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 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.

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.

In 2008, EPA released the Integrated Science Assessment for Oxides of Nitrogen- Health Criteria (Final Report) and the Integrated Science Assessment for Sulfur - Health Criteria (Final Report). 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 Report).

• Learn more about the 2008 Final Report: Integrated Science Assessment for Oxides of Nitrogen and Sulfur (Ecological Criteria)
• Learn about the 2008 Final Report: Integrated Science Assessment for Sulfur (Health Criteria)
• Learn about the 2008 Final Report: Integrated Science Assessment for Oxides of Nitrogen (Health Criteria)
• Learn about the Nitrogen Dioxide Secondary Air Quality Standards
• Learn about the Sulfur Dioxide Secondary Air Quality Standards

History of the Oxides of Nitrogen & Sulfur Ecological Criteria Assessments