NOx in the Air: Multiple Effects
The emission of nitrogen oxides into the air can contribute to a wide range of human health and environmental impacts. Its contribution to some problems are better understood than others.Those identified as being of concern in a recent EPA report include:
- Ground-level ozone. NOx reacts with volatile organic compounds in the presence of sunlight to form ozone, a key component of smog. Ozone contributes to a variety of respiratory problems for the young, elderly, those with lung ailments or people who exert themselves during periods of high ozone concentrations in the air.
- Acid deposition. Sulfur dioxide and NOx are the two key air pollutants that cause acid rain. Acid rain can change soil chemistry which, in turn, can reduce growth in some trees, as well as their ability to resist disease. As lakes and streams become acidified, they may lose fish biodiversity, including some sensitive species such as trout. Sudden acidic "pulses" can release aluminum-which is highly toxic to fish-into streams, especially during spring spawning periods when rain and snow melt contribute large amounts of acid at once. It can damage a wide range of materials, from galvanized steel and copper to stone in buildings and monuments.
- Drinking water nitrate. High nitrate levels in drinking water is a health hazard, particularly for infants, as it can contribute to the "blue baby" syndrome. High levels in water may also increase cancer risks for all ages. Atmospheric nitrogen deposition in sensitive watersheds can increase stream water nitrate concentrations, where it can remain in the water and be carried long distances downstream.
- Global warming. Nitrous oxide, a breakdown form of NOx,is a greenhouse gas, and anthropogenic emissions contribute about 2 percent of the "greenhouse effect" relative to total anthropogenic emissions of greenhouse gases in the United States.
- Nitrogen dioxide. Exposure to NO2, another breakdown form of NOx, is associated with a variety of acute and chronic health effects. Those of most concern include mild changes in airway responsiveness and pulmonary function in individuals with pre-existing respiratory illnesses as well as an increase in respiratory illnesses in children. All areas of the United States monitoring NO2 are below the EPA's threshold for health effects.
- Nitrogen saturation of terrestrial ecosystems. Nitrogen can accumulate in watersheds with high atmospheric deposition. In most ecosystems, nitrogen deposition has a fertilizing effect that accelerates plant growth. Although this is often considered beneficial, it can also cause adverse changes such as shifts in plant species composition, decreases in species diversity and nitrate leaching to surface and ground waters. Studies in prairie ecosystems have shown that increased nitrogen loadings lead to an increased abundance of nonnative species, the loss of native species and the disruption of ecosystem functioning. Some trees and plants in nitrogen-saturated areas may be more susceptible to insect and disease attacks.
- Particulate matter. NOx compounds react with other compounds in the atmosphere to form nitrate particles and acid aerosols. Because of their small size, nitrate particles have the ability to be transported hundreds, even thousands, of miles though the atmosphere. Small particles can also penetrate deeply in the lungs where they may contribute to a range of adverse health effects.
- Stratospheric ozone depletion. Stratospheric ozone protects people, plants and animals on the Earth's surface from ultraviolet radiation. Nitrous oxide, which is very stable in the lower atmosphere (troposphere) slowly migrates to the stratosphere where solar radiation breaks it into nitric oxide (NO) and nitrogen (N). The NO reacts with ozone to form nitrogen dioxide and molecular oxygen, so additional nitrous oxide emissions would result in some decrease in stratospheric ozone.
- Toxic products. Airborne particles derived from NOx emissions react in the atmosphere to form various nitrogen-containing compounds, some of which may be mutagenic.
- Visibility and regional haze. NOx emissions lead to the formation of compounds that can interfere with the transmission of light, limiting visual range and color discrimination. Most visibility and regional haze problems can be traced to airborne particles in the atmosphere that include carbon compounds, nitrate and sulfate aerosols and soil dust. The major cause of visibility impairment in the eastern United States is sulfates, while in the West the other particle types play a greater role.
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