Ground-level ozone, widely known as smog, is formed when sunlight triggers chemical reactions between nitrogen oxide and volatile organic compounds emitted from industrial plants, electric utilities, motor vehicles and other sources. Ozone pollution travels with the breeze and can affect downwind counties and states. Breathing ozone particularly irritates the respiratory tract. Short-term exposure can exacerbate asthma and trigger attacks and other health problems. The U.S. Environmental Protection Agency lowered the annual 8-hour ozone standard from .075 parts per million based on a three-year average to .070 ppm in 2015.
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Note: These are year-over-year data. Single-year data do not reflect compliance with federal air quality standards, which are based on the three-year average in a region.