VOCs are also produced naturally by trees in cities and forests. While smog is commonly found around our cities, prevailing winds and weather patterns move this health hazard over long distances. Both Atlantic Canada and the Windsor-Quebec Corridor inherit the polluted air of industrialized urban centres along the Eastern United States. Periods of smog can last several days, and often occur when pollutants are trapped over a region on hot, sunny days. Weather, climate and land surfaces also affect smog levels.
Rain can clean the air of the pollutants that cause smog.
But this may result in polluted rain water or acid rain. Wind can blow smog away. If cities are surrounded by hills or mountains, the air flow is trapped and smog levels may increase. This is the case in the Lower Fraser Valley, around Vancouver. In most of Canada, smog problems are particularly bad on hot, sunny days.