Air quality refers to how clean or polluted the air is, measured by the concentration of pollutants like ground-level ozone, particulate matter, nitrogen dioxide, and sulphur dioxide. Environment and Climate Change Canada monitors air quality and issues the Air Quality Health Index (AQHI), which helps Canadians understand health risks from air pollution and make decisions about outdoor activities. Provincial governments regulate many emission sources, while municipalities can affect air quality through bylaws on idling, woodburning, and land-use planning that reduces vehicle dependence. Poor air quality causes respiratory problems, cardiovascular disease, and premature deaths—making it a public health issue as well as an environmental one. Climate change is expected to worsen air quality through more frequent wildfires and heat events.