On 31 August 2014 the US Environmental Protection Agency’s (EPA) Office of Air Quality Planning and Standards released the final version of the policy assessment for the review of the ozone National Ambient Air Quality Standard (NAAQS).
Established in 2008, the current standards for ground-level ozone (O3) include a primary standard to protect public health of 75 parts per billion (ppb) as an eight-hour average, and a secondary standard to protect public welfare (vegetation and ecosystems), set identically to the primary standard. These 2008 standards are now under review, as required by the Clean Air Act.
The report recommends lowering the primary ozone standard to a level within the range of 60-70 ppb. As a level of 70 ppb would provide little margin of safety, the policy advice is that the level should be set lower. The combined occurrence of respiratory symptoms and lung function decrements has been reported to occur at levels above 65 ppb, and 60 ppb corresponds to the lowest exposure concentration demonstrated to result in lung function decrements and pulmonary inflammation.
With regard to the secondary standard, it is concluded that it is appropriate to consider a revised secondary standard in terms of a cumulative, seasonal, concentration-weighted form, called the W126 index. A range of levels from 17 to 7 ppm-hours as cumulated daily means over a three-month period is recommended. The Clean Air Scientific Advisory Committee (CASAC) noted that a level of 15 ppm-hours is requisite to protect median crop yield loss to no more than 5 per cent and that a level below 10 ppm-hours is required to reduce foliar injury prevalence. It also noted that a level of 7 ppm-hours limits median relative biomass loss for trees to no greater than 2 per cent and offers additional protection against crop yield loss and foliar injury.