Smoke from Canadian wildfires pushed Toronto and Detroit near the top of a live global air-pollution ranking as thick haze spread across the Great Lakes and parts of the northeastern United States.
During the August 2025 smoke event, Toronto briefly ranked as the second-most polluted major city monitored by IQAir, while Detroit was listed fifth. Toronto recorded a US Air Quality Index reading of approximately 158, and Detroit reached about 147, placing both cities in the unhealthy range at that point in time.
The rankings represented a live snapshot rather than an annual assessment of each city’s normal air quality. However, they illustrated how rapidly wildfire smoke can transform conditions in cities located hundreds or thousands of miles from the flames.
Smoke Spread Across the Great Lakes
The polluted air was carried into southern Ontario and the US Midwest by winds moving smoke from numerous fires burning across Canada.
Large fires in provinces including Manitoba, Saskatchewan and parts of western and northern Canada produced enormous smoke plumes. Atmospheric circulation then transported fine particles toward major population centers around the Great Lakes.
Toronto, Detroit, Chicago, Montreal, Minneapolis and New York were among the cities affected during different stages of the smoke event. Visibility declined, skies appeared pale or orange, and some residents reported smelling smoke even though the active fires were far away.
Wildfire smoke can travel across international borders and remain suspended in the atmosphere for days. Changes in wind direction, temperature and atmospheric pressure determine whether the smoke stays high above the ground or descends to the level where people breathe.
Official Canadian wildfire and air-quality information is available through the Government of Canada’s Air Quality Health Index. US residents can follow local conditions through the AirNow Fire and Smoke Map.
What the Pollution Rankings Actually Mean
Descriptions such as “the world’s most polluted city” usually come from a live ranking of major cities monitored at a particular moment.
The ranking can change from hour to hour as wind patterns, traffic, weather and wildfire smoke concentrations shift. It should not be confused with an annual pollution ranking, which compares long-term exposure over months or years.
During the smoke event, Toronto and Detroit temporarily recorded worse air than many cities that normally experience higher average pollution levels. That did not mean Toronto had permanently become one of the world’s most polluted cities.
It meant that the concentration of fine particles had risen sharply enough to place the cities near the top of the live list during that period.
IQAir’s global air-quality ranking tracks real-time pollution levels in major cities. The figures can provide a useful overview, although residents should also follow official local health advisories because different countries use slightly different air-quality scales.
Fine Particles Create the Main Health Concern
The most serious pollutant in wildfire smoke is generally fine particulate matter known as PM2.5.
These particles have a diameter of 2.5 micrometers or less, making them many times smaller than the width of a human hair. Because of their tiny size, they can travel deep into the lungs and may enter the bloodstream.
Short-term exposure can cause coughing, throat irritation, headaches, stinging eyes, chest discomfort, wheezing and difficulty breathing. Smoke can also worsen asthma, chronic obstructive pulmonary disease and other respiratory conditions.
Exposure may place additional strain on the cardiovascular system. People with heart disease can experience an increased risk of symptoms or medical emergencies when particle pollution rises.
Health Canada states that there is no known completely safe level of exposure to some pollutants in wildfire smoke. Health risk generally rises as smoke concentrations and exposure duration increase.
Some People Face Greater Risks
Wildfire smoke can affect anyone, including healthy adults. Certain groups are more likely to experience serious effects.
Children breathe more air relative to their body size and often spend more time being physically active outdoors. Their lungs are also still developing.
Older adults may have existing heart, lung or circulation problems that make it harder for the body to tolerate polluted air. Pregnant people, outdoor workers and individuals with diabetes may also face greater risks.
People with asthma, heart disease, chronic lung disease or a history of stroke should follow medical advice and take air-quality alerts seriously. Anyone who experiences severe breathing difficulty, chest pain, confusion or bluish lips should seek urgent medical care.
Even people without existing health conditions may develop symptoms when smoke reaches unhealthy levels, especially during outdoor exercise or physically demanding work.
Why Wildfire Smoke Can Become Trapped Near Cities
Smoke does not always remain close to the fire that produced it.
Hot air can lift smoke high into the atmosphere, where winds transport it over long distances. The particles may later descend as weather systems shift.
An atmospheric inversion can make conditions worse. This occurs when a layer of warmer air sits above cooler air near the ground. The warm layer acts like a lid and prevents smoke and local pollution from dispersing upward.
When that happens, PM2.5 can accumulate near roads, homes and workplaces. Air quality may remain poor until winds strengthen, rain removes some particles or the atmospheric pattern changes.
Smoke concentrations can also vary significantly within the same city. One neighborhood may temporarily record worse conditions than another because of wind direction, elevation, local traffic or the placement of monitoring stations.
How Residents Can Reduce Smoke Exposure
People should check local air-quality readings before exercising, working or spending extended periods outdoors.
