The World Meteorological Organization said the quality of the air people breathe was interlinked with climate change, and the two issues needed to be tackled together.

Wildfires in the Amazon, Canada and Siberia have brought home how air quality can be impacted on a vast scale, the WMO said in its fifth annual Air Quality and Climate Bulletin.

"Climate impacts and air pollution respect no national borders -- as exemplified by intense heat and drought which fuels wildfires, worsening air quality for millions of people," said WMO deputy secretary-general Ko Barrett.

The bulletin looked at the interplay between air quality and the climate, highlighting the role of tiny particles called aerosols in wildfires, winter fog, shipping emissions and urban pollution.

Particles with a diameter of less than 2.5 micrometres (PM 2.5) are considered particularly harmful since they can penetrate deep into the lungs or cardiovascular system.

Wildfires in 2024 led to above-average PM 2.5 levels in Canada, Siberia and central Africa, the WMO said. The biggest PM 2.5 surge, however, was in the Amazon basin.

- Wildfire season stronger, longer -

"The wildfire season has the tendency to be stronger and longer every year as a result of climate change," said WMO scientific officer Lorenzo Labrador, who coordinated the bulletin.

Wildfires in Canada have ended up causing air pollution in Europe.

"We had that last year and this year as well. So you have a degradation in air quality across continents when the meteorological conditions are right," Labrador told a press conference.

"What we have from these fires is essentially a witches' brew of components that pollute the air."

The World Health Organization estimates that air pollution causes more than 4.5 million premature deaths each year.

The WMO called for improved monitoring and better policies to safeguard human and environmental health -- and reduce agricultural and economic losses.

The bulletin highlighted pollution hotspots in northern India.

It said the Indo-Gangetic Plain, home to more than 900 million people, had seen a marked rise in air pollution and winter fog episodes, which are growing in frequency and duration due to pollution, notably from agricultural biomass burning.

"Persistence of fog is no longer a simple, seasonal weather event -- it is a symptom of escalating human impact on the environment," it said.

- Dramatic improvements in China -

PM 2.5 levels continued to decline in eastern China last year, which the WMO put down to sustained mitigation measures.

When countries take measures to combat poor air quality, the improvement can be clearly seen in meteorological data, said Paolo Laj, the WMO's global atmosphere chief.

"Look at Europe, Shanghai, Beijing, cities in the United States: many cities have taken measures and you see in the long term, a strong decrease" in recorded air pollution, he told AFP.

"Over a 10-year period, Chinese cities have improved their air quality in a dramatic way. It's really impressive what they have done."

Laj said there was no all-purpose measure that could bring about drastic change, such as switching to electric cars, "but when measures are taken, it works".

In Europe, "we don't realise what we were breathing in 20 years ago, but it was much worse than today", he added.