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Nitrogen dioxide (NO2) is part of the highly reactive nitrogen oxide group, like nitrous or nitric acid. It forms from large vehicle emissions, power plants and other large machinery. 
Yellow or colourless at lower temperatures, NO2 appears as a reddish-brown gas above 21°C, with an acrid smell. It can be introduced into the atmosphere or the environment through natural mechanisms, which include entry from the stratosphere, bacterial metabolisms, volcanoes and lightning. A powerful greenhouse gas, it is only present in trace quantities in our atmosphere and determines ozone concentrations.
When breathed in, it diffuses into our lungs and reacts with anti-oxidants and fat molecules to produce reactive species that can cause inflammation and other kinds of damage. Acute harm from nitrogen dioxide exposure only occurs at high concentrations, ranging from mild irritation of the respiratory tract, to edema, bronchitis, pneumonia and death to asphyxiation from fluid build-up in the lungs. Rest assured, this is not likely to happen outside of an occupational setting.
Nonetheless, this molecule has been shown to cause respiratory symptoms in people with asthma and other pre-existing respiratory conditions. The main sources for daily exposure include cigarette smoke, and gas stoves. The latter is a large problem in lower-income countries, where the amount of NO2 inhaled at home can seriously harm children and adults alike. It is difficult to assess how beneficial NO2 reductions would be, versus how costly they would be, and thus the WHO has issued no guidelines for curbing its emissions. This is despite respiratory illness having become the leading cause of death on the world today.

You might also like: Global CO2 Data

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