In Canada smog is a concern in most major urban centres but, because it travels with the wind, it can affect sparsely populated areas as well. Even healthy young adults breathe less efficiently on days when the air is heavily polluted, especially if exercising outdoors. The two key components of smog that affect our health are fine, airborne particles and ground-level ozone. These particles give smog its color. Depending on the types of particles present, the air can appear yellowish-brown, or even white.
Particles can also obscure the landscape, blocking out distant scenery or buildings.
Recent studies have shown that every major Canadian urban centre has levels of airborne particles high enough to pose a health risk. Unlike the ozone that forms naturally in the stratosphere, ground-level ozone does not provide protection from the sun's harmful UV rays, nor does it find its way to the upper atmosphere. More than half of all Canadians live in areas where ground-level ozone may reach unacceptable levels during the summer months. Periods of high ozone often last several days and frequently occur when a stagnant air mass traps pollutants over a region. Not only does ground-level ozone pose a problem to humans, it is also believed to reduce the productivity of agricultural crops and to have significant effects on the health of Canada's forests.