The increase in pollution caused by rural fires and heatwaves could undermine international efforts to improve air quality and protect human health and ecosystems, warns a new report from the World Meteorological Organization (WMO).
Released on September 7, on the occasion of the International Day of Clean Air for Blue Skies, the annual bulletin on Air Quality and Climate analyzes the complex interconnections between air quality and climate change and argues that the two areas should be addressed through complementary and coordinated policies.
The report also stresses the need to strengthen atmospheric monitoring of pollutants such as fine particles, namely PM2.5, and tropospheric ozone, as well as aerosols such as black carbon and microplastics. Although they are widely dispersed and can be harmful to ecosystems, these pollutants remain insufficiently monitored.
PM2.5 particles are composed of microscopic particles and droplets with diameters of less than 2.5 micrometers. Because they are small enough to penetrate deeply into the lungs and enter the bloodstream, they pose a health risk. They can originate from industrial and agricultural activities, residential heating and transport, but also from rural fires and dust storms.
Extreme fires aggravate exposure to pollution
According to the WMO, extreme fires are increasing, with significant health consequences due to the high toxicity of smoke. While environmental regulations in Europe and North America have contributed to reducing industrial and transport PM2.5 emissions, exposure to particles originating from fires has risen.
One of the studies cited in the bulletin indicates that extreme wildfire smoke episodes have tripled globally since the 1990s, contributing to an estimated nearly 100,000 additional deaths per year between 2010 and 2018. The WMO notes, however, that this figure may be underestimated.
Current air quality risk assessment models may also fail to adequately capture the greater toxicity of wildfire smoke. An epidemiological study concluded that conventional models, based on total PM2.5 concentrations, can underestimate by up to 93% the mortality attributable to fires.
“Meeting the World Health Organization air quality guidelines will become progressively more difficult, as climate extremes favorable to fires are expected to increase in the future,” the bulletin warns.
In 2025, PM2.5 concentrations were above the long-term average in northern Canada, in parts of the Russian Federation, and in central-west Africa, due to increased fire activity. Northwestern Spain also stood out as a hotspot following the exceptional fires recorded in late summer 2025.
In the Amazon, biomass burning was relatively lower in 2025 than in previous years. In China, PM2.5 levels remained below the long-term average, reflecting the reduction of emissions associated with human activities. In India, levels remained above average due to biomass burning and other pollution sources.
Heat waves promote ozone formation
Tropospheric ozone is another of the main problems identified by the WMO. This pollutant affects human health, agriculture and ecosystems and tends to worsen during heat waves.
Prolonged exposure to ozone is associated with increased mortality, especially due to respiratory diseases. High temperatures, stagnant atmospheric conditions and intense solar radiation promote the formation of this pollutant at the surface.
Recent studies conducted during heat waves in the southeastern United States, Europe and China have demonstrated this relationship and point to the possibility of tens of thousands of additional premature deaths linked to increased ozone exposure.
“Going forward, it is anticipated that health risks related to ozone will intensify,” the bulletin says.
Black carbon and microplastics also concern
The report also draws attention to aerosols, whose particles can travel long distances, crossing oceans and continents, and alter the way the atmosphere reflects sunlight. They can also modify the chemical properties of soils and bodies of water, contaminating environments that previously had low levels of pollution.
One example is black carbon, or soot, which deposits on snow and ice. By reducing the amount of solar radiation reflected by these surfaces, it contributes to accelerating their melting.
Microplastics are another cause for concern. Formed by the degradation of plastics due to sunlight exposure and other factors, they are present in the atmosphere, soils and oceans. A simulation using an Earth system model estimates 51 million tonnes of microplastics on land and 22 million tonnes in the oceans, which can, in turn, serve as a source of microplastics for the atmosphere.
The WMO notes, however, that there are large differences between the available estimates due to the lack of continuous monitoring.
“As plastic production, and especially the mismanagement of plastic waste, continue to rise, it will be crucial to monitor the atmospheric pathways of microplastic deposition in remote environments to understand their global impacts on the Earth system,” the bulletin concludes.
