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 two-thirds or 68 per cent of NO_x emitted in Ontario in 2006 came from the transportation sectors. Other industrial processes were the second largest source of NO_x emissions, accounting for approximately 11 per cent.

Ontario Nitrogen Oxides Emissions by Sector (Emissions from Point/Area/Transportation Sources, 2006 Estimates)
Pie Chart	Category	Percent
	Other Transportation	40%
	Road Vehicles	28%
	Other Industrial Processes	11%
	Utilities	8%
	Cement and Concrete	5%
	Miscellaneous	6%
	Smelters/Primary Metals	2%

Note: 2006 is the latest complete inventory. Emissions may be revised with updated source/sector information or emission estimation methodologies as they become available.

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	Pollutant Concentration Breakpoints (ppb)	Nitrogen Dioxide (NO2)
Very Good	0 - 15	0 -50	No health effects are expected in healthy people.
Good	16 - 31	51 -110	Slight odour.
Moderate	32 - 49	111 - 200*	Odour.
Poor	50 - 99	201 - 524	Air smells and looks brown. Some increase in bronchial reactivity in asthmatics.
Very Poor	100 or over	525 or over	Increasing sensitivity for asthmatics and people with bronchitis.

* Ontario 1-hour AAQC for NO₂ = 200 ppb

Note: ppb = parts per billion.

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