On October 1, 2015 the US Environmental Protection Agency (EPA) introduced new regulations lowering limits for National Ambient Air Quality Standards (NAAQS) of ground-level ozone from 75 to 70 parts per billion (ppb). The ruling became effective on December 28, 2015. The EPA justified this move by citing a close link between ozone and numerous respiratory and pulmonary health concerns.
While the majority of states affected by this regulation today are in the US Midwest or the East and West Coasts, the EPA projects that nearly all US counties will need to meet these standards in 10 years.
Ozone forms when precursor compounds such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the atmosphere. The EPA explicitly sets standards for six pollutants: ground-level ozone, particulate matter, carbon monoxide, lead, nitrogen dioxide and sulfur dioxide. As a result of recent studies, the EPA ordered a reduction in ground level ozone concentrations (8-hour average) to 70 ppb.
The new regulations are designed to allow those areas not currently in compliance with the new standards some time to comply. The amount of time allowed varies based on the ozone concentration at the local level. In general, state recommendations are required by October 2016 (one year after rule) with final rules by October 2017. Individual State Implementation Plans (SIPs) are required by October 2018, with regions graded from Marginal to Extreme (5 groups) having from three to 20 years to reach attainment.