Integrated Science Assessment (ISA) Ecological Criteria Assessment for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter
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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." This ISA uses the term “oxides of nitrogen” to refer to all forms of oxidized nitrogen (NOY) compounds, including nitric oxide (NO), nitrogen dioxide (NO2), and all other oxidized nitrogen containing compounds formed from NO and NO2. EPA’s National Ambient Air Quality Standard currently 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.” Oxides of sulfur are defined here to include sulfur monoxide (SO), sulfur dioxide (SO2), sulfur trioxide (SO3), disulfur monoxide (S2O), and sulfate (SO42−). However, SO, SO3, and S2O are present at much lower ambient levels than SO2 and SO42− and are therefore not included in the ISA. EPA’s National Ambient Air Quality Standard currently uses SO2 as the indicator for the larger group of oxides of sulfur.
Particulate matter (PM) is a complex mixture of extremely small particles and liquid droplets suspended in the air. Particulate matter is composed of some or all of the following components: nitrate (NO3−), SO42−, ammonium (NH4+), metals, minerals (dust), and organic and elemental carbon (C). EPA’s National Ambient Air Quality Standard currently uses PM 2.5 and PM10 as the indicator for the larger group of particulate matter.
Direct effects of gaseous SOX and NOY in the ambient air on vegetation, include injury to leaves and plants. However, there is little evidence that such effects occur at current levels of exposure in the U.S. Additionally, NOY, SOX, and PM, at current levels that occur in the U.S., contribute to total nitrogen and sulfur deposition. In turn, nitrogen and sulfur deposition cause alteration of the biogeochemistry and the physiology of organisms from multiple taxonomic groups (e.g.; diatoms, lichens, herbaceous plants and trees), resulting in declines in biodiversity in terrestrial, freshwater, wetland, and estuarine ecosystems in the U.S.
EPA Action
The Clean Air Act requires EPA to periodically review the science for six major air pollutants, including nitrogen oxides, sulfur oxides, particulate matter, carbon monoxide, ozone and lead. EPA's Center for Public Health and Environmental Assessment (formerly National Center for Environmental Assessment) develops Integrated Science Assessments (ISAs) that summarize the science related to the human 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 in conjunction with other technical assessments, thus provide the policy-relevant scientific information necessary to conduct a review of the adequacy of the current 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 sulfur oxides in 1971, a 3-hour average of SO2 at 500 ppb, also to protect against plant phytotoxicity. 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 2020, EPA released an update to the 2008 version of the Integrated Science Assessment for Oxides of Nitrogen and Sulfur - Ecological Criteria (Final). Due to the overlap of effects of air pollutants on ecosystems, particulate matter was added in to the report such that the ecological assessments were combined into a single criteria report, called the Integrated Science Assessment (ISA) for Oxides of Nitrogen, Oxides of Sulfur and Particulate Matter Ecological Criteria (Final Report) released in October 2020. In addition, in late 2019 EPA released the Integrated Science Assessment for Particulate Matter (Final Report), which included human health and other non-ecological aspects of welfare (e.g.; visibility and climate).
- Learn more about the Integrated Science Assessment (ISA) for Oxides of Nitrogen, Oxides of Sulfur and Particulate Matter Ecological Criteria (Final Report, 2020)
- Learn about the Integrated Science Assessment for Particulate Matter (Final Report, 2019)
- Learn about the Integrated Science Assessment for Sulfur Oxides - Health Criteria (Final Report, 2017)
- Learn about the Integrated Science Assessment for Oxides of Nitrogen- Health Criteria (Final Report, 2016)
- Read the Preamble to the Integrated Science Assessments (2015)
- Learn about the Nitrogen Dioxide and Sulfur Dioxide Secondary Air Quality Standards
- Learn about the Particulate Matter Air Quality Standards
History of the Combined Ecological Criteria Assessments
Peer-Reviewed Literature
EPA used the Health and Environmental Research Online (HERO) database to augment the underlying literature supporting the final assessment.
See: ISA for NOx/SOx/PM - Ecological Criteria (2020 Final Project Page)
HERO is a searchable database of more than 1.6 million scientific studies and other references used to develop EPA assessments aimed at understanding the health and environmental effects of pollutants and chemicals.
