In the three cities, the air quality index (AQI), which measures a range of pollutants, spiked above the level of 150 which is considered unhealthy.

In Paris and Los Angeles it was 44 and 34 respectively.

The extreme pollution in the three Balkan capitals coincided with the drop in temperatures.

In the Serbian capital Belgrade, the concentration of particles with a diameter of 2.5 microns or less (PM2.5) had reached 98 micrograms per cubic meter. It was more than 19 times the World Health Organisation's annual guideline.

The PM2.5 particles pose a severe health risk if inhaled over long periods of time as they are tiny enough to get into the bloodstream.

In Sarajevo, the capital of neighbouring Bosnia, the situation has been worsening over the past few days, especially in the evenings, due to the tens of thousands of homes using coal and wood as fuel.

In some parts of the city, the concentration of PM2.5 particles exceeded 300 micrograms per cubic metre.

Experts say the Balkan cities suffer from a chronic air pollution due to several reasons, citing notably burning of fossil fuels -- coal in particular -- in various forms and outdated technologies.

The start of the heating season aggravates the problem, they say.

"With burning of low-quality coal and wood, and what is particular for our region, burning of all kinds of waste -- old furniture, clothes, plastic -- and, in the worst case, using motor oil for heating, we get the picture," said Dejan Lekic of the Belgrade-based National Ecological Association.

"Combined with heavy traffic in major cities this means extreme pollution," he told AFP.

Air pollution is the cause of 20 percent of premature deaths in 19 Balkan cities, according to a UN study from 2019.

People living in the region lose in average up to 1.3 years of their life due to air pollution, the study showed.

burs-cbo/ljv/giv