Chief Causes for Concern

NOx…

• Is one of the main ingredients involved in the formation of ground-level ozone, which can trigger serious respiratory problems.
• Reacts to form nitrate particles, acid aerosols, as well as NO₂, which also cause respiratory problems.
• Contributes to formation of acid rain.
• Contributes to nutrient overload that deteriorates water quality.
• Contributes to atmospheric particles that cause visibility impairment most noticeable in national parks.
• Reacts to form toxic chemicals.
• Contributes to global warming.

NOx and the pollutants formed from NOx can be transported over long distances, following the pattern of prevailing winds in the U.S. This means that problems associated with NOx are not confined to areas where NOx are emitted. Therefore, controlling NOx is often most effective if done from a regional perspective, rather than focusing on sources in one local area.

Since 1983, monitored levels of NO₂ have decreased 21 percent. These downward trends in national NO₂ levels are reflected in all regions of the country. Nationally, average NO₂ concentrations are well below the NAAQS and are currently at the lowest levels recorded in the past 20 years. All areas of the country that one violated the NAAQS for NO₂ now meet that standard. Over the past 20 years, national emissions of NOx have declined by almost 15 percent.