selected air pollutants considered harmful to public health, including the health of “sensitive” populations such as people with asthma, children, and the elderly. Secondary NAAQS are intended to protect public welfare, such as crops and other vegetation. EPA has, for the most part, set the secondary standards at the same levels as the primary standards.1

EPA has set NAAQS for six pollutants (see Table 3-1): nitrogen dioxide (NO2), ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), lead (Pb), and particulate matter (PM; PM10 and PM2.5).2 Units of measure for the standards are parts per million (ppm) by volume, milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (µg/m3). Pollutants for which there are NAAQS are known as criteria pollutants because EPA prepares “criteria documents” for them describing their sources and effect.

Each NAAQS is composed of an indicator (e.g., PM2.5), a level or concentration (e.g., 65 µg/m3), an averaging time (e.g., 24 hours), and a statistical form (e.g., 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor). Although the concentration of the standard tends to receive the most public attention, the averaging time and statistical form are also important determinants of stringency.

The CAA mandates that the NAAQS be reviewed at 5-year intervals, although in practice, the interval often takes much longer—sometimes more than a decade. A typical review cycle for each pollutant involves developing a criteria document, a compendium of current knowledge about the pollutant, followed by development of a “staff paper,” which summarizes knowledge considered most pertinent to decision making and proposes a range of actions for the EPA administrator’s consideration. The range of actions and preferred options may include no change to the existing standard. Both documents are made available to the public for comment and are formally reviewed by the independent Clean Air Scientific Advisory Committee (CASAC) of the EPA Science Advisory Board, which complements the Committee with additional experts to form a review panel for each pollutant. The EPA administrator, after a lengthy period of public comment, makes the final decision on the standard to be promulgated. Although CASAC approval of (“closure on”) the two documents is sought before the EPA administrator proposes


A separate secondary standard has been set for sulfur dioxide (SO2).


PM is regulated by mass concentration within aerodynamic size classes defined by the upper-bound particle size sampled with 50% efficiency by the inlets of air monitors (expressed in µm; PM10, PM2.5, and, PM10-2.5) (EPA 2004c).

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