Kiera Jefferson
Nitrogen dioxide (NO2) is a pungent gas that contributes to air pollution. It is a member of the NOx family of nitrogen oxides and is composed of one nitrogen atom and two oxygen atoms. NO2 is primarily generated through the burning of fossil fuels, with road traffic being the principal outdoor source. Exposure to NO2 irritates the airways and aggravates respiratory diseases, particularly in vulnerable subpopulations such as people with asthma or pre-existing medical conditions. While air quality standards and regulations have helped reduce NO2 emissions, it remains a significant air pollutant, impacting human health and the environment.
| Characteristics | Values |
|---|---|
| Chemical composition | One atom of nitrogen and two atoms of oxygen |
| State | Gas at ambient temperatures |
| Colour | Brownish-red |
| Smell | Pungent |
| Health effects | Irritation of airways, aggravation of respiratory diseases, increased risk of asthma, inflammation, increased susceptibility to pulmonary infections, emphysema-like changes, etc. |
| Sources | Burning of fossil fuels (coal, oil, methane gas, diesel, etc.), emissions from cars, trucks, buses, power plants, industrial processes, etc. |
| Vulnerable populations | People living near emission sources, people with pre-existing medical conditions (asthma, COPD, cardiovascular disease, diabetes, lung cancer), people of colour |
| Standards and regulations | National air quality standards, federal Clean Air Act, EPA's rules to reduce emissions, state/local/tribal government plans |
| Concentration in urban areas | Can exceed a total concentration (NO + NO2) of 500 μg/m3 |
| Formation | Direct emissions from combustion sources, atmospheric reactions between nitric oxide (NO) and other air pollutants requiring sunlight |
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What You'll Learn
Nitrogen dioxide is a pungent gas, NO2, that contributes to smog
Nitrogen dioxide (NO2) is a pungent gas composed of one nitrogen atom and two oxygen atoms. It is a member of the NOx family of nitrogen oxides, which also includes nitric oxide (NO) and nitrous oxide (N2O). NO2 is formed through the burning of fossil fuels such as coal, oil, methane gas, and diesel at high temperatures. As a result, the largest sources of NO2 emissions are trucks, buses, cars, and power plants.
NO2 is unhealthy to breathe and can irritate the airways in the human respiratory system. Exposure to high concentrations of NO2 over short periods can aggravate respiratory diseases, especially asthma, leading to coughing, wheezing, and difficulty breathing. Prolonged exposure to elevated NO2 concentrations may also contribute to the development of asthma and increase susceptibility to respiratory infections. Scientific evidence suggests that exposure to NO2 could potentially cause asthma in children.
In addition to its direct health impacts, NO2 contributes to smog and particle pollution. It is a key agent in the formation of several airborne toxic substances, including nitric acid (HNO3), fine particles, peroxyacetyl nitrate, and nitrosamines. NO2 also plays a role in the formation of ozone (O3), another important trace gas. The reddish-brown haze characteristic of smoggy air in California, for example, is partly due to NO2 and fine airborne particulate matter.
Ambient air quality standards have been established to define the maximum amount of NO2 that can be present in outdoor air without harming human health. Efforts to reduce NO2 emissions, such as the implementation of state and federal regulations, have led to decreases in NO2 concentrations in many areas. However, monitors show that the highest concentrations of outdoor NO2 are still found in large urban regions.
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NO2 is a nitrogen oxide formed by burning fossil fuels
Nitrogen dioxide (NO2) is a reddish-brown gas with a pungent, acrid odour. It is one of the several oxides of nitrogen formed by the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures. At normal temperatures, oxygen and nitrogen gases do not react together.
NO2 is a significant air pollutant, contributing to particle pollution and the chemical reactions that make ozone. It is one of six widespread air pollutants for which there are national air quality standards to limit their levels in the outdoor air.
As of 2020, human-made sources in the US emit 7.64 million short tons of nitrogen oxides per year, mainly from burning fossil fuels. Trucks, buses, and cars are the largest sources of NO2 emissions, followed by diesel-powered non-road equipment, industrial processes such as oil and gas production, and coal-fired power plants.
NO2 has harmful effects on human health, especially for those living near emission sources or with pre-existing medical conditions such as asthma, COPD, cardiovascular disease, or lung cancer. Scientific evidence suggests that exposure to NO2 could cause asthma in children.
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NO2 is a key agent in forming toxic air substances
Nitrogen dioxide (NO2) is a pungent gas composed of nitrogen and oxygen. It is a member of a family of chemicals known as nitrogen oxides or NOx. NO2 is formed when fossil fuels such as coal, oil, methane gas, or diesel are burned at high temperatures. The primary sources of NO2 emissions are cars, trucks, and buses, followed by power plants and off-road equipment.
Secondly, NO2 contributes to the formation of other toxic air substances. It reacts with other chemicals in the atmosphere, including water, oxygen, and other air pollutants, to produce acid rain, particulate matter, and ozone. Acid rain can harm sensitive ecosystems such as lakes and forests. Particulate matter, particularly nitrate particles, contributes to the reddish-brown haze associated with smog and reduces visibility. Ozone (O3), formed through photochemical reactions involving NO2, is another toxic air pollutant with harmful health effects.
