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December 2004 The six principal pollutants Trends toward improvement Natasha Kassulke Ambient air quality can be monitored at stacks and remote locations. © Sherry Sanford

The Environmental Protection Agency has set national air quality standards for six principal pollutants (also called criteria pollutants): nitrogen dioxide (NO2), ozone (O3), sulfur dioxide (SO2), particulate matter (PM), carbon monoxide (CO) and lead (Pb).

Each year, EPA looks at these pollutant levels and the amount of emissions to see how both have changed over time. Nationwide monitoring has found that concentrations of most criteria pollutants improved from 1983 through 2002. Monitoring sites are established near areas of high pollution or near pollution sources.

Continuous monitors measure ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), fine particle matter in the size of 2.5 microns (PM2.5), particulate matter as big as 10 microns (PM10) and nitrogen oxides (NOx). These monitors produce hourly readings and also measure wind speed and direction, temperature, solar radiation and barometric pressure.

Ozone (O3)

At ground level, ozone it is created by a chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of heat and sunlight.

Ozone can be good or bad depending on where it is located in the atmosphere. Good ozone occurs naturally in the stratosphere 10 to 30 miles above the earth's surface and forms a layer that protects life on earth from the sun's harmful rays. In the lower atmosphere, ozone is considered bad.

Vehicle exhaust, industrial emissions, gasoline vapors and chemical solvents are some major sources of NOx and VOCs that form ozone. Wind can carry ozone hundreds of miles from its source. Ozone may damage plants and trigger health problems even at low levels – people with respiratory illnesses such as asthma are most affected. Peak ozone levels typically occur during hot and dry summertime conditions.

To reduce ozone, some communities are reducing NOx emissions from power plants and industrial combustion sources as well as introducing low-emission cars and trucks, using cleaner gasoline and improving vehicle inspection programs to reduce VOCs.

Particle pollution (PM)

Although particulate matter emissions from industrial sources have decreased significantly since the Clean Air Act was enacted in 1970, EPA found that in 2002 over 15 million people still live in areas with PM concentrations above the national standard.

As a result, efforts have been stepped up to reduce PM, the main ingredient of haze, soot and airborne dust. Particle pollution occurs year-round and is a mixture of microscopic solids (metals, soil, dust and allergens) and liquid droplets (including acids and organic chemicals) suspended in air.

The particle size is linked to its potential for causing health problems. Particles less than 10 microns in diameter pose the greatest problems because they attach to toxins and are inhaled deep into your lungs and even get into your bloodstream. Larger particles are less of a concern, but can irritate your eyes, nose and throat. Small particles of concern include the fine particles 2.5 microns in diameter and coarse particles of 2.5 to 10 microns in diameter.

People with heart or lung disease, asthma, and older adults and children are at the greatest risk. PM also is the major source of haze that reduces visibility. Atmospheric deposition, particles that are carried over long distances by wind or water, can make lakes and streams acidic, damage forests and crops, and change the nutrient balance in coastal waters.

PM sources include cars, trucks, buses, factories, construction sites, tilled fields, unpaved roads, stone crushing and wood burning. Other particles may be formed in the air as gaseous chemicals combine.

Carbon monoxide (CO)

Carbon monoxide is a colorless, odorless gas formed when carbon in fuels is not burned completely. Motor vehicle exhaust contributes about 56 percent of CO emissions nationwide. Other sources include off-road vehicles (construction equipment and boats), industrial processes and forest fires.

The highest levels of CO in the outside air typically occur during the colder months when air pollution becomes trapped near the ground beneath a layer of warm air. Carbon monoxide at high levels is poisonous even to healthy people.

Improvements in vehicle and emissions control technology have greatly reduced emissions of carbon monoxide over the past 20 years, but the number of cars and trucks on the road and the miles they are driven have doubled in the same period.

Approaches to reduce CO have included establishing national ambient air quality standards, requiring national controls for motor vehicle emissions (including tailpipe emissions, new vehicle technologies and clean fuels programs) and requiring reductions from large industrial facilities.

Sulfur dioxide (SO2)

Sulfur dioxide easily dissolves in water to form weak sulfuric acid, the main ingredient in acid rain. Sulfur is prevalent in raw materials including crude oil, coal and ore that contains metals like aluminum, copper, zinc, lead and iron.

Over 65 percent of SO2 released to the air – more than 13 million tons per year according to EPA – comes from electric utilities, especially those that burn coal. Other sources are petroleum refineries, cement manufacturing, metal processing plants, trains, large ships and some off-road diesel equipment.

SO2 contributes to respiratory illness, acid rain formation and haze as it is transported over long distances by wind.

Wisconsin's early research and negotiations with utilities and industry crafted acid rain law and policy to reduce SO2 emissions years before tighter federal standards.

Over the past 30 years, sulfur dioxide emissions have decreased by more than a third by installing pollution control equipment at coal-fired power plants, reducing pollution from industrial processing facilities, reducing the average sulfur content of fuels burned, using cleaner fuels and crafting tough acid rain law through negotiations with utilities and industry.

Nitrogen dioxide (NO2)

Nitrogen oxides (NOx) is a generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of these nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as a reddish-brown layer over many urban areas.

Nitrogen oxides form when fuel is burned at high temperatures. Primary sources are motor vehicles, electric utilities and other industrial, commercial and residential furnaces and boilers that burn fuels.

NO2 is one of the main ingredients in forming ground-level ozone, causes respiratory problems, contributes to acid rain formation, deteriorates water quality and causes haze. NOx and the pollutants formed from NOx can be transported over long distances by prevailing winds.

Since 1970, EPA has tracked emissions of the six principal air pollutants. All have decreased significantly except for NOx, which has increased about 10 percent over this period. To reduce emissions, EPA requires motor vehicle manufacturers to redesign car and truck engines and install pollution controls. To reduce acid rain, EPA devised a strategy to cut NOx emissions from coal-fired power plants. Since NOx can be transported long distances, the Act also requires "upwind" states to implement programs that will help "downwind" states meet ozone standards.

Lead (Pb)

Lead levels in the air have decreased dramatically since 1978, primarily due to emission reductions from cars and trucks with the phase out of leaded gasoline. Today, metal processing plants are responsible for most lead in the air, according to EPA.

Children and infants are at the greatest risk of lead exposure. They can be exposed to lead through air, paint chips and lead-contaminated food or water. Lead causes damage to kidneys, liver, brain, nerves and other organs. Lead exposure also may lead to osteoporosis and reproductive disorders. Excessive exposure can lead to mental retardation, behavioral disorders, memory problems and mood changes. It causes high blood pressure, increased heart disease and anemia.

High levels of lead are still of concern in urban areas with high levels of traffic, trash incinerators, or other industry, as well as areas near lead smelters, battery plants or industrial facilities that burn fuel.

Natasha Kassulke is the associate editor of Wisconsin Natural Resources magazine.