Prompted by a mandatory five-year review of particle pollution standards under the Clean Air Act, the EPA proposed an update to the National Ambient Air Quality Standards to lower the limit for fine particle pollution from 15 µg/m3 to 12–13 µg/m3. The EPA also proposed an alternate limit of 11 µg/m3. The proposal stipulates that an area’s three-year average amount of fine particle pollution must not exceed the limits, which 99% of counties already meet. The 24-hour fine particle standard of 35 µg/m3 will not be altered, but the proposal would set a 24-hour fine particle limit for better visibility, at either 28 or 30 deciviews. Coarse particle limits would not be affected. Following a 63-day comment period, final standards will be published by December 14 and are expected to take effect in early 2015.

Source: EPA