Integrated Science Assessment (ISA) for Oxides of Nitrogen - Health Criteria

The Issue

The oxides of nitrogen, or nitrogen oxides, comprise a group of highly reactive gases that includes nitrogen dioxide (NO₂), nitric oxide (NO), gaseous forms of nitrous acid, gases produced from reactions involving NO (HONO), NO₂  and nitric acid (HNO₃) and other oxidized nitrogen compounds. EPA’s National Ambient Air Quality Standards (NAAQS) use NO2 as the indicator for the larger group of nitrogen oxides. NO₂ forms quickly from emissions from cars, trucks and buses, power plants, and off-road equipment. In addition to contributing to the formation of ground-level ozone and fine particle pollution, exposure to oxides of nitrogen has been linked with a number of adverse human health effects, most notably on the respiratory system.

EPA Action

The Clean Air Act requires EPA to periodically review the science for six major air pollutants, including oxides of nitrogen. EPA's Center for Public Health and Environmental Assessment (CPHEA) develops ISAs that summarize and evaluate the science related to the human health and welfare effects caused by these pollutants. ISAs provide a comprehensive evaluation and synthesis of the policy-relevant scientific literature published since the last NAAQS review and serve as the scientific foundation for establishing and reviewing the primary (health-based) and secondary (welfare-based) NAAQS. 

In 2016, EPA released the Integrated Science Assessment for Oxides of Nitrogen - Health Criteria (Final) to assess the human health effects of exposures to oxides of nitrogen  and to serve as the scientific foundation for review of the primary NO₂ NAAQS. The 2016 assessment was an update to the 2008 ISA, reflecting more recent scientific evidence. The ecological effects of oxides of nitrogen were assessed separately as part of the 2020 Integrated Science Assessment (ISA) for Oxides of Nitrogen, Oxides of Sulfur and Particulate Matter Ecological Criteria (Final Report).

• Learn about the Integrated Science Assessment for Oxides of Nitrogen- Health Criteria (2016)
• Learn about the Integrated Science Assessment for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter - Ecological Criteria (2020)
• Learn about the Nitrogen Dioxide Primary Air Quality Standards
• Learn about the Nitrogen Dioxide and Sulfur Dioxide Secondary Air Quality Standards

History of the NOx Health Criteria Assessments

Peer Review Literature

Explore the literature cited in the 2016 ISA on the Health and Environmental Research Online (HERO) database:  ISA-NOx -Primary Health (2016) Project Page