Smog is a type of visible air pollution resulting from the interaction of sunlight with pollutants including nitrogen oxides and volatile organic compounds, creating ground-level ozone and particulate matter. The term originated combining 'smoke' and 'fog' to describe industrial pollution in early 20th century cities. Smog causes respiratory problems, aggravates asthma and other conditions, and reduces visibility. Smog alerts warn vulnerable populations when air quality deteriorates to unhealthy levels. Canadian cities experience smog primarily during hot summer days when conditions favour ozone formation. Reducing smog requires controlling emissions from vehicles, industry, and other sources. Air quality improvements have reduced smog frequency in many Canadian cities compared to past decades.