Some 3.7 million premature deaths annually are attributed to outdoor air pollution. About 80% of those deaths are due to heart disease and stroke, while another 20% are from respiratory illnesses and cancers related to exposure to fine particulate matter (PM2.5), the most health-harmful air pollutant.
According to WHO’s most recent survey of 1600 cities worldwide, only 12% of the urban population surveyed live in areas that comply with WHO air quality guideline levels for PM2.5. Average particulate air pollution levels in many developing cities can be 4-12 times higher than WHO air quality guideline levels, putting many at risk of long-term health problems.
Ground-level ozone, produced by the atmospheric interaction of a mix of air pollutants, including methane and NOx, is another health risk, raising rates of asthma and chronic respiratory illness as well as other sorts of breathing problems and reduced lung function. Ozone also reduces crop productivity in peri-urban areas, where ozone levels may often be heaviest.
In most low and middle-income cities where historical comparisons are possible, air pollution has become worse over the past several years. Many factors contribute to this increase, including increased urban power demand, which drives up power plant emissions, and soaring use of private motor vehicle transport.
Building heating and cooling systems, often energy inefficient, also play a role as do industrial emissions, when industries are located in close proximity to cities and homes. In low income and some middle-income cities, the incineration of solid waste, burning of agricultural waste in periurban areas, and the use of solid fuels (coal and biomass) for cooking and heating are also serious urban air pollution issues.
Some 25% of households in least developed cities are reliant on solid fuels for cooking. Those households face a double air pollution burden – outdoors as well as inside the home.