Environmental Engineering FOURTH EDITION

414 ENVIRONMENTAL ENGINEERING

compliance dates for automobile emission standards. In 1990, amendments focused
on toxic air pollutants and control of all vehicular emissions. The 1970 amendments,
however, are the basis for existing federal clean air legislation.

National Ambient Air Quality Standards (NAAQS)

The EPA is empowered to determine allowable ambient concentrations of those air
contaminants that are pollutants throughout the United States; these are the National
Ambient Air Quality Standards (NAAQS) and historically have been the focus of
the nationwide strategy to protect air quality. The primary NAAQS are intended to
protect human health, the secondary NAAQS to “protect welfare.” The latter levels
are actually determined as those needed to protect vegetation. These standards are
listed in Table 21-1.
NAAQS are set on the basis of extensive collections of information and data on the
effects of these air pollutants on human health, ecosystems, vegetation, and materials.
These documents are called “criteria documents” by the EPA, and the pollutants for
which NAAQS exist are sometimes referred to as “criteria pollutants.” Data indicate
that all criteria pollutants have some threshold below which there is no damage.
Under the Clean Air Act, most enforcement power is delegated to the states by
the EPA. However, the states must show the EPA that they can clean up the air to the

Table 21-1. Selected National Ambient Air Quality Standards

Pollutant

Primary standard Secondary standard

PPm I@m3 PPm wdm3

Particulate matter less than

10 pm in diameter: PMlo

24-h average

Annual geometric mean

3-h maximum

24-h average

Annual arithmetic mean

Nitrogen oxides

Annual arithmetic mean

Carbon monoxide

1 h maximum

8hmaximum

1 h maximum

Quarterly arithmetic mean

Sulfur dioxide

Ozone

Lead

150

50

0.5 1300

0.14 365

0.03 80

0.053 100

9 10,000

35 40,000

0.12 210

1.5

150

50

0.02 60

0.1 260

Same as primary

Same as primary

Same as primary

Same as primary