The U.S Environmental Protection Agency (EPA) has announced tougher limits for permissible amounts of ozone in the air, in line with the agency's mandate under the Clean Air Act. The new limits allow ozone exposure at 75 parts per billion (ppb), down from the 84 ppb limit put in place more than 10 years ago. The Washington Post reports that the new limits are higher than those that EPA advisers had recommended for pollution-forming ozone.
According to the EPA, ground-level ozone (commonly known as "smog") is not emitted directly into the air, but forms when emissions of nitrogen oxides and volatile organic compounds "cook" in the sun. Power plants, motor vehicle exhaust, industrial facilities, gasoline vapors and chemical solvents are the major human-made sources of these emissions.
The Los Angeles Times reports that under the new standards, "counties in the Southeast, the Midwest and a few parts of California, including inland portions of the San Francisco Bay Area, could find their air declared dirty."