While much of the smog here is generated locally, air pollution from the United States contributes about 50 per cent of ground level ozone. This valley, which includes the City of Vancouver, is bordered by the Coastal Mountains to the north and the Cascade Mountains to the southeast. Motor vehicles in the Vancouver area are the major source of NOx and VOC emissions. Cross border pollution contributes between 50 and 90 percent of this region's smog problem. Children and active adults are also at a greater risk because they typically spend more time outdoors.
Children tend to be more sensitive than adults because they breathe faster.
Sensitive people may experience symptoms after only one or two hours outdoors. Exposure to ozone can irritate the nose and throat and cause chest tightness, coughing and wheezing. Reduction in lung function has been observed in clinical studies with concentrations as low as 82 ppb. Health studies have focused on two groups of airborne particles that have been proved to be harmful, even at very low levels. These very fine particles cause the greatest concern.