Greta Jimenez
Nitrogen dioxide (NO2) is a pungent reddish-brown gas and a major air pollutant. It is one of a group of highly reactive gases known as nitrogen oxides (NOx). NO2 is primarily released into the air through the burning of fossil fuels such as coal, oil, methane gas, and diesel at high temperatures. It is also formed indoors when fuels like wood or gas are burned. NO2 is a harmful pollutant that can cause a range of adverse health effects, especially for those living near emission sources, people with pre-existing medical conditions, and vulnerable populations such as young children and the elderly.
| Characteristics | Values |
|---|---|
| Chemical composition | One atom of nitrogen and two atoms of oxygen |
| Colour | Brownish-red |
| Smell | Pungent |
| Temperature | Gas at ambient temperatures |
| Health effects | Irritates airways, coughing, wheezing, difficulty breathing, hospital admissions, asthma, lung irritation, diminished immune responses to respiratory infections |
| Formation | Burning of fossil fuels (coal, oil, methane gas, diesel) at high temperatures, burning of wood or gas, combustion sources, chemical reactions with other air pollutants |
| Sources | Trucks, buses, cars, power plants, industrial sites, on-road vehicles, off-road equipment, tobacco smoke, gas-, wood-, oil-, kerosene- and coal-burning appliances |
| Standards | National Ambient Air Quality Standard (NAAQS), Clean Air Act |
| Concentration | Can exceed a total concentration (NO + NO2) of 500 μg/m3 in dense urban areas, 1-hour standard of 0.18 ppm in 2007, 1-hour standard of 0.100 ppm |
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What You'll Learn
Nitrogen dioxide sources
Nitrogen dioxide (NO2) is a highly reactive gaseous air pollutant composed of nitrogen and oxygen. It is formed by the burning of fuel and fossil fuels such as coal, oil, methane gas (natural gas), or diesel at high temperatures. NO2 is one of the principal nitrogen oxides associated with combustion sources.
Outdoor Sources of Nitrogen Dioxide
The primary outdoor source of nitrogen dioxide is road traffic. Trucks, buses, and cars are the largest sources of NO2 emissions. Other significant sources include:
- Diesel-powered non-road equipment
- Industrial processes, such as oil and gas production
- Industrial boilers and other movable engines
- Coal-fired power plants
- Gas-fired power plants
- Facilities that extract, process, or transport oil and gas
Levels of nitrogen dioxide are higher on or near heavily travelled roadways and in large urban regions.
Indoor Sources of Nitrogen Dioxide
The presence and use of indoor sources are the primary determinants of indoor nitrogen dioxide levels. The use of a gas stove is a dominant activity influencing indoor NO2 concentrations. Other important indoor sources include:
- Tobacco smoke
- Gas-, wood-, oil-, kerosene-, and coal-burning appliances such as stoves, ovens, space heaters, water heaters, and fireplaces, particularly unflued or poorly maintained appliances
- Combustion space heaters
- Gas heaters
- Gas-fired heating furnaces
- Candles and mosquito coils
Occupational exposures can also be elevated in certain indoor spaces, including underground parking garages and ice arenas with inadequate ventilation from the exhaust emissions of propane- and petrol-fuelled ice-resurfacing machines.
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Health impacts of nitrogen dioxide pollution
Nitrogen dioxide (NO2) is a gaseous air pollutant composed of nitrogen and oxygen. It is formed when fossil fuels such as coal, oil, methane gas (natural gas), or diesel are burned at high temperatures. NO2 is one of the most prevalent nitrogen oxides, along with nitric oxide (NO), and the combination is referred to as NOx. NO2 is a primary pollutant, and road traffic is its principal outdoor source.
The health impacts of nitrogen dioxide pollution are significant and far-reaching. As a respiratory irritant, NO2 can cause coughing, wheezing, and difficulty breathing, leading to increased hospital admissions and visits to emergency rooms. Those who live near emission sources, such as heavily travelled roadways, industrial sites, and power plants, are at a higher risk of exposure. Vulnerable subpopulations with pre-existing medical conditions, including asthma, chronic obstructive pulmonary disease (COPD), cardiovascular disease, diabetes, and lung cancer, face elevated risks from nitrogen dioxide pollution. Additionally, young children, the elderly, and people of colour are more susceptible to its harmful effects.
