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Air Quality

Air Pollutants and Monitoring - Criteria Pollutants

National Ambient Air Quality Standards for SO2

On June 22, 2010, EPA strengthened the primary NAAQS for SO2.

2010 National Ambient Air Quality Standard for SO2 (June 22, 2010)

Standard Type Averaging Time Level
Primary 1 - Hour 1 75 Parts per Billion
Secondary 3 - Hour 2 0.5 Parts per Million

1 To attain this standard, the 3-year average of the 99th percentile of the daily maximum 1-hour average at each monitor within an area must not exceed 75 ppb.

2 Not to be exceeded more than once per year.

History and Implementation of the standard for SO2

EPA first set standards for SO2 in 1971. EPA set a 24-hour primary standard at 140 parts per billion (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).

On June 22, 2010, EPA strengthened the primary National Ambient Air Quality Standard (NAAQS) for the SO2 standard by establishing a new 1-hour standard at a level of 75 ppb.

The EPA then revoked the two existing primary standards of 140 ppb evaluated over 24- hours, and 30 ppb evaluated over an entire year because they will not add additional public health protection given a 1-hour standard at 75 ppb.

On August 1, 2011, EPA proposed adding a new secondary SO2 standard at a level the same as the primary standard. However, on March 20, 2012, EPA decided to retain the current SO2 and NO2 secondary standards, and not set a new secondary standard.


For more information please contact the Bureau of Air Quality at (803) 898-4123 or by email.