The Issue

Sulfur dioxide (SO2) is one of a group of reactive gases known as "sulfur oxides." Emissions of SO2 declined by approximately 79% between 1990 and 2014 subsequent to several U.S. air quality regulatory programs. Coal-fired power plants remain the dominant anthropogenicanthropogenicMade by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities. source, emitting 3,200,000 tons of SO2 annually in 2014. Smaller sources of nationwide SO2 emissions include industrial processes such as extracting metal from ore, petroleum refining, and chemical processing.

SO2 is linked with a number of adverse effects on the respiratory system, particularly for exercising asthmatic individuals. In 2010, EPA revised the primary (health-based) standard by establishing a one-hour standard at a level of 75 parts per billion (ppb).

EPA Action

EPA's Nitrogen Dioxides (NOx) and Sulfur Oxides (SOx) research efforts are focused on improving emissions estimates, determining health and ecological effects, and improving modeling capabilities. The Clean Air Act requires EPA to periodically review the science for six major air pollutants, including NOx and SOx. EPA's National Center for Environmental Assessment develops Integrated Science Assessments (ISAs) that summarize the science related to the health and ecological effects caused by these pollutants. ISAs provide a comprehensive review of the policy-relevant scientific literature published since the last National Ambient Air Quality (NAAQS) review and are a critical part of the scientific basis for establishing the NAAQS.

In 2017, EPA released the Integrated Science Assessment for Sulfur Oxides - Health Criteria (Final). But because of the overlap of these two air pollutants in the environment, the ecological assessments were combined into a single criteria report, called the Integrated Science Assessment for Oxides of Nitrogen and Sulfur - Ecological Criteria (Final Report, 2008). An update to the 2008 ecological criteria assessment is currently under development.

• Learn about the Integrated Science Assessment for Sulfur Oxides - Health Criteria (2017)
• Read the Preamble to the Integrated Science Assessments (2015)
• Learn about the Integrated Science Assessment for Oxides of Nitrogen and Sulfur - Ecological Criteria (2008)
• Learn about the Sulfur Dioxide Primary Air Quality Standards
• Learn about the Sulfur Dioxide Secondary Air Quality Standards

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History of the Sulfur Oxides Health Criteria Assessments

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Peer-Reviewed Literature

EPA used the Health and Environmental Research Online (HERO) database to augment the underlying literature supporting the final assessment.

See: ISA-Sulfur Oxides-Health Criteria (2017 Final Project Page)

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