"Across the world, city air is often thick with exhaust fumes, factory smoke or soot from coal burning power plants," says Dr. Maria Neira of the World Health Organization. "In many countries there are no air quality regulations and, where they do exist, national standards and their enforcement vary markedly."
WHO estimates more than two million people die every year from breathing in tiny particles present in indoor and outdoor air pollution.
PM10 particles, measuring 10 micrometers or less, are an important indicator of urban air pollution and the health risks associated with the complex mixtures of pollutants typically found in cities.
The smaller PM10 particles are able to penetrate deep into the lungs, and also to cross into the blood, causing damage in many organ systems. Exposure to these particles can cause heart disease, lung cancer, asthma, and acute lower respiratory infections.
The WHO air quality guidelines for PM10 particles is 20 micrograms per cubic metre (µg/m3) as an annual average, but the new data shows that average PM10 in some cities has reached up to 300 µg/m3.
The great majority of urban populations have an average annual exposure to PM10 particles in excess of the WHO Air Quality guidelines. Only a few cities currently meet the guideline values, the data show.
For 2008, the estimated mortality attributable to outdoor air pollution in cities amounts to 1.34 million premature deaths. If the WHO guidelines had been universally met, an estimated 1.09 million deaths could have been prevented in 2008.
The number of deaths attributable to urban air pollution has increased from the previous estimation of 1.15 million deaths in 2004.
The increase in the mortality estimated to be attributable to urban air pollution is linked to recent increases in air pollution concentrations and in urban population size, as well as improved data availability and methods employed.
WHO is calling for greater awareness of health risks caused by urban air pollution, implementation of effective policies and close monitoring of the situation in cities.
A reduction from an average of 70 µg/m3 of PM10 to an annual average of 20 µg/m3 of PM10 is expected to yield a 15 percent reduction in mortality - considered a major public health gain.
In developed as well as 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.
"Local actions, national policies and international agreements are all needed to curb pollution and reduce its widespread health effects," said Dr. Michal Krzyzanowski, head of the WHO European Centre for Environment and Health in Bonn, Germany.