On October 1, 2015, the U.S. Environmental Protection Agency (EPA) finalized a new, more stringent national ambient air quality standard (NAAQS) for ground-level ozone of 70 parts per billion (ppb). National Ambient Air Quality Standards for Ozone, 80 Fed. Reg. 65292 (Oct. 26, 2015).

Smog Over Los Angeles. The proposed rule would lower the standard for ground-level ozone from 75 ppb to between 65 and 70. (Photo Credit: Alan Clements, at www.chem3400.blogspot.com)

Smog Over Los Angeles. The final rule lowered the standard for ground-level ozone from 75 to 70 parts per billion. (Photo Credit: Alan Clements, at http://www.chem3400.blogspot.com)

Ozone, the main component of smog, is not emitted directly into the air but results when emissions of precursors, such as nitrogen oxides, volatile organic compounds, carbon monoxide and methane, “cook” in the sun. Typical sources of these emissions include electric utilities and motor vehicle exhaust. Ozone causes or aggravates a variety of respiratory conditions, such as asthma, emphysema and bronchitis. The federal Clean Air Act requires EPA to adopt primary and secondary NAAQS for six “criteria” pollutants, including ozone, and revise the standards every five years. 42 U.S.C. 7409. Primary standards must be sufficient to protect public health from the risks of ozone in the ambient air, and secondary standards must be sufficient to protect “public welfare”—e.g., trees, plants and ecosystems. EPA’s current primary and secondary standards for ozone are both 75 ppb.

EPA’s November 2014 proposed rule suggested reducing the standards to between 65 to 70 ppb and requested comments on reducing the standard to as low as 60 ppb, a level that would have put some urban areas of Minnesota into “nonattainment” under the Clean Air Act New Source Review program. However, EPA decided to set the revised primary and secondary standards at the upper end of the proposed range—70 ppb. The agency also retained the standards’ indicators (O3), forms (fourth-highest daily maximum, averaged across three consecutive years) and averaging times (eight hours). In promulgating the final rule, EPA stated that clinical studies and risk and exposure analyses made clear that a standard of 70 ppb will protect public health and “essentially eliminate exposures that have been shown to cause adverse health effects, protecting 99.5 percent of children from even single exposures to ozone at 70 ppb.” The new standard takes effect 60 days after publication in the Federal Register.