Long-term exposure to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. Scientific evidence suggests that exposure to NO2 could be a factor in causing asthma in children. A 2022 review of multiple studies found a strong association between elevated levels of NO2 and adverse respiratory health outcomes.
Nitrogen dioxide also plays a role in the formation of other toxic substances and pollutants, such as ozone (O3), nitric acid (HNO3), peroxyacetyl nitrate, nitrosamines, and nitro-polycyclic aromatic hydrocarbons (nitro-PAHs). These secondary pollutants further contribute to diminished air quality and can have additional health impacts. High levels of NO2 and other nitrogen oxides can result in a reddish-brown haze, characteristic of smoggy air, particularly in large urban regions.
While emissions of nitrogen dioxide have decreased in recent years due to cleanup efforts and stricter regulations, it remains a significant concern for human health. Continuous monitoring and adherence to air quality standards are crucial to mitigate the adverse health effects of NO2 pollution and protect vulnerable populations.
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Nitrogen dioxide and asthma
Nitrogen dioxide (NO2) is a gaseous air pollutant composed of nitrogen and oxygen. It is one of a group of highly reactive gases known as oxides of nitrogen or nitrogen oxides (NOx). NO2 is formed when fossil fuels such as coal, oil, methane gas (natural gas), or diesel are burned at high temperatures. The burning of fuels is the primary source of NO2 emissions, with trucks, buses, and cars being the largest emitters. Other sources include industrial processes, power plants, and off-road equipment.
Nitrogen dioxide pollution has harmful effects on human health, particularly the respiratory system. It can irritate the airways and aggravate respiratory diseases, especially asthma. People with asthma are more susceptible to the health impacts of NO2 exposure. Short-term exposure to high concentrations of NO2 can lead to respiratory symptoms such as coughing, wheezing, or difficulty breathing, and can trigger asthma attacks. It can also increase the risk of hospital admissions and emergency room visits for people with asthma.
Long-term exposure to elevated levels of NO2 has been linked to the development of asthma, particularly in children. Multiple studies have found a strong association between elevated NO2 levels and an increased risk of asthma. This is a global concern, as nitrogen dioxide air pollution contributes to the burden of paediatric asthma cases worldwide. Children living in areas with high traffic-related air pollution, for example, have an increased risk of developing asthma due to NO2 exposure.
Vulnerable subpopulations, including people with pre-existing medical conditions such as asthma, are at higher risk of adverse health effects from nitrogen dioxide pollution. Additionally, those who live near emission sources, such as heavily travelled roadways or industrial sites, are also at higher risk. However, it is important to note that everyone is susceptible to the health impacts of NO2 pollution.
While there have been improvements in air quality in recent years due to cleaner power plants and vehicles, many people still breathe unhealthy levels of nitrogen dioxide. Reducing NO2 emissions and improving ventilation in indoor spaces can help mitigate the health risks associated with nitrogen dioxide pollution, especially for individuals with asthma.
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Nitrogen dioxide and other nitrogen oxides
Nitrogen dioxide (NO2) is a gaseous air pollutant composed of nitrogen and oxygen. It is a member of a group of highly reactive gases known as nitrogen oxides or NOx. Nitrogen oxides are formed when fossil fuels such as coal, oil, methane gas (natural gas) or diesel are burned at high temperatures. NO2 is the most prevalent nitrogen oxide, along with nitric oxide (NO). Other nitrogen oxides include nitrous oxide (N2O), nitrous acid (HONO) and nitric acid (HNO3).
NO2 and other nitrogen oxides in the outdoor air contribute to particle pollution and to the chemical reactions that make ozone. They are primarily emitted from combustion sources, such as cars, trucks, buses, power plants, and off-road equipment. NO2 emissions are highly correlated with other nitrogen oxides and several other traffic-related pollutants. As such, control measures that reduce NO2 emissions will also reduce emissions of other nitrogen oxides.
