Cities in parts of Europe have been suffering from some of the worst air quality in the world.
Winter smog has become a big issue in the Western Balkans.
Serbia is the country with the highest rate of pollution-related deaths in Europe, according to the Global Alliance on Health and Pollution.
Jasna Cizler is a cycling campaigner in the capital Belgrade, who believes two-wheeled transport holds the key to cleaner air.
Produced by Guy Delauney