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Mississauga occupies a strategic position within Canada's most populous urban corridor, situated on the northwestern shore of Lake Ontario at the heart of the Greater Toronto Area. This city of over 700,000 residents lies on the flat to gently rolling terrain of the Iroquois Plain, with elevations ranging from 76 to 156 meters above sea level, creating minimal natural ventilation barriers. Mississauga's geography creates a complex air quality environment shaped by its immediate proximity to Lake Ontario's moderating influence, which can trap pollutants during temperature inversions while also providing cleansing lake breezes. The city exists within a pronounced urban-rural gradient, transitioning from dense commercial and residential development in its eastern sections near Toronto to more suburban and light industrial areas westward toward agricultural regions of Halton and Peel. Mississauga's location within Ontario's Golden Horseshoe industrial belt means it receives transported pollution from Hamilton's steel industry to the southwest and Toronto's urban emissions to the east, while also generating its own from the Pearson International Airport corridor, Highway 401/403/407 transportation networks, and light manufacturing districts. The Credit River valley running through the city creates microclimates that can concentrate vehicle emissions, while the lake's thermal mass influences local wind patterns that alternately disperse and trap airborne particulates depending on seasonal conditions.

Mississauga's air quality follows distinct seasonal patterns shaped by Lake Ontario's influence and regional weather systems. Winter months from December through February typically see the poorest air quality due to temperature inversions that trap pollutants close to the ground, compounded by increased residential heating emissions and vehicle exhaust in cold conditions. Sensitive groups should limit prolonged outdoor exertion during winter cold snaps when particulate matter concentrations peak. Spring brings improvement as increasing solar radiation breaks up inversions and prevailing westerly winds disperse pollutants, though March and April can experience dust from agricultural activities and road sanding residue. Summer offers generally good air quality with lake breezes providing natural ventilation, but occasional heat waves in July and August can trigger ground-level ozone formation from vehicle and industrial emissions reacting with sunlight, particularly on hot, stagnant days. Autumn represents the most favourable season for outdoor activity, with September through November typically experiencing clean air as cooler temperatures reduce ozone formation while remaining above inversion-prone conditions. However, late autumn can bring increased particulate matter from increased residential heating and reduced atmospheric mixing. Throughout the year, weather hazards like lake-effect snow in winter or humidity in summer don't directly worsen pollution but can exacerbate respiratory symptoms during already poor air quality episodes.