Wildfire smoke triggers alerts across 20 US states

Smoke from wildfires burning across Canada and northern Minnesota blanketed large parts of the United States on Thursday, pushing air pollution to dangerous levels and placing millions of people under health alerts from the Upper Midwest to the East Coast.

More than 20 states were affected as dense smoke travelled south and east on north-westerly winds. The sharpest deterioration was recorded around the Great Lakes, where parts of Minnesota, Wisconsin, Michigan, Illinois and Ohio experienced air quality ranging from unhealthy to hazardous. The polluted air was expected to spread towards Washington and other Mid-Atlantic communities during the day.

Detroit registered some of the worst air pollution among major cities worldwide, with smoke reducing visibility and giving the sky a grey-orange appearance. Authorities advised residents to remain indoors, close windows and avoid strenuous outdoor activity. Children, older people, pregnant women and those with asthma, heart disease or other respiratory conditions faced the greatest risk.

Air quality advisories covered all of New York state, including New York City, after smoke from more than 100 Canadian fires reached the region. Pollution readings moved into categories considered unhealthy for sensitive groups and, in some areas, unhealthy for the wider population. New Jersey, Pennsylvania and parts of New England also issued warnings.

New York officials opened cooling centres and distributed protective masks through public facilities and transport locations as the smoke coincided with high temperatures. Residents were urged to shorten outdoor journeys, move exercise indoors and monitor local pollution readings. Schools, parks and government departments were told to adjust activities where conditions posed a health threat.

The Environmental Protection Agency’s Air Quality Index classifies readings from zero to 50 as good and 51 to 100 as moderate. Levels above 100 can harm sensitive groups, while readings exceeding 150 are considered unhealthy for everyone. Conditions above 300 are classified as hazardous and can prompt emergency health warnings.

Fine particulate matter known as PM2.5 is the main danger from wildfire smoke. The particles are small enough to travel deep into the lungs and enter the bloodstream. Exposure can irritate the eyes and throat, trigger coughing and wheezing, worsen asthma and increase stress on the cardiovascular system.

Health specialists recommended properly fitted N95 or KN95 masks for people who must spend time outdoors. Cloth masks provide little protection against fine smoke particles. Households were advised to keep doors and windows shut, use air conditioning on recirculation settings and operate air purifiers or heating and cooling systems fitted with efficient filters.

The smoke originated primarily from large fires burning across northern Ontario, where dry vegetation, hot weather and strong winds complicated firefighting operations. More than 180 fires were active in the province, with smoke carried across the international border by atmospheric circulation. Fires in northern Minnesota also contributed to the pollution affecting the Midwest.

Several wilderness fires in Minnesota were ignited by lightning and had burned tens of thousands of acres. Evacuations and access restrictions were imposed around affected areas, while crews faced difficult conditions in remote forests. Fire managers warned that some deeply burning areas could remain active for an extended period without sustained rainfall.

A high-pressure weather system helped trap pollution closer to ground level, increasing exposure in populated areas. Shifting winds then transported the smoke through Chicago, Detroit, Cleveland, Buffalo, Pittsburgh, Philadelphia and New York, producing a broad corridor of haze visible in satellite imagery.

Meteorologists expected rainfall and an approaching cold front to provide some relief in parts of the Northeast during the weekend. Conditions could fluctuate before then because pollution levels depend on wind speed, atmospheric mixing and fire activity hundreds of miles away.

The smoke also raised concerns ahead of major outdoor events in the New York-New Jersey region, including the World Cup final scheduled for Sunday. More than 80,000 spectators were expected at the stadium, while tens of thousands more could attend public screenings. Forecasts indicated that rain and changing winds might improve conditions before the match, though visitors were advised to monitor official alerts.