In many cities air pollution is reaching levels that threaten people's health, according to an unprecedented compilation of air quality data released by the World Health Organisation. The information includes data from nearly 1,100 cities across 91 countries, including capital cities and cities with more than 100,000 residents. WHO estimates more than two million people die every year from breathing in tiny particles present in indoor and outdoor air pollution. In both developed and developing countries, the largest contributors to urban outdoor air pollution include motor transport, small-scale manufacturers and other industries, burning of biomass and coal for cooking and heating, as well as coal-fired power plants.