Changes

===Sulfur dioxide===

In November 2009, the EPA proposed new [[National Ambient Air Quality Standards]] for [[sulfur dioxide]], the first time since 1971 that the agency has recommended tightening controls on SO₂ to protect public health. The old limits measured sulfur dioxide averages over 24-hour and one-year periods. The new rule would require one-hour measurements, such that a spike of emissions above a new limit — between 50 and 100 parts per billion in one hour — would no longer be acceptable.<ref>Renee Schoof, [http://www.stltoday.com/stltoday/news/stories.nsf/sciencemedicine/story/433E1BA4259929798625767C00178FFB?OpenDocument "EPA proposes sulfur dioxide limits for first time since 1971,"] McClatchy Newspapers, November 29, 2009.</ref>

===Ozone===