Minnesota recorded the most intense smoke pollution in its history this week as fires in and near the Boundary Waters Canoe Area Wilderness combined with smoke drifting south from more than 900 wildfires in Canada. State readings of fine particulate pollution reached levels the monitoring network had never measured.
The highest reading came at Ely, in the state's north-east, at roughly 797 micrograms of fine particulate per cubic metre. West Duluth recorded about 636, Virginia 604, Grand Portage 576 and Duluth proper 574. The previous record for the region was 476, and the Minnesota Pollution Control Agency has been keeping these records since 2014.
The Twin Cities metro set its own marks: about 383 micrograms at St. Michael, 365 in Minneapolis, 362 at Apple Valley, 343 at Blaine, 309 in the East Phillips neighbourhood of Minneapolis, 307 at St. Paul's Harding High School and 286 at Lakeville. Thursday was the first time on record that the metro area has been placed in the "hazardous" category, the most severe rating in use. For part of that afternoon, Minneapolis had the worst air quality of any major city in the world.
Those numbers are hard to read without a reference point. The World Health Organization's guideline for fine particulate over a 24-hour period is 15 micrograms per cubic metre; the Ely reading is about 53 times that. It also sits beyond the upper end of the US Environmental Protection Agency's air quality index, a scale that tops out well below what northern Minnesota recorded this week.
The Minnesota Department of Natural Resources, which has documented smoke observations in the state since 1976, described the event as the worst and most widespread smoke outbreak on record, pending laboratory confirmation of the readings by the pollution control agency. It said the outbreak was substantially worse than any in at least 50 years, and exceeded the smoky summer of 2025 in both frequency and intensity.
The fires began around July 7 and the heaviest smoke settled over the state between July 14 and 17, peaking on the 15th and 16th. More than 50,000 acres had burned by July 16 with minimal containment. The Superior National Forest has reported the worst wildfire conditions it has seen in 20 years.
Air of this quality is a health exposure rather than an inconvenience. Particulate this fine passes deep into the lungs and crosses into the bloodstream, and studies of previous smoke episodes have found measurable increases in emergency admissions for asthma, heart attack and stroke in the days following a spike. State health officials advised residents across most of Minnesota to stay indoors, run air filtration where available and avoid outdoor exertion during the worst of it.
The smoke has since spread well beyond the state, contributing to unhealthy air in Detroit, Chicago and across the Great Lakes, and drawing a threat from President Trump to add the cost of the pollution to tariffs on Canada.