Residents of Sarajevo were advised by public health officials to stay home on Thursday, as flights were grounded and winter smog blanketed Bosnia-Herzegovina's capital.
The Swiss air quality technology company IQAir, which assesses air quality in real time, put Sarajevo's air quality index, or AQI, at a "very unhealthy" 225, ranking the city as the third most-polluted in the world.
If people have to go outside, they should wear masks with a filter to avoid damaging their health, authorities said.
"This is horrible; we are choking, particularly people with heart conditions," said pensioner Hajra Subasic. "I can barely breathe; you see how hard it is for me to speak."
The thick, brown fog has been spreading across the city since Wednesday evening.
By Thursday, hardly anything could be seen from the top of the Trebevic mountain in the valley basin. Only the top of the 172-metre-high Avaz Twist Tower building, which houses the country's media group of the same name, was visible.
Sarajevo and other major cities in the Balkans are traditionally very polluted during winter months as countries there rely on coal and wood for heating.
Experts have warned of the effects on people's health because bad air is linked to various diseases and premature deaths.
Nestled in a valley surrounded by mountains, Sarajevo is particularly prone to air pollution. Bad urban planning and the proliferation of tall buildings have made the problem worse in recent years.
"Nobody is doing anything," fumed Subasic. "I can't believe this is happening in Bosnia's capital and they (authorities) can't fix it."
Balkan states generally have poor records in environment protection despite promises to improve the situation as they seek to move closer to European Union membership. Balkan rivers are often clogged with garbage and recycling is almost non-existent.
The air in neighboring Serbia's capital, Belgrade, was also deemed "unhealthy" on Thursday with the air quality index put at 170.
In Sarajevo, planes were grounded during the morning hours because of the fog and drivers were warned to take additional care because of low visibility. Many people were seen wearing face masks for at least some protection.
A Sarajevo resident, who gave only her first name as Nada, said she tries to evade pollution by going to the mountains around the city where the air is clean and from where Sarajevo can be seen covered in a cloud of fog below.
"I accumulate some strength there but I have no idea how long it will last," she said. "It's even worse in our flats because of the heating and dry air and because we can't open our windows."
