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Integrated Science Assessment (ISA) for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter (Ecological Criteria)

The Issue

Nitrogen dioxide (NO2) is one of a group of highly reactive gases known as "oxides of nitrogen" or "nitrogen oxides." Other oxides of nitrogen include nitrous acid and nitric acid. EPA’s National Ambient Air Quality Standard uses NO2 as the indicator for the larger group of oxides of nitrogen.

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

Particulate matter (PM) is a complex mixture of extremely small particles and liquid droplets suspended in the air. PM can harm the environment by changing the natural nutrient and chemical balance of the soil.

EPA Action

The Clean Air Act requires EPA to periodically review the science for six major air pollutants, including oxides of nitrogen, oxides of sulfur and particulate matter. EPA's National Center for Environmental Assessment develops Integrated Science Assessments (ISAs) that summarize the science related to the health and welfare (e.g., climate, visibility, ecosystem) effects caused by these pollutants. ISAs provide a comprehensive assessment of the most policy-relevant scientific literature published since the last National Ambient Air Quality Standards (NAAQS) review and are a critical part of the scientific basis used to support retaining or revising the primary (health-based) and secondary (welfare-based) NAAQS.

EPA first set secondary NAAQS for oxides of nitrogen in 1971 , an annual average of NO2 at 0.053 parts per million (53 ppb) to protect against plant phytoxicity. EPA first set secondary NAAQS for oxides of sulfur in 1971, a 3-hour average of SO2 at 500 ppb, also to protect against plant phytotxicity. In 2012, EPA jointly reviewed the secondary NAAQS for oxides of nitrogen and sulfur because of their associated transformation products in the ambient air, contribution to deposited nitrogen and sulfur, and their joint contribution to ecological effects. In that review, EPA decided to retain the secondary NAAQS for NO2 and SO2.

In December 2012, the U.S. EPA announced its final decisions on the most recent review of secondary NAAQS for particulate matter. The U.S. EPA retained the 24-hour and annual PM2.5 standards and the 24-hour PM10 standard to address visibility and non-visibility welfare effects.

In 2008, EPA released the Integrated Science Assessment for Oxides of Nitrogen and Sulfur - Ecological Criteria (Final Report) and in 2009, EPA released the Integrated Science Assessment for Particulate Matter (Final Report).

History of the Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter Assessments

Date Milestone
Mar 2017 EPA released the first draft Integrated Science Assessment for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter - Ecological Criteria (EPA/600/R-16/372) for a public review and comment period.
Aug 2015 EPA released preliminary draft materials that were discussed in a teleconference workshop to help inform the development of the draft assessment.
Dec 2009 EPA released the document Integrated Science Assessment for Particulate Matter (Final, 2009) (EPA/600/AR-08/139F).
Dec 2008 EPA released the document Integrated Science Assessment for Oxides of Nitrogen and Sulfur – Ecological Criteria (Final, 2008) (EPA/600/R-08/082F).
Aug 1993 EPA released the document Air Quality Criteria for Oxides of Nitrogen (Final, 1993) (EPA/600/8-91/049).
1982 EPA released the documents Air Quality Criteria for Oxides of Nitrogen (Final, 1982) (EPA/600/8-82/026) and Air Quality Criteria for Particulate Matter and Sulfur Oxides (Final, 1982) (EPA/600/8-82/029).