Sulfur dioxide (SO2) is one of a group of highly reactive gasses known as “oxides of sulfur.” The largest sources of SO2 emissions are from fossil fuel combustion at power plants (73%) and other industrial facilities (20%). Smaller sources of SO2 emissions include industrial processes such as extracting metal from ore, and the burning of high sulfur containing fuels by locomotives, large ships, and non-road equipment. SO2 is linked with a number of adverse effects on the respiratory system.
EPA first set standards for SO2 in 1971. EPA set a 24-hour primary standard at 140 ppb and an annual average standard at 30 ppb (to protect health). EPA also set a 3-hour average secondary standard at 500 ppb (to protect the public welfare). In 1996, EPA reviewed the SO2 NAAQS and chose not to revise the standards.
In 2010, EPA revised the primary SO2 NAAQS by establishing a new 1-hour standard at a level of 75 parts per billion (ppb). EPA revoked the two existing primary standards because they would not provide additional public health protection given a 1-hour standard at 75 ppb.
For more information about SO2:
Basic Information - Basics about SO2 air pollution.
Health - Effects of SO2 air pollution.
SO2 Primary Standards - Links to technical information related to setting the national air quality standards for SO2 to protect public health.
SO2 Secondary Standards - Links to technical information related to setting the national air quality standards for SO2 to protect public welfare.
SO2 Designations - Regional, state and local information related to SO2 nonattainment.
SO2 Implementation - Programs and requirements for reducing SO2 air pollution.
Regulatory Actions - Links to proposed and final rules, fact sheets, and other rulemaking documents.
Guidance and Data - Links to policies and guidance on SO2 designations and implementation.
Nonattainment Areas - Status of nonattainment areas (the Green Book)
Air Quality Trends - Progress made in reducing SO2 in the air.
Air Emission Sources - Summarizes SO2 air emissions by source at national, state and local levels.
State Implementation Plan (SIP) Status and Information - identifies how states and EPA work together to ensure that the agency's National Ambient Air Quality Standards (NAAQS) are met and maintained.
Acid Rain - The overall goal of EPA's Acid Rain Program is to achieve significant environmental and public health benefits through reductions in emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx), the primary causes of acid rain.