How Nitrogen Oxides Affect the Way We Live and Breathe

Published by the US EPA
Office of Air Quality Planning & Standards
September 1998

EPA tracks emissions of six principal air pollutants carbon monoxide, lead, nitrogen oxides, particulate matter, sulfur dioxide, and volatile organic compounds. All have decreased significantly since passage of the Clean Air Act in 1970 - except for nitrogen oxides.

Table of Contents
NOx: What is it? Where does it come from?
Chief Causes for Concern
Health and Environmental Impacts of NOx

NOx: What is it? Where does it come from?

Nitrogen oxides, or NOx, is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO₂) along with particles in the air can often be seen as a reddish-brown layer over many urban areas.

Nitrogen oxides form when fuel is burned at high temperatures, as in a combustion process. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.

Motor Vehicles, 49%; Utilities, 27%; Industrial/Commercial/Residential, 19%; All Other Sources, 5%
courtesy of EPA - Environmental Protection Agency

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.

NOx emissions are increasing.

Since 1970, EPA has tracked emissions of the six principal air pollutants - carbon monoxide, lead, nitrogen oxides, particulate matter, sulfur dioxide, and volatile organic compounds. Emissions of all of these pollutants have decreased significantly except for NOx which has increased approximately 10 percent over this period.

Health and Environmental Impacts of NOx

Nitrous oxide causes a wide variety of health and environmental impacts because of various compounds and derivatives in the family of nitrogen oxides, including nitrogen dioxide, nitric acid, nitrous oxide, nitrates, and nitric oxide.

	Ground-level Ozone (Smog) - is formed when NOx and volatile organic compounds (VOCs) react in the presence of heat and sunlight. Children, people with lung diseases such as asthma, and people who work or exercise outside are susceptible to adverse effects such as damage to lung tissue and reduction in lung function. Ozone can be transported by wind currents and cause health impacts far from original sources. Millions of Americans live in areas that do not meet the health standards for ozone. Other impacts from ozone include damaged vegetation and reduced crop yields
	Acid Rain - NOx and sulfur dioxide react with other substances in the air to form acids which fall to earth as rain, fog, snow or dry particles. Some may be carried by wind for hundreds of miles. Acid rain damages; causes deterioration of cars, buildings and historical monuments; and causes lakes and streams to become acidic and unsuitable for many fish.
	Particles - NOx reacts with ammonia, moisture, and other compounds to form nitric acid and related particles. Human health concerns include effects on breathing and the respiratory system, damage to lung tissue, and premature death. Small particles penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory disease such as emphysema and bronchitis, and aggravate existing heart disease.
	Water Quality Deterioration - Increased nitrogen loading in water bodies, particularly coastal estuaries, upsets the chemical balance of nutrients used by aquatic plants and animals. Additional nitrogen accelerates "eutrophication," which leads to oxygen depletion and reduces fish and shellfish populations. NOx emissions in the air are one of the largest sources of nitrogen pollution in the Chesapeake Bay.
	Global Warming - One member of the NOx, nitrous oxide, is a greenhouse gas. It accumulates in the atmosphere with other greenhouse gasses causing a gradual rise in the earth's temperature. This will lead to increased risks to human health, a rise in the sea level, and other adverse changes to plant and animal habitat.
	Toxic Chemicals - In the air, NOx reacts readily with common organic chemicals and even ozone, to form a wide variety of toxic products, some of which may cause biological mutations. Examples of these chemicals include the nitrate radical, nitroarenes, and nitrosamines.
	Visibility Impairment - Nitrate particles and nitrogen dioxide can block the transmission of light, reducing visibility in urban areas and on a regional scale in our national parks. Reference: http://www.epa.gov/air/urbanair/nox/hlth.html

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