Skopje's high levels of pollution are caused by a combination of smoke from houses, emissions from the industry, from buses and other forms of public transport, as well as from cars, and a lack of interest in caring for the environment. Central heating is often not affordable, and so households often burn firewood, as well as used car tyres, various plastic garbage, petroleum and other possible flammable waste, which emits toxic chemicals harmful to the population, especially to children and the elderly.