Nitrogen dioxide

The chemical formula for nitrogen dioxide is NO₂.

Nitrogen dioxide is a reddish-brown, pungent, acidic gas that is corrosive and strongly oxidising.

Nitrogen dioxide is not usually released directly into the air. Nitrogen dioxide forms when nitrogen oxide (NO) and other nitrogen oxides (NO_x) react with other chemicals in the air to form nitrogen dioxide.

The main source of nitrogen dioxide resulting from human activities is the combustion of fossil fuels (coal, gas and oil) especially fuel used in cars. It is also produced from making nitric acid, welding and using explosives, refining of petrol and metals, commercial manufacturing, and food manufacturing.

Natural sources of other nitrogen oxides include volcanoes and bacteria.

The main health effect of nitrogen dioxide is on the respiratory system. Inhalation of nitrogen dioxide by children increases their risk of respiratory infection and may lead to poorer lung function in later life. There is also an association between nitrogen dioxide concentrations in the air and increases in mortality and hospital admissions for respiratory disease. Nitrogen dioxide can decrease the lungs’ defences against bacteria making them more susceptible to infections. It can also aggravate asthma.

• young children
• asthmatics of all ages (but especially children)
• adults with heart and respiratory disorders.

The national environmental standard for nitrogen dioxide is 200 µg/m³ as a 1-hour average.

The national ambient air quality guideline for nitrogen dioxide is 100 µg/m³ as a 24-hour average.

Nitrogen dioxide is toxic to plants in short-term concentrations of 120 µg/m³. It reduces plant growth. When sulphur dioxide and ozone are also present, the effects on vegetation are worse. Along with sulphur dioxide, nitrogen dioxide can cause acid rain. Acid rain is not a problem in New Zealand.

Nitrogen dioxide forms acids in the presence of moisture and these can be corrosive to building materials at high concentrations.

Nitrogen dioxide can form secondary particles called nitrates that cause haze and reduce visibility. Nitrogen dioxide is the gas that makes summer smog look brownish in colour.

The 1-hour national environment standard for nitrogen dioxide is occasionally breached. In most places nitrogen dioxide levels are below the standard and guideline values. 

Nitrogen dioxide can be a localised problem around congested or high traffic roads.