The Clean Air Act requires EPA to set national ambient air quality standards for “criteria pollutants.” Currently, sulfur dioxide (SO2) and five other major pollutants are listed as criteria pollutants. The others are ozone, lead, carbon monoxide, nitrogen oxides, and particulate matter. The law also requires EPA to periodically review the standards and revise them if appropriate to ensure that they provide the requisite amount of health and environmental protection and to update those standards as necessary.
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). In 1996, EPA reviewed the SO2 NAAQS and chose not to revise the standards.
In 2010, EPA revised the primary SO2 standards by establishing a new 1-hour standard at a level of 75 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.
In 2012, EPA took final action to retain the current secondary stamdard for SO2 of 500 ppb averaged over three hours, not to be exceeded more than once per year