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Air Quality: EPA's Integrated Science Assessments (ISAs)

Oxides of Nitrogen & Sulfur - Ecological Criteria

Sulfer & Oxides of Nitrogen Report Cover
(PDF, 898 pp, 42 MB,
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The Issue
 Nitrogen dioxide (NO2) is one of a group of highly reactive gases known as "oxides of nitrogen," or "nitrogen oxides (NOx)." Other nitrogen oxides include nitrous acid and nitric acid. EPA’s National Ambient Air Quality Standard uses NO2 as the indicator for the larger group of nitrogen oxides.

Sulfur dioxide (SO2) is one of a group of highly reactive gases known as “oxides of sulfur.” When deposited, NOX and SOX can cause terrestrial and aquatic ecosystems to become acidified; this can lead to changes in diversity or loss of fish or plant populations. Nitrogen deposition can as also cause algal blooms or changes in ecosystem biodiversity. Airborne NOX and SOX can also injure leaves and plants.

EPA first set standards for NO2 in 1971, setting both a primary standard (to protect health) and a secondary standard (to protect the public welfare) at 0.053 parts per million (53 ppb), averaged annually. EPA first set standards for SO2 in 1971, setting a 3-hour average secondary standard at 500 ppb to protect the public welfare. In 2012, EPA jointly reviewed the secondary NAAQS for oxides of nitrogen and sulfur because oxides of nitrogen and sulfur in the ambient air, and their associated transformation products, such as deposited nitrogen and sulfur, are linked from an atmospheric chemistry perspective and can jointly contribute to ecological effects. In that review, EPA decided to retain the secondary NAAQS for NO2 and SO2.

EPA Action
EPA's NOx and 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 2008, EPA released the Integrated Science Assessment for Oxides of Nitrogen- Health Criteria (Final) and the Integrated Science Assessment for Sulfur - Health Criteria (Final). But because of the overlap of these 2 air pollutants in the environment, the reports were combined into a single ecological criteria report, called the Integrated Science Assessment for Oxides of Nitrogen and Sulfur - Ecological Criteria (Final). [See the history of the pollutant reports for more detailed information on the NOx/SOx assessments]

[Update] NCEA and the Office of Air Quality Planning and Standards held a joint workshop, from March 4- 6, 2014, to inform the planning for EPA’s upcoming review of the secondary National Ambient Air Quality Standard (NAAQS) for sulfur oxides (SOx) and nitrogen oxides (NOx). The scope of this review included gas-phase NOx and SOx, nitrogen deposition, sulfur deposition, ecosystem acidification and ecosystem eutrophication. This NOx/SOx workshop should provide EPA the opportunity to consider key science and policy issues around which the review will be structured.

Workshop announcement:

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