Air pollution contributes to a wide variety of adverse health effects. EPA has established NAAQS for six of the most common air pollutants – carbon monoxide, lead, ground‐level ozone, particulate matter, nitrogen dioxide, and sulfur dioxide – known as “criteria” air pollutants (or simply “criteria pollutants”). The presence of these pollutants in ambient air is generally due to numerous diverse and widespread sources of emissions. The primary standards are set to protect public health. EPA also sets secondary standards to protect public welfare from adverse effects of criteria pollutants, including protection against visibility impairment, or damage to animals, crops, vegetation, or buildings (Table 3.9).
Table 3.9 National ambient air quality standards for criteria pollutants.
Source: From USEPA (2009b).
| National ambient air quality standards for criteria pollutants | |||
| Pollutant | Primary standard | Secondary standard | Regulation allowance |
| Ozone (O3) | Eight‐hour average concentration 0.075 ppm | Same as primary | Three‐year average of the annual fourth‐highest daily maximum concentration at or below the standard. |
| Carbon monoxide (CO) | Eight‐hour average concentration 9 ppm (10 mg/m3) | N/A | Not to be exceeded more than once per year |
| One‐hour average concentration 35 ppm (40 mg/m3) | N/A | Not to be exceeded more than once per year | |
| Nitrogen dioxide (NO2) | One‐hour average concentration 0.100 ppm | Same as primary | Three‐year average of 98th percentile concentration at or below the standard |
| Eight‐hour average concentration 0.75 ppm | Same as primary | Annual arithmetic mean 0.053 ppm | |
| Particulate matter (PM10) | Twenty‐four‐hour average concentration 150 μg/m3 | Same as primary | Not to be exceeded more than once per year on average over a three‐year period. |
| Particulate matter (PM2.5) | Twenty‐four‐hour average concentration 35 μg/m3 | Same as primary | Three‐year average of 98th percentile concentration at or below the standard |
| Annual arithmetic mean: 15 μg/m3 | Same as primary | Three‐year average at or below the standard | |
| Sulfur dioxide (SO2) | Twenty‐four‐hour average concentration 0.14 ppm | Three‐hour concentration 0.5 ppm | 0.03 ppm annual arithmetic mean. Not to be exceeded more than once per year (secondary standard) |
| Lead | Rolling thee month average 0.15 μg/m3 | Same as primary | Not to be exceeded. |
As required by the CAA (USEPA 2010a), EPA periodically conducts comprehensive reviews of the scientific literature on health and welfare effects associated with exposure to the criteria air pollutants (USEPA 2006a, b, 2008a, 2009a, b, 2010b). The resulting assessments serve as the basis for making regulatory decisions about whether to retain or revise the NAAQS that specify the allowable concentrations of each of these pollutants in the ambient air (USEPA 2009b).
The primary standards are set at a level intended to protect public health, including the health of at‐risk populations, with an adequate margin of safety. In selecting a margin of safety, EPA considers such factors as the strengths and limitations of the evidence and related uncertainties, the nature and severity of the health effects, the size of the at‐risk populations, and whether discernible thresholds have been identified below which health effects do not occur. In general, for the criteria air pollutants, there is no evidence of discernible thresholds (USEPA 2006a, b, 2008a, 2009a, b, 2010b).
The secondary standards were to protect the public from known or anticipated adverse effects. The time schedule for their achievement was to be determined by state and local governments. Both primary and secondary standards had to be consistent with air quality criteria Table (3.9). In addition, standards had to prevent the continuing deterioration of air quality in any portion of an air quality region. However, new industrial and commercial operations in the region cannot be expected zero pollution, while an economic growth in the region could fail to press for a decrease in contaminant emissions from sources already in the region. To circumvent this obvious conflict between ambient air quality and anticipated economic growth, regulation were proposed in late August 1974 to prevent serious deterioration of air quality in areas where the air is already cleaner than required by Federal standards. A threshold classification is to be put into effect by states subject to an EPA review (USEPA 1975). In no case will the ambient air quality of an area violate federal primary and secondary standards. The classifications are as follows:
- Class I: Areas where almost no change from current air quality will be allowed (e.g. national parks, monuments, and areas of national heritage).
- Class II: Areas where moderate change will be allowed, but where stringent air quality constraints are desirable (e.g. residential areas).
- Class III: Areas where substantial industrial growth will be allowed and where the increase in concentration of pollutants up to the federal standards will be insignificant (e.g. designated industrial complex).
The CAA does not require EPA to establish primary NAAQS at a zero‐risk level, but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety. In all NAAQS reviews, EPA gives particular attention to exposures and associated health risks for at‐risk populations (also see Chapter 5). Standards include consideration of providing protection for a representative sample of persons comprising at‐risk populations rather than to the most susceptible single person in such groups. Even in areas that meet the current standards, individual members of at‐risk populations may at times experience health effects related to air pollution (Batson and Schwartz 2008; Kampa and Castanas 2008; Latza et al. 2008; Salvi 2007; Wigle et al. 2007).
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