EPA’s Proposed Changes to Ground-Level Ozone Standards
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The EPA is required by the Clean Air Act (CAA) to review outdoor air quality standards for ozone every 5 years “by following a set of open, transparent steps and considering the advice of a panel of independent experts.” In this most recent review, more than 1,000 scientific studies published since the last review in 2008 were examined, and the EPA says they show that current standards are not protective enough. “Studies indicate that exposure to ozone at levels below 75 ppb [parts per billion]—the level of the current standard—can pose serious threats to public health, harm the respiratory system, cause or aggravate asthma and other lung diseases, and is linked to premature death from respiratory and cardiovascular causes.”
Based on these findings, the EPA proposed to lower the ground-level ozone limits to within a range of 65 to 70 ppb and will also be taking comments on setting a level as low as 60 ppb. These new limits would apply to both primary standards for public health and secondary standards for public welfare and were proposed based on the following.
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Proposed Primary Standards
- The EPA is required to set primary air quality standards to reduce risk sufficiently to protect public health with an “adequate margin of safety,” including the health of at-risk groups, and the EPA administrator considers factors such as “the nature and severity of health effects, the size of the at-risk groups affected, and the degree of certainty and uncertainty in the science.”
- The EPA is required to set standards that are “requisite,” meaning they are neither more nor less stringent than necessary to accomplish the end result and that the EPA is not required to set primary standards at a zero-risk level.
- Although EPA staff experts, independent science advisors, and the Clean Air Scientific Advisory Committee (CASAC) concluded that scientific evidence supports a standard within a range of 60 to 70 ppb, the administrator did not include a standard of 60 ppb in the proposed range because of increasing uncertainty in the scientific evidence about the additional health improvements realized at lower ozone concentrations. However, the EPA is accepting comments on all aspects of the proposal, including on levels as low as 60 ppb and on retaining the existing standard.
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Proposed Secondary Standards
- In addition to primary standards, under the CAA, the EPA is also required to set secondary standards to protect the public welfare, which includes “plants, trees, and ecosystems.” According to the EPA, “New studies add to the evidence showing that repeated exposure to ozone stunts the growth of trees, damages plants, and reduces crop yield.”
- The proposed secondary standard would provide protection against the cumulative exposures of ground-level ozone “that can damage plants and trees during the consecutive three months in the growing season when daytime ozone concentrations are the highest and plant growth is most affected.”
- The EPA is proposing to define this necessary protection in terms of a “W126 index,” which is “a seasonal index often used to assess the impact of ozone on ecosystems and vegetation.” The proposed range is 13 to 17 parts per million-hours (ppm-hours), averaged over 3 years. To achieve this range, the EPA proposed setting an 8-hour secondary standard at a level within the range of 65 to 70 ppb. The EPA also analyzed data from air quality monitors and found that “setting a standard in a W126 form would not provide additional protection beyond an 8-hour standard.”
- The EPA is accepting comments on setting this secondary standard, on defining a target protection level in terms of a W126 index value as low as 7 to 13 ppm-hours, and on retaining the existing 8-hour secondary standard.