include asthmatics, individuals with bronchitis or emphysema, children and the elderly. SO2 is also a primary contributor to acid rain, which causes acidification of lakes and streams and can damage trees, crops, historic buildings and statues. In addition, sulfur compounds in the air contribute to visibility impairment in large parts of the country. This is especially noticeable in national parks.

Sulfur dioxide (SO2) is released primarily from burning fuels that contains sulfur (like coal, oil and diesel fuel). Stationary sources such as coal- and oil-fired power plants, steel mills, refineries, pulp and paper mills, and nonferrous smelters are the largest releasers.

NAAQS: 0.03 ppm (annual mean), 0.14 ppm (24-hr average) and 0.50 ppm (3-hr average)

More in Scorecard's chemical profile of sulfur dioxide.

VOLATILE ORGANIC COMPOUNDS (VOC)
Volatile organic compounds (VOC) are defined by the Clean Air Act as chemicals that participate in forming ozone (O3). Ozone is a respiratory toxicant. The class of VOCs includes many specific chemicals which may also cause adverse health effects in their own right (such as cancer or reproductive toxicity).

VOCs are emitted from diverse sources, including automobiles, chemical manufacturing facilities, drycleaners, paint shops and other commercial and residential sources that use solvent and paint. VOC emissions form O3 through complex chemical reactions with oxides of nitrogen (NOx) in the presence of sunlight.

More in Scorecard's chemical profile of volatile organic compounds.