Unlike the ozone that forms naturally in the stratosphere, ground-level ozone does not provide any significant protection from the sun's harmful UV rays, nor does it find its way to the upper atmosphere. Most Canadians in urban areas live where ground-level ozone may reach unacceptable levels during the summer months. Periods of high ozone can last several days and frequently occur when a stagnant air mass traps pollutants over a region. Recent studies have shown that every major Canadian urban centre has levels of ground-level ozone high enough to pose a health risk. Not only is ozone a problem for humans, it is also known to damage vegetation and cause the deterioration of some natural and synthetic materials, including paints and dyes.
Ozone is also a powerful greenhouse gas, which contributes to climate change.