Air pollution is reaching dangerous levels in several cities in the Balkans with the Bosnian capital of Sarajevo hit worst and enveloped in dense smog coming from coal-fired plants, cars and coal heaters still used in many households.
Martin Tais, a climate change expert said: "Sarajevo is a large city, with 150,000 cars and some 100,000 houses that use coal for heating. The problem comes back to us every year.
"It is a city that was built at a height of 500 metres above sea level and is surrounded by high mountains.
"It is, therefore, a valley where polluted substances are trapped through a phenomenon known as temperature inversion, which forms a cover."