On November 25, 2014, the U.S. Environmental Protection Agency (EPA) proposed a new, more stringent national ambient air quality standard (NAAQS) for ground-level ozone, the main component of smog. National Ambient Air Quality Standards for Ozone, _____ Fed. Reg. ____ (proposed Nov. 25, 2014).

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

Ozone 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 parts per billion (ppb), and the agency is proposing to reduce both standards to between 65 to 70 ppb. EPA has also solicited comments on reducing the standard to as low as 60 ppb. Public comments may be submitted through February 23, 2014, and EPA expects to promulgate a final rule by October 2015. The rule has potential implications for Minnesota. Depending on where EPA sets the revised standards, ozone levels in parts of Minnesota, particularly the metro area, could fall into “nonattainment” under the Clean Air Act New Source Review program. This would require the Minnesota Pollution Control Agency to amend Minnesota’s Clean Air Act implementation plan to include measures sufficient to meet the new standards.

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