This month, the US EPA updated to 70 parts per billion (ppb) the National Ambient Air Quality Standards for ground-level ozone. To meet the standard, the fourth-highest maximum daily eight-hour ozone concentration each year, averaged over three years, must not exceed 70 ppb. The changes are to both the primary (public health) and secondary (public welfare) standards. For the secondary standards, the EPA has set the limit of the W126 index, which determines cumulative exposure that damages plants in the growing season, to 17 ppm-hours. The EPA will immediately team with state, tribal and local agencies to implement the standards. According to the Clean Air Act, the EPA must designate areas meeting or not meeting the standards as attainment or nonattainment areas, respectively. States with nonattainment areas must develop emission inventories and a program for further air quality safeguards for the areas. Based on their ozone levels, nonattainment areas will have from 2020 to 2037 to meet the primary standard.

Source: EPA