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 which was set up in 1974. This network has enabled monitoring equipment to be established in more than 50 cities of the world in 35 countries. Initially sulphur dioxide, particulate matter and lead were monitored, but in 1991 the network was expanded to measure carbon monoxide, nitrogen dioxide and ozone.

The most severe air pollution monitored occurs in developing countries, but even megacities in developed countries have at least one major air pollutant exceeding health guidelines. Although London compares fairly favourably with other world megacities, exceedences of health guidelines do occur, most readily for nitrogen dioxide, ozone and particulate matter. The WHO/UNEP survey has revealed that seven megacities had three or more pollutants which exceeded WHO health protection guidelines, Mexico City, Beijing, Cairo, Jakarta, Los Angeles, Sao Paulo and Moscow. Mexico City was classified as having serious problems for sulphur dioxide, particulate matter, carbon monoxide and ozone in addition to moderate to heavy pollution for lead and nitrogen dioxide. Mexico City suffers in particular due to its high altitude and climate which results in poor ventilation, in addition to the large number of old and poorly maintained vehicles.

The worst pollutant affecting megacities as a whole is particulate matter, particularly amongst Asian megacities. This is of particular concern as high levels of particulates are known to increase mortality rates. In contrast, many of the megacities have reduced emissions of sulphur dioxide through changes in fuel use from high sulphur coal and oil to cleaner fuels such as natural gas.