In November, the US EPA proposed updates to the National Ambient Air Quality Standards for ground-level ozone. Both the primary (public health) and secondary (public welfare: protecting trees, plants and ecosystems) standards for the average level over any eight-hour period would be 65–70 ppb. Both are currently 75 ppb. Current and proposed federal rules, such as the Mercury and Air Toxics Standards, and the Clean Power Plan will facilitate most counties’ attaining the primary standard by 2025, but California will have more time to meet it. The EPA is proposing to define the secondary standard by a “W126 index,” which determines effects of ozone on plants and ecosystems. The proposed W126 level is 13–17 ppm-h averaged over three years. The EPA is accepting comments on keeping the current standards, reducing levels to as low as 60 ppb for the primary standards, and establishing a W126 level within 7–13 ppm-h for the secondary standards. The agency will reach a final decision by October 1, 2015.

Source: US EPA