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Berwyn, Illinois, nestled within Cook County, occupies a geographically significant position in the Chicago metropolitan area. Located approximately 11 miles west of downtown Chicago, it sits along the Illinois River's western edge, though the river itself is not directly adjacent to the city. The terrain is predominantly flat, characteristic of the Illinois River Valley, with an average elevation of around 600 feet above sea level. This flatness, while facilitating urban development, can unfortunately exacerbate air quality issues by limiting vertical mixing of pollutants. Berwyn’s urban character is defined by its dense residential neighborhoods, a legacy of its early 20th-century growth as a commuter suburb. The surrounding landscape transitions from urban to a mix of agricultural fields and scattered industrial zones, particularly to the west and southwest. The proximity to Chicago’s industrial belt, historically centered around steel and manufacturing, contributes to regional pollution sources. The urban–rural gradient is relatively sharp; Berwyn quickly gives way to more rural landscapes, though the influence of Chicago’s sprawling urban footprint remains evident. The city’s location within the broader Midwest corridor, known for its agricultural activity, also means it can be affected by dust and particulate matter carried by prevailing winds. The lack of significant topographic features hinders natural air cleansing processes, making Berwyn susceptible to pollutant accumulation, especially during periods of stagnant weather. The city’s dense population and reliance on vehicular transportation further compound these challenges.

Berwyn’s air quality experiences a distinct seasonal cycle heavily influenced by meteorological patterns. Spring, while bringing relief from winter’s cold, often sees elevated levels of pollen and dust, impacting those with respiratory sensitivities. The warming temperatures can also trigger increased ozone formation, particularly during sunny days with light winds. Summer months are frequently characterized by periods of stagnant air, leading to the build-up of pollutants from both local sources and those transported from elsewhere. Temperature inversions, common during summer nights, trap pollutants near the ground, worsening air quality. Autumn typically offers improved conditions as temperatures cool and wind patterns become more variable, dispersing pollutants. However, agricultural burning in surrounding areas can occasionally contribute to particulate matter pollution. Winter presents a mixed picture. While cold temperatures can reduce ozone formation, the increased use of heating systems, particularly those burning fossil fuels, can elevate particulate matter levels. Fog, a frequent occurrence during winter, traps pollutants close to the ground, creating localized air quality concerns. Sensitive groups, including children, the elderly, and individuals with pre-existing respiratory conditions, should be particularly mindful of air quality forecasts during the spring and summer months, limiting outdoor exertion on days with poor air quality. During winter, those with heart or lung conditions should exercise caution during periods of heavy fog or when heating systems are in frequent use.