Nitrogen dioxide concentrations in the U.S., averaged over 2005 (left) and 2014 (right)
Images from the NASA Scientific Visualization Studio
Nitrogen dioxide (NO2) is one of a group of highly reactive gasses called nitrogen oxides. They contribute to air pollution and can be harmful to human health. Nitrogen oxides include (EPA, 2013e):
- Nitrogen dioxide (NO2)
- Nitrogen oxide (NO)
- Nitrous acid (HNO2)
- Nitric acid (HNO3)
EPA’s National Ambient Air Quality Standards (NAAQS) use nitrogen dioxide (NO2) as the indicator for the larger group of nitrogen oxides as it is of greatest concern. Nitrogen dioxide is a nonflammable, reddish-brown gas with a strong, harsh odor.
Nitrogen oxides are broken down quickly in the environment by reacting with compounds in the atmosphere, often leading to the formation of other important pollutants (EPA, 2013e).
- NO2 can react with ammonia, water, and other compounds to form particulate matter
- NO2 can react with volatile organic compounds (VOCs) in the presence of heat and sunlight to form ozone
Health Effects | Top
Populations sensitive to NO2 air pollution
- People with existing lung conditions, including:
- Their lungs are still developing and they are more likely to have asthma
- Older adults
- They are more likely to have undiagnosed lung conditions
Symptoms of exposure to NO2 (ATSDR, 2002; NLM, 2015; EPA, 2013m)
Exposure to low levels of nitrogen oxides can cause:
- Eye, nose, throat, and lung Irritation
- Shortness of breath
- Decreased ability to breath deeply or vigorously
- Worsened asthma symptoms
Exposure to NO2 has also been linked to increases in respiratory illnesses as well as the number of emergency department visits and hospital admissions for respiratory causes (EPA, 2013m). In animals, exposure during pregnancy has resulted in toxic effects to the fetus. Some recent studies have identified possible associations between maternal exposure to nitrogen dioxide and adverse birth outcomes (EPA, 2013m).
Sources | Top
Nitrogen dioxide is released into the atmosphere from motor vehicle exhaust; burning coal, oil, and natural gas; arc welding; electroplating; engraving; and dynamite blasting (Figure 1). People who live near fossil fuel-burning power plants or areas with heavy motor vehicle traffic may be exposed to higher levels of NO2. Concentrations of nitrogen dioxide within 50 meters of a roadway may be approximately 30 - 100% higher than concentrations away from roadways (EPA, 2013e). Households that burn wood or use kerosene heaters and gas stoves may have higher levels of NO2 indoors. Nitrogen oxides are also found tobacco smoke, so smokers and people who breathe in second-hand smoke may also be exposed to higher levels (ATSDR, 2002).
Figure 1: 2014 nitrogen oxides emissions in Utah by source sector
In 2014, 47% of nitrogen oxides releases in Utah came from mobile sources (e.g., motor vehicles) (Figure 1) (EPA, 2016e). Releases from fuel combustion were the second largest source of nitrogen oxides emissions, over 80% of which were due to electricity generation in coal-fired power plants. A county level distribution of NO2 emissions is presented in Figure 2 (note that emission levels do not indicate air quality)
Figure 2: 2014 Utah NO2 emissions by county (tons per square mile)
Standards and Trends | Top
There are two primary National Ambient Air Quality Standards (NAAQS) for nitrogen dioxide: 53 parts per billion (ppb) averaged over a year (set in 1971), and 100 ppb one hour standard averaged over three years (set in 2010) (EPA, 2013f). All areas in the nation meet the annual NAAQS, with concentrations measuring well below 53 ppb. The current average annual concentrations are approximately 10 – 20 ppb. In 2012, EPA designated all areas of the country as unclassifiable/attainment for the 2010 hourly NAAQS of 100 ppb. Available air quality data show that all monitored areas in the country meet the standard and there is no evidence of violations, but the existing nitrogen dioxide monitoring network does not provide adequate evidence to determine if the new NAAQS is being met in all areas (EPA, 2013e). Nationally, nitrogen dioxide levels in air have decreased by 59% between 1980 and 2015 (Figure 3).
Figure 3: Average national NO2 concentration, 1980 – 2015
A similar trend can be seen when examining the Southwest region states of Arizona, Colorado, New Mexico, and Utah (Figure 4), as well as data from nitrogen dioxide monitors along the Wasatch Front (Figure 5).
Figure 4: Average NO2 concentration in the Southwest, 2000 – 2015
Figure 5: 3-year averages of the 98th percentile of the daily maximum 1-hour average NO2 concentrations along the Wasatch Front, 1993 – 2014