EPA's National Ambient Air Quality Standards: The Standard Review/Reevaluation Process

Fact Sheet

• The Clean Air Act directs EPA to identify and set national ambient air quality standards for pollutants that cause adverse effects to public health and the environment. EPA has set national air quality standards for six common air pollutants---ground-level ozone (smog), carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, and particulate matter (measured as PM₁₀ and PM_2.5.)

• For each of these six pollutants, EPA has set health-based or "primary" standards to protect public health, and welfare-based or "secondary" standards to protect the environment (crops, vegetation, wildlife, buildings and national monuments, visibility, etc).

• EPA is required by the Clean Air Act to review the health and welfare-based standards at least once every five years to determine whether revisions to the standards are necessary to continue to protect public health and the environment.

• EPA undertakes an extensive scientific and technical assessment process during the standard review for any pollutant. The first step in the process is the preporation of the Agency's "criteria document," an extensive assessment of scientific data pertaining to the health and environmental effects associated with the pollutant under review.

• EPA then prepares a document (known as a "staff paper") that interprets the most relevant information in the "criteria document" and identifies 1) factors EPA staff believes should be considered in the standard review; 2) uncertainties in the scientific data; and 3) ranges of alternative standards the staff believes should be considered. The "staff paper" is compiled by technical staff to assess the policy implications of the science. It represents the views of the staff and, in final form, is ultimately used as the basis for staff recommendations to the EPA Administrator.

• Drafts of both the "criteria document" and the "staff paper," which are based on thousands of peer-reviewed scientific studies, receive extensive review by representatives of the scientific community, industry, public interest groups and the public, as well as the Clean Air Scientific Advisory Committee (CASAC)---a Congressionally mandated group of independent scientific and technical experts.

• As part of its mandate, CASAC also makes recommendations to EPA on the adequacy of the standards. Based on the scientific assessments and taking into account the recommendations of CASAC, the EPA Administrator must judge whether or not proposing revisions to the standards is appropriate.

• Before making a decision, the EPA Administrator goes through an extensive public review and comment process. EPA reviews and extensively analyzes issues raised in public comments before announcing a final decision. As with every proposed and final rule, all other relevant federal agencies are given the opportunity to participate in the process.

• Since 1980, EPA has completed reviews of six national ambient air quality standards. Only one of those reviews resulted in a revised standard.

• EPA has completed its reviews of the national air quality standards for ozone and particulate matter. The Administrator signed both the new particulate and ozone standards in July 1997.

• As a result of this review, EPA is phasing out and replacing the previous 1-hour primary ozone standard (health-based) with a new 8-hour standard to protect against longer exposure periods. In establishing the 8-hour standard, EPA is setting the standard at 0.08 parts per million (ppm) and defines the new standard as a "concentration-based" form, specifically the 3-year average of the annual 4th-highest daily maximum 8-hour ozone concentration. The standards were last revised in 1979.

• EPA is also revising the primary (health-based) PM standards by adding a new annual PM_2.5 standard set at 15 micrograms per cubic meter (µ/m³) and a new 24-hour PM_2.5 standard set at 65 µ/m³. EPA is retaining the annual PM₁₀ standard of 50 µ/m³ and adjusting the PM₁₀ 24-hour standard of 150 µ/m³ by changing the form of the standard.

• The health-based standard for lead was last revised in 1978 and is set at 1.5 micrograms per cubic meter (µ/m³), averaged over a calendar quarter (once every three months). The secondary standard for lead is the same as the primary standard.

• There are two health-based standards for carbon monoxide---an 8-hour standard set at 9 ppm and a 1-hour standard set at 35 ppm. Both standards are not to be exceeded more than once per year. EPA announced its decision to retain the current standards for carbon monoxide in 1994. There is no secondary standard for carbon monoxide.

• EPA proposed to retain the current national standards for nitrogen dioxide in 1995. The primary and secondary standards for nitrogen dioxide are both set a 0.053 ppm, measured as an annual average. EPA issued its final decision on the nitrogen dioxide standards in October 1996.

• EPA announced its final decision to retain the current health-based standards for sulfur dioxide in May 1996. The two primary standards are set at 0.14 ppm, 24-hour average and 0.030 ppm, averaged annually. The 24-hour standard may not be exceeded more than once per year and the annual standard is never to be exceeded. In January 1997, EPA proposed a new program to address the potential health risks posed to asthmatics by short-term peak levels of sulfur dioxide in localized situations. EPA retained the secondary standard for sulfur dioxide in 1993, which is set at 0.50 ppm, averaged over a three-hour period. The secondary standard may not be exceeded more than once per year.