Nitrogen dioxide is a sharp-smelling, reddish-brown, reactive gas made up of one nitrogen atom and two oxygen atoms. It forms primarily during high-temperature combustion and in the atmosphere when nitric oxide reacts with oxygen in the air.
Nitric oxide, whose molecules consist of just one atom each of nitrogen and oxygen, accounts for roughly 90 percent of direct combustion emissions of nitrogen oxides, with nitrogen dioxide making up the remainder.
The main outdoor sources of nitrogen dioxide are road vehicles, especially diesel engines, along with electric power plants, industrial boilers and large ships. Indoor sources include unvented or poorly vented gas stoves and heaters as well as tobacco smoke.
Nitrogen dioxide in the atmosphere damages natural ecosystems in various ways. It reacts with moisture and oxygen to form acid rain, which strips essential nutrients from the soil, weakens trees and creates toxic conditions in waterbodies for fish and other aquatic life.
When exposed to sunlight, nitrogen dioxide acts as a building block for ground-level ozone, the main ingredient in smog, which damages plant tissue and other organisms through direct atmospheric exposure.
Nitrogen dioxide also transforms in the air into tiny nitrate particles, which are a critical part of PM2.5, the microscopic soot and dust pollution that can remain suspended in the air for days and spread across long distances. Eventually, these particles settle onto land and water, where they disrupt natural habitats, overload waterbodies with unwanted nutrients and harm plant and animal life.
Nitrogen dioxide is also a major public health hazard in itself, in addition to all the hazards from the secondary pollutants it creates. It is a potent respiratory irritant, in contrast to nitric oxide, which has relatively low direct toxicity for humans. It irritates the airways, reduces lung function, triggers coughing and wheezing, and increases susceptibility to respiratory infections. Children, older adults, and people with existing lung conditions are especially at risk. Over the long term, repeated exposure can lead to chronic breathing problems and even increase the risk of developing asthma.
Overall emissions of nitrogen dioxide have dropped substantially in much of the world during recent decades due to cleaner electric power plants and the installation of catalytic converters on automobiles. However, older diesel vehicles and residential heating systems remain major sources of emissions.