Furthermore, NO2 is a precursor to the formation of nitric acid (HNO3) and peroxyacetyl nitrate. It also plays a role in the creation of fine particles, nitrosamines, and nitro-polycyclic aromatic hydrocarbons (nitro-PAHs). These toxic substances have adverse effects on human health and the environment.
The presence of NO2 in the air is closely linked to other nitrogen oxides, particularly nitric oxide (NO). While NO2 can be directly emitted from combustion sources, much of the NO2 in the atmosphere is formed through photochemical reactions between NO and other air pollutants, requiring the presence of sunlight. Therefore, control measures that reduce NO2 emissions will also effectively decrease the levels of other harmful NOx compounds.
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NO2 is a health risk, especially for those near emission sources
Nitrogen dioxide (NO2) is a pungent gas composed of nitrogen and oxygen atoms. It is formed when fossil fuels such as coal, oil, methane gas, or diesel are burned at high temperatures. NO2 is a significant air pollutant, contributing to particle pollution and the formation of ozone.
NO2 is a health risk as it damages the human respiratory system. Scientific evidence suggests that exposure to NO2 increases the likelihood of hospital admissions and may cause asthma in children. A 2022 review of multiple studies also linked elevated NO2 levels to heart and lung harm, adverse pregnancy and birth outcomes, and a potentially higher risk of kidney and neurological issues, autoimmune disorders, and cancer.
People living near emission sources, such as heavily travelled roadways, industrial sites, and power plants, are at an even greater risk from NO2 pollution. This is because NO2 emissions are highly correlated with other traffic-related pollutants and oxides of nitrogen. Consequently, those residing in large urban regions like the Northeast corridor, Chicago, and Los Angeles are exposed to higher concentrations of NO2.
While air quality standards and cleanup efforts have helped reduce NO2 emissions, it remains a concern for many. Vulnerable subpopulations, including individuals with pre-existing medical conditions such as asthma, COPD, cardiovascular disease, and people of colour, are at a particularly higher risk from NO2 exposure.
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Regulations to reduce NO2 emissions are improving air quality
Nitrogen dioxide (NO2) is a pungent gas that contributes to the reddish-brown haze characteristic of smoggy air. It is a major air pollutant, primarily released into the air through the burning of fossil fuels such as coal, oil, methane gas, and diesel. NO2 emissions come from cars, trucks, buses, power plants, and off-road equipment.
Breathing air with high NO2 concentrations can irritate the human respiratory system and aggravate respiratory diseases, especially asthma. Prolonged exposure to high levels of NO2 may even contribute to the development of asthma and increase susceptibility to respiratory infections. As such, people with asthma, children, and the elderly are at greater risk of adverse health effects from NO2.
Recognizing the dangers of NO2, the US Environmental Protection Agency (EPA) has implemented regulations to reduce NO2 emissions and improve air quality. The Clean Air Act, for instance, has been instrumental in cutting pollution while allowing for economic growth. The Act mandates that new industrial facilities are designed with good pollution control in mind, ensuring that as new, cleaner facilities are built, the country's industrial base becomes cleaner overall.
The EPA's Clean Air Interstate Rule (CAIR) has achieved significant reductions in power plant emissions of SO2 and NOx, which contribute to fine particles in the air. CAIR is a major reason why most areas in the East have met the 1997 and 2006 air quality standards for fine particles. The EPA has also set emissions standards for passenger cars, trucks, heavy-duty trucks, and buses, leading to the adoption of modern automotive technologies that have resulted in cleaner and more efficient vehicles.
Additionally, the Mercury and Air Toxics Standards for power plants require reductions in mercury emissions and other toxic air pollutants, protecting Americans from a host of illnesses and premature deaths. Air quality regulators have also focused on NO2 as a marker for controlling ambient levels of NOx, as control measures that reduce NO2 emissions will also reduce emissions of other NOx species.
While challenges remain, particularly in areas with poor local air quality, these regulations are making a significant difference. Over forty years of clean air policies have improved air quality and public health in the United States.
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Frequently asked questions
Nitrogen dioxide (NO2) is a pungent gas that is one of a group of highly reactive gases known as nitrogen oxides or NOx. NO2 is composed of one atom of nitrogen and two atoms of oxygen and is a gas at ambient temperatures. It is primarily generated during fossil fuel combustion, so thermal power plants and automobiles are the dominant sources.
Nitrogen dioxide is unhealthy to breathe. It can irritate the airways in the human respiratory system and cause coughing, wheezing, or difficulty breathing. Short-term exposure can aggravate respiratory diseases, especially asthma, and lead to hospital admissions. Longer exposures to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections.
Trucks, buses, and cars are the largest sources of NO2 emissions, followed by diesel-powered non-road equipment, industrial processes such as oil and gas production, industrial boilers, and coal-fired power plants. Road traffic is the principal outdoor source of nitrogen dioxide. The burning of fuels like wood or gas indoors can also produce NO2.
