Air Quality: Smog

In 1993, a Smog Alert Program was initiated by the Ontario Ministry of Environment and by Environment Canada.

Smog Advisories are issued when wide-spread, elevated levels of air pollution are forecast to occur within the next 24 hours. In addition, Smog Watches are called when there is a 50% chance that smog may occur within the next few days.

Smog Formation

Smog (smoke/fog) is a hazy condition that results when smoke and fog combine.

Smog, (photochemical) is formed by chemical reactions driven by energy from the sun. On hot sunny days, as morning light builds in large cities, air monitors record increased concentrations of volatile organic compounds and of nitrogen oxides. Later in the morning the concentrations of these contaminants decrease. A similar pattern (build up of contaminants followed by a decrease) is repeated during the period of heavy evening traffic.


Smog consists mainly of nitrogen oxides, organic compounds, ozone and suspended particles; it may be observable from midday to late afternoon. In addition to an observable haze, the following may also occur – irritation of eyes, breathing problems, adverse effects to plant growth.