Smoke from raging wildfires in northwestern Ontario has severely degraded air quality across parts of the northeastern United States, with Chicago recording "Very Unhealthy" levels of fine particulate pollution on Wednesday. The deteriorating conditions come as smoke continues to drift south, prompting health advisories in several major cities ahead of Sunday's FIFA World Cup final in neighboring New Jersey. Toronto briefly ranked as the world's most polluted major city on Wednesday, while New York City also issued air quality alerts, urging residents to reduce strenuous outdoor activities as thick wildfire smoke spread across the region. Chicago's air quality reaches unhealthy levels According to Swiss air quality monitoring company IQAir, Chicago's Air Quality Index (AQI) reached 171, placing it in the "Unhealthy" category. The city's PM2.5 concentration measured 166 µg/m³, which IQAir said is 33.2 times higher than the World Health Organization's annual PM2.5 guideline value. The forecast suggests conditions may gradually improve over the coming days, with AQI levels expected to drop to 62 on Friday, 66 on Saturday, and 85 on Sunday, although air quality is still forecast to remain less than ideal.PM2.5 driving the pollution IQAir's latest readings for Chicago showed: PM2.5: 166 µg/m³ Ozone (O₃): 68 µg/m³ Nitrogen dioxide (NO₂): 22.5 µg/m³ Fine particulate matter (PM2.5) is considered the primary pollutant responsible for the hazardous conditions. These microscopic particles can penetrate deep into the lungs and even enter the bloodstream, increasing the risk of respiratory and cardiovascular problems. AccuWeather rates PM2.5 as 'Very Unhealthy' AccuWeather also classified Chicago's PM2.5 pollution as "Very Unhealthy." Its latest air quality data showed: PM2.5: AQI 212 (105 µg/m³) – Very Unhealthy PM10: AQI 197 (235 µg/m³) – Very Unhealthy NO₂: AQI 43 (21 µg/m³) – Fair Ozone: AQI 13 (40 µg/m³) – Excellent Carbon Monoxide: AQI 2 (224 µg/m³) – Excellent Sulphur Dioxide: AQI 2 (2 µg/m³) – Excellent AccuWeather warned that sensitive groups are likely to experience immediate health effects and should avoid outdoor activities. It added that even healthy individuals may experience difficulty breathing and throat irritation, recommending people stay indoors and postpone outdoor exercise if possible.IQAir health recommendations With pollution levels elevated, IQAir advised residents to: Avoid outdoor exercise. Keep windows closed to prevent polluted air from entering homes. Wear a well-fitting mask when outdoors. Run an air purifier indoors where available. Authorities across affected regions continue to monitor air quality as wildfire smoke shifts with changing weather patterns. Public health officials have urged residents, particularly children, older adults and people with heart or lung conditions, to limit prolonged exposure until conditions improve.