3.15 Outdoor Air Quality Standards in the United States 77
3.15 Outdoor Air Quality Standards in the United States
The source of outdoor air pollution may be classified into three broad categories: thestation-
ary sources,the mobile sources,and the natural sources. Examples of stationary sources include
power plants, factories, and dry cleaners. The mobile sources of air pollution consist of cars,
buses, trucks, planes, and trains. As the name implies, the sources of natural air pollution could
include windblown dust, volcanic eruptions, and forest fires. The Clean Air Act, which sets the
standard for six major air pollutants, was signed into law in 1970. The Environmental Pro-
tection Agency is responsible for setting standards for these six major air pollutants: carb-
on monoxide (CO), lead (Pb), nitrogen dioxide (NO 2 ), ozone (O 3 ), sulfur dioxide (SO 2 ), and
particulate matter (PM). The EPA measures the concentration level of these pollutants in many
urban areas and collects air quality information by actual measurement of pollutants from
thousands of monitoring sites located throughout the country. According to a study performed
by the EPA (1997), between 1970 and 1997, the U.S. population increased by 31% and the
vehicle miles traveled increased by 127%. During this period, the total emission of air pollu-
tants from stationary and mobile sources decreased by 31% because of improvements made in
the efficiency of cars and in industrial practices, along with the enforcement of the Clean Air
Act regulations.
However, there are still approximately 107 million people who live in areas with
unhealthy air quality. The EPA is continuously working to set standards and monitor theThe EPA sets the standard for
MCL in our drinking water.
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