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174 � Transportation Statistics Annual Report 1996
BOX 8-1: AIR QUALITY STANDARDS AND ATTAINMENT LEVELS
The National Ambient Air Quality Standards (NAAQS) are a set of atmospheric concentration levels (not to be confused
with emissions levels) for six critical air pollutants: carbon monoxide (CO), lead, nitrogen dioxide (NO 2), ozone
(O 3), fine particles (PM-10), and sulfur dioxide (SO 2). These standards are defined by the U.S. Environmental Protection
Agency (EPA) as concentration levels above which there is significant damage to public health and welfare.
Primary standards protect against adverse health effects, while secondary standards protect against other effects,
such as damage to vegetation and buildings. Current primary and secondary standards are shown in the table.
Since there may be significant day-to-day or even hour-to-hour variation in air quality, standards for some pollutants
are defined in terms of peak periods. Health impacts are more closely connected with the worst concentration
levels than with average levels. For example, the standard for CO is not attained if the eight-hour concentration
level is exceeded during any eight-hour period within a year, or if the higher one-hour concentration level is exceeded
during any one-hour period. For O 3 the standard is not attained if the maximum one-hour concentration exceeds
the standard more than one day a year. 1
Monitoring of ambient concentrations takes place at over 4,000 monitoring sites, known as the Aerometric
Information Retrieval System, operated mostly by state and local agencies. While not all sites monitor each pollutant,
each is monitored at 300 or more sites.
Areas where concentrations of one or more pollutants persistently exceed the standards levels are designated
as nonattainment areas. A nonattainment area is classified as marginal, moderate, serious, severe, or
extreme depending on the number of days and the amount by which the concentration standards are exceeded.
In nonattainment areas, steps must be taken to reduce emissions. As of February 8, 1996, there were 182
regions designated as nonattainment areas for one or more of the six pollutants: 75 for O 3, 35 for CO, 43 for
1 U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Quality and Emission Trends Report, 1992,
EPA-454/R-93-031 (Research Triangle Park, NC: October 1993).
Primary standard Secondary standard
Standard level Standard level
Pollutant Type of average concentration Type of average concentration
Carbon monoxide 8-hour 9 ppm No secondary standard
1-hour 35 ppm No secondary standard
Lead Maximum quarterly 1.5 ug/m Same as primary standard
average
Nitrogen dioxide Annual arithmetic 0.053 ppm Same as primary standard
mean
Ozone Maximum daily 1-hour 0.12 ppm Same as primary standard
average
PM-10 Annual arithmetic mean 50 ug/m Same as primary standard
24-hour 150 ug/m Same as primary standard
Sulfur dioxide Annual arithmetic mean 80 ug/m 3-hour 1,300 ug/m
24-hour 365 ug/m
KEY: ppm = parts per million; ug/m = micrograms per cubic meter.
NATIONAL AMBIENT AIR QUALITY STANDARDS
SOURCE: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Quality and Emissions Trends Report, 1992
(Research Triangle Park, NC: 1993).