Nitrogen dioxide is one of six common air pollutants called criteria pollutants, which are subject to primary and secondary National Ambient Air Quality Standards under the Federal Clean Air Act. Ambient air quality standards define the maximum amount of pollutant that can be present in outdoor air without harming human health. In 2007, the California Air Resources Board lowered the state one-hour standard for NO2 to 0.18 ppm, retaining the annual average standard of 0.030 ppm. The national standard was more recently revised in 2010 following evidence for adverse effects in asthmatics at lower NO2 concentrations.
Nitrogen dioxide causes a range of harmful effects on the lungs, including coughing, wheezing, and difficulty breathing. Those who live near emission sources are at higher risk of exposure. Other vulnerable subpopulations include people with pre-existing medical conditions such as asthma, chronic obstructive pulmonary disease (COPD), cardiovascular disease, diabetes, and lung cancer, as well as people of colour. High nitrogen dioxide concentrations are associated with the use of candles, mosquito coils, incense burning, and the burning of fuels like wood or gas indoors.
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Reducing nitrogen dioxide emissions
Nitrogen dioxide (NO2) is a gaseous air pollutant and a highly reactive nitrogen oxide (NOx) compound. It is formed when fossil fuels such as coal, oil, methane, and diesel are burned at high temperatures. NO2 is a primary outdoor pollutant, with road traffic being the principal source. Indoor sources of NO2 include tobacco smoke and the burning of fuels like wood or gas in appliances such as stoves, ovens, and heaters.
To reduce NO2 emissions, governments and regulatory bodies have implemented various measures:
- The US EPA has established national and regional rules to reduce NO2 and NOx emissions, helping state and local governments meet the National Ambient Air Quality Standard (NAAQS).
- The EPA identifies areas that do not meet the national NO2 standards, and state, local, and tribal governments develop plans to reduce NO2 levels in those areas.
- The federal Clean Air Act in the US has driven down nitrogen dioxide emissions, leading to improved air quality nationwide. Power plants, industrial sites, and on-road vehicles are now cleaner.
- Stricter regulations have been imposed on exhaust gas purity, especially for coal-fired boilers, to reduce NOx emissions.
Additionally, specific methods and technologies have been proposed to reduce NO2 emissions:
- Oxidation methods can remove nitrogen oxides, sulfur dioxide, and mercury from flue gases simultaneously with high efficiency. Ozone is commonly used as an oxidizer for nitrogen oxides in commercial installations.
- Injection of sodium chlorite solution into the flue gas upstream of the wet flue gas desulphurization (WFGD) absorber enhances the removal efficiency of nitrogen oxides and mercury.
- Platinum metal catalysts are used to control automobile exhaust gases, and they have potential for use in other emission sources like power station boilers and industrial boilers.
- Improved ventilation can help reduce indoor NO2 concentrations, especially in spaces with combustion sources such as gas appliances, stoves, and heaters.
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Frequently asked questions
Nitrogen dioxide (NO2) is a pungent gas composed of nitrogen and oxygen. It is a member of a group of highly reactive gases known as nitrogen oxides or NOx.
Nitrogen dioxide is primarily released into the air through the burning of fossil fuels such as coal, oil, methane gas, diesel, and wood. Cars, trucks, and buses are the largest sources of NO2 emissions, followed by industrial processes such as oil and gas production, and coal-fired power plants.
Nitrogen dioxide pollution can irritate the airways in the human respiratory system and aggravate respiratory diseases, especially asthma. Exposure can cause coughing, wheezing, and difficulty breathing, and in some cases, lead to hospitalisation. Longer-term exposure to elevated concentrations of NO2 may also increase the risk of developing asthma and respiratory infections.
Nitrogen dioxide is regulated by air quality standards that define the maximum amount of pollutant that can be present in outdoor air without harming human health. For example, the US EPA has established national and regional rules to reduce NO2 emissions, and the Clean Air Act sets primary and secondary standards to prevent adverse effects on human health and the environment.
