Background. The Clean Air Act (CAA), which was last amended in 1990, requires EPA to set NAAQS for wide-spread pollutants from numerous and diverse sources considered harmful to public health and the environment. The Clean Air Act established two types of national air quality standards:

• Primary standards set limits to protect public health, including the health of at-risk populations such as people with pre-existing heart or lung disease (such as asthmatics), children, and older adults.

• Secondary standards set limits to protect public welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings.

The Clean Air Act requires periodic review of the science upon which the standards are based and the standards themselves.

Also under the CAA, each state must develop a plan—the State Implementation Plan (SIP)—describing how it will attain and maintain the NAAQS—i.e., a plan as to how it will clean up polluted areas and keep them clean. In general, the SIP is a collection of programs (monitoring, modeling, emission inventories, control strategies, etc.) and documents (policies and rules) that the state uses to attain and maintain the NAAQS. A state must engage the public in approving its plan prior to sending it to EPA for approval.

8-hour ozone non-attainment areas (2008 standard) as of 2 July 2014. Non-attainment areas are those where the standards are not met. Click to enlarge.

The current O₃ standards were established in 2008 at the end of the previous review cycle. These standards include a primary O₃ standard of 75 ppb, and a secondary O₃ standard set identical to the primary standard. These 2008 standards are now under review, as required by the CAA.

In January 2010, EPA proposed strengthening the 8-hour primary ozone standard, designed to protect public health, to a level within the range of 60-70 ppb. EPA also proposed a distinct cumulative, seasonal secondary standard, designed to protect sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas. EPA also proposed to set the level of the secondary standard within the range of 7-15 ppm-hours. (Earlier post.)

In September 2011, President Obama requested that EPA withdraw the agency’s draft Ozone National Ambient Air Quality Standards (NAAQS) rulemaking reflecting these more stringent standards. At that time, the President said that since work was already underway to update the science that would result in the reconsideration of the ozone standard in 2013, he did not support asking state and local governments to begin implementing a new standard that will soon be reconsidered.

The first external review draft of this current PA was produced in August 2012, with the second external review draft released in January 2014.

The PA is based on the scientific and technical information assessed and presented in the Integrated Science Assessment for Ozone (ISA); the Health Risk and Exposure Assessment for Ozone (HREA); and the Welfare Risk and Exposure Assessment for Ozone (WREA). The PA, said the staff, is intended to “bridge the gap” between the relevant scientific evidence and technical information and the judgments required of the EPA Administrator in determining whether to retain or revise the current standards.

Findings. Staff found that current science provides even more comprehensive evidence of the deleterious effects of O₃ exposure on human health. Secondary oxidation products, which develop as a result of O₃ exposure, initiate numerous responses at the cellular, tissue, and whole organ level of the respiratory system. These key initiating events have the potential to result in a variety of adverse respiratory effects, as well as effects outside the respiratory system (e.g., cardiovascular effects).

Ozone inhalation poses the greatest risk to people in certain lifestages (i.e., children, older adults), people with asthma, people with certain genetic variants (related to oxidative stress and inflammation), people with diets limited in certain nutrients (antioxidant vitamins C and E), and people experiencing the largest exposures (e.g., outdoor workers, children).

While staff concluded that while the 75 ppb standard provides “important improvements in public health protection,” the available health evidence and exposure/risk information “call into question the adequacy of the public health protection provided by the current standard.”

Specific to the recommended new range of 60-70 ppb for the revised primary standard, staff concluded that:

• A level of 70 ppb is below the O₃ exposure concentration that has been reported to elicit a broad range of respiratory effects. A level of 70 ppb is also just below the lowest exposure concentration at which the combined occurrence of respiratory symptoms and lung function decrements have been reported (72 ppb). A level of 70 ppb is also above the lowest exposure concentration demonstrated to result in lung function decrements and pulmonary inflammation (60 ppb).

  Compared to the 75 ppb standard, 70 ppb would be expected to (1) reduce the occurrence of exposures of concern to O₃ concentrations that result in respiratory effects in healthy adults by about 45 to 95%, almost eliminating the occurrence of multiple exposures at or above 70 ppb; (2) reduce the occurrence of moderate-to-large O₃-induced lung function decrements by about 15 to 35%; (3) more effectively maintain short- and long- term O₃ concentrations below those present in the epidemiologic studies that reported significant O₃ health effect associations in locations likely to have met the current standard; and (4) reduce the risk of O₃-associated mortality and morbidity, particularly the risk associated with the upper portions of the distributions of ambient O₃ concentrations.

• A level of 65 ppb is below the lowest exposure concentration at which the combined occurrence of respiratory symptoms and lung function decrements has been reported. A level of 65 ppb is above the lowest exposure concentration demonstrated to result in lung function decrements and pulmonary inflammation.

  Compared to a standard with a level of 70 ppb, a revised standard with a level of 65 ppb would be expected to (1) further reduce the occurrence of exposures of concern (by about 80 to 100% compared to the current standard), decreasing exposures at or above 60 ppb and almost eliminating exposures at or above 70 and 80 ppb; (2) further reduce the occurrence of FEV1 decrements > 10, 15, and 20% (by about 30 to 65%, compared to the current standard); (3) more effectively maintain short- and long-term O₃ concentrations below those present in the epidemiologic studies that reported significant O₃ health effect associations in locations likely to have met the current standard; and (4) further reduce the risk of O₃-associated mortality and morbidity, particularly the risk associated with the upper portion of the distribution of ambient O₃ concentrations.

• A level of 60 ppb corresponds to the lowest exposure concentration demonstrated to result in lung function decrements and pulmonary inflammation. Compared to a standard with a level of 70 or 65 ppb, a revised standard with a level of 60 ppb would be expected to (1) further reduce the occurrence of exposures of concern (by about 95 to 100% compared to the current standard), almost eliminating exposures at or above 60 ppb; (2) further reduce the occurrence of FEV1 decrements > 10, 15, and 20%, (by about 45 to 85% compared to the current standard); (3) more effectively maintain short- and long-term O₃ concentrations below those present in the epidemiologic studies that reported significant O₃ health effect associations in locations likely to have met the current standard; and (4) further reduce the risk of O3-associated mortality and morbidity, particularly the risk associated with the upper portion of the distribution of ambient O₃ concentrations.

With regard to the secondary standard, staff concluded that it is appropriate to give consideration to a range of levels from 17 to 7 ppm-hr.

Resources

• Policy Assessment for the Review of the Ozone NAAQS, Final Report (August 2014)