Nitrogen Dioxide (NO₂)

What is Nitrogen Dioxide?

NO₂ is a reddish-brown gas with a pungent and irritating odour. It transforms in the air to form gaseous nitric acid and toxic organic nitrates. NO₂ also plays a major role in atmospheric reactions that produce ground-level ozone, a major component of smog. It is also a precursor to nitrates, which contribute to increased respirable particle levels in the atmosphere.

What are the sources of NO₂?

All combustion in air produces oxides of nitrogen (NO_x), of which NO₂ is a major product. Approximately 63 per cent of NO_x comes from the transportation sector in Ontario. A large part of the remaining 37 per cent comes from power generation, primary metal production and incineration. Natural sources of NO_x include lightning and the aerobic activity of soil bacteria. These natural sources, however, are small compared to emissions caused by human activity.

Ontario Nitrogen Oxides Emissions by Sector (Emissions From Human Activity, 2000 Estimates)
Pie Chart	Category	Percent
	Road Vehicles	35%
	Other Transportation	28%
	Utilities	15%
	Other Processes	12%
	Miscellaneous Area Sources	8%
	Primary Metals	2%

What are the effects of NO₂?

NO₂ can irritate the lungs and lower resistance to respiratory infection. Sensitivity increases for people with asthma and bronchitis. NO₂ chemically transforms into nitric acid and, when deposited, contributes to lake acidification. NO₂, when chemically transformed to nitric acid, can corrode metals, fade fabrics and degrade rubber. It can damage trees and crops, resulting in substantial losses.

The following table shows the health effects of different Air Quality Index levels caused by nitrogen dioxide.

Health effects of different Air Quality Index (AQI) levels caused by nitrogen dioxide
Category	AQI	Nitrogen Dioxide (NO2)
Very Good	0 - 15	No health effects are expected in healthy people.
Good	16 - 31	Slight odour.
Moderate	32 - 49	Odour.
Poor	50 - 99	Air smells and looks brown. Some increase in bronchial reactivity in asthmatics.
Very Poor	100 or over	Increasing sensitivity for asthmatics and people with bronchitis.

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