A city with an estimated population of more than ten million people. Megacities are the largest urban populations in the world and some have significant air pollution problems. The World Health Organisation (WHO) and the United Nations Environment Programme (UNEP) operate an air pollution monitoring network as part of the Global Environment Monitoring System. The most severe air pollution monitored occurs in developing countries, but megacities in developed countries have at least one major air pollutant exceeding health guidelines. The worst pollutant affecting megacities as a whole is particulate matter, particularly amongst Asian megacities.