UN's WMO Marks Clean Air Day With Warning That Extreme Wildfire Smoke Events Have Tripled Since 1990s

Extreme wildfire smoke events have tripled across the globe since the 1990s, according to a new bulletin from the World Meteorological Organization (WMO), which warns that pollution from wildfires and heatwaves risks undermining international efforts to improve air quality.

The UN's weather and climate agency released the findings to mark the International Day of Clean Air for Blue Skies on 7 September, an annual observance designated by the UN General Assembly in 2019 to raise awareness of air pollution and promote international action to improve air quality.

Wildfire Smoke Rising Even as Other Pollution Falls

The WMO's Air Quality and Climate Bulletin found that while regulation has cut industrial and transport pollution across Europe and North America, exposure to fire-related PM2.5 has been climbing in the opposite direction.

A study cited in the bulletin found that extreme fire-smoke events have tripled worldwide since the 1990s, contributing to an estimated 100,000 additional deaths annually between 2010 and 2018. The WMO says this figure may be an underestimate.

Fire activity pushed PM2.5 levels above long-term averages in 2025 across northern Canada, parts of the Russian Federation, western-central Africa and north-western Spain. China's PM2.5 levels stayed below the long-term average, reflecting declining emissions from human activity.

Health Risk Models 'May Underestimate' Wildfire Deaths by 93%

The bulletin's most striking claim is that the tools used to assess how dangerous wildfire smoke actually is may themselves underestimate the risk.

Standard air quality risk models, which are based on total PM2.5 concentration, may underestimate wildfire-attributable deaths by as much as 93 per cent, the report says. This is because they do not adequately capture the greater toxicity of fire-derived particles compared with other sources of pollution.

The bulletin adds that meeting World Health Organization air quality guidelines will become progressively more difficult as extreme fire weather is expected to increase in the future.

Heatwaves Adding to Ozone and Aerosol Risks

Heatwaves are compounding the problem by driving up ground-level ozone, which the bulletin says is a growing health concern. Long-term exposure to ozone pollution is associated with increased mortality, primarily through respiratory diseases.

Recent studies have examined ozone formation during heatwaves in the south-eastern United States, Europe and China. The report also flagged gaps in monitoring of aerosols such as black carbon and microplastics.

Black carbon settling on snow and ice reduces how much sunlight those surfaces reflect, which can speed up melting. One Earth system model estimated 51 million tonnes of microplastics on land and 22 million tonnes in the ocean, though the WMO says there are large variations in the estimates because of insufficient sustained monitoring.

Wildfire Smoke Travels Far Beyond Hotspots

The findings matter well beyond the countries named as hotspots.

Aerosol particles can travel far from their sources, including across oceans and continents, meaning pollution can affect areas far from where it was produced.

The WMO's findings suggest conventional risk assessments could underestimate the health impact of wildfire smoke by up to 93 per cent.

The WMO bulletin warned that 'as extreme fire weather is expected to increase in the future, meeting World Health Organization air quality guidelines will become progressively more difficult.'