Outdoor activity should be reduced when the air reaches unhealthy levels, particularly for children, older adults and people with heart or lung conditions. Strenuous exercise causes a person to breathe more deeply and rapidly, increasing the amount of smoke entering the lungs.
Windows and doors should remain closed when outdoor air quality is poor, provided indoor temperatures can remain safe. Air conditioning can be used in recirculation mode to reduce the amount of outdoor smoke entering the building.
Portable air cleaners fitted with high-efficiency particulate air filters can lower indoor particle concentrations. The cleaner should be appropriately sized for the room where it will operate.
Activities that produce additional indoor pollution should also be limited. Burning candles, smoking, frying food, using wood-burning appliances and vacuuming without an effective filter can increase particle levels inside the home.
Health Canada offers detailed guidance through its wildfire smoke and health resource.
Do Masks Protect Against Wildfire Smoke?
A properly fitted N95, KN95 or similar respirator can reduce exposure to fine particles when a person must go outside.
Loose cloth masks and ordinary surgical masks generally provide much less protection against PM2.5 because smoke particles can pass through gaps around the face or through materials that do not filter very small particles effectively.
A respirator must fit closely around the nose and mouth to work properly. Facial hair, an incorrect size or a loose nose seal can reduce its effectiveness.
Masks do not filter every gas found in wildfire smoke, but a well-fitted particulate respirator can still significantly reduce the amount of PM2.5 inhaled.
People with breathing or cardiovascular conditions may find respirators uncomfortable and should discuss their use with a healthcare professional.
Drivers Should Keep Vehicle Air on Recirculation
Smoke can enter cars and other vehicles through ventilation systems.
Drivers travelling through smoky areas should keep windows closed and switch the climate-control system to recirculation mode. This reduces the amount of polluted outdoor air pulled into the cabin.
Vehicle filters should be replaced according to the manufacturer’s instructions. A clogged or low-quality cabin filter may remove fewer particles.
Drivers should also be prepared for reduced visibility. Dense smoke can make roads, vehicles and traffic signals harder to see, especially around sunrise, sunset or at night.
A vehicle should not be treated as a long-term clean-air shelter because smoke can still enter, and extended idling in enclosed areas creates a separate carbon-monoxide danger.
Smoke Can Affect Indoor Air for Days
A home may continue to contain smoke particles after outdoor air begins to improve.
Particles can enter through open windows, doors, ventilation systems and small gaps in the building. They can then settle on floors, furniture and other surfaces before becoming airborne again.
Once official readings improve, windows can be opened temporarily to replace polluted indoor air with cleaner outdoor air. Air cleaners should continue operating until indoor conditions are comfortable.
People without air conditioning or filtration may use public libraries, shopping centers or designated clean-air spaces when local authorities make them available. Remaining in an overheated home is not safe, particularly when wildfire smoke occurs during a heat wave.
Balancing heat and smoke protection can be difficult. Residents should follow local health instructions and seek a cooler, filtered location whenever possible.
Canadian Fires Can Affect Air Far Beyond Canada
The smoke over Toronto and Detroit demonstrated the cross-border nature of wildfire pollution.
Canadian wildfire smoke has repeatedly reached the United States and, in some cases, crossed the Atlantic Ocean. A fire does not need to threaten a city directly to affect the health of its residents.
During major smoke events, millions of people can experience elevated PM2.5 concentrations at the same time. Schools may move activities indoors, outdoor events may be postponed, flights may be disrupted and health services may see more patients with respiratory symptoms.
Research and public-health agencies warn that extreme wildfire seasons can create effects that continue long after the visible flames are controlled. Smoke exposure can affect communities far from evacuation zones and may occur repeatedly during the same summer.
Climate Change Is Increasing the Smoke Threat
Individual wildfires can begin through lightning, human activity, equipment failures or other causes. The behavior and severity of a fire are also influenced by temperature, wind, vegetation and soil moisture.
A warming climate can increase the likelihood of hot, dry conditions that allow vegetation to ignite and burn more intensely. In some regions, it can also lengthen the period during which major wildfires are possible.
This does not mean every fire is directly caused by climate change. It means the environmental conditions that support large, destructive fires are becoming more common in many areas.
As smoke events become more frequent, cities far from traditional wildfire zones may need better air monitoring, public clean-air facilities and emergency plans for schools, workplaces and outdoor events.
What Toronto and Detroit Residents Should Remember
The most important action during a smoke event is to follow current local air-quality information rather than judging conditions by appearance alone.
Air may contain harmful particles even when the sky looks relatively clear or there is no noticeable smell. Conditions can also deteriorate quickly when a smoke plume moves closer to the surface.
Residents should reduce outdoor exertion, keep indoor air as clean as possible, use an effective respirator when outdoor exposure cannot be avoided and pay attention to respiratory or cardiovascular symptoms.
Toronto and Detroit’s positions near the top of the global pollution ranking were temporary, but the underlying health risk was real. The event showed how quickly distant wildfires can affect millions of people across two countries.
