Pollution spikes are common in the city of nearly 400,000, especially during thermal inversions when a layer of warm air sits above a layer of cold air, trapping smog over the mountain-ringed city.

Individual heating, fuelled by wood and coal, is the largest source of these emissions, worsened by polluting vehicles.

According to Swiss firm IQAir, which also produces air purifiers, levels of PM2.5 pollutants -- carcinogenic microparticles small enough to enter the bloodstream -- reached nearly 300 micrograms per cubic metre on Friday, far exceeding the World Health Organization's recommended daily exposure limit.

By Friday afternoon, although the air quality index had improved, the pollution level remained "hazardous".

Environmental expert at the Sarajevo Meteorological Institute Enis Krecinic said the air quality was "extremely polluted".

"In our ranking as well, which has six air quality categories, the index is as bad as it gets," Krecinic told AFP.

Even though the hourly peaks of the slightly larger PM10, fine particles with a diameter of 10 micrometres or less, were below recent levels, the daily concentrations on Thursday were the highest since the start of winter, he said.

Authorities once again issued an alert on Friday, a common occurrence on smoggy winter days, with cleaner air expected by Sunday.

Sarajevo's all-time worst recorded air pollution was an hourly reading of 756 micrograms per cubic meter of PM10 particles in December 2016, Krecinic said.

Bosnia has one of Europe's worst rate of premature deaths due to fine particles, according to data from the European Environment Agency, with 199 premature deaths per 100,000 inhabitants in 2023.