"Implementation of revised ozone standards by the U.S. Environmental Protection Agency (EPA) is now moving forward, after the agency designated 52 areas with just over 200 counties or partial counties and two tribal areas as 'nonattainment' for the standards. The standards-- formally known as National Ambient Air Quality Standards (NAAQS) for ground-level ozone-- are standards for outdoor (ambient) air. In 2015, EPA tightened both the primary (health-based) and secondary (welfare-based) standards from 75 parts per billion (ppb) to 70 ppb after concluding that protecting public health and welfare requires lower concentrations of ozone than were previously judged to be safe. Ozone aggravates heart and lung diseases and may contribute to premature death; the primary standard addresses these concerns. Ozone can also have negative effects on forests and crop yields, which the secondary NAAQS is intended to protect."
CRS Report for Congress, R43092