Editor's Note: The World Health Organization answers questions on air quality and health.

What are the health consequences of air pollution?

Air pollution increases the risk of respiratory and heart disease in the population. Both short- and long-term exposure to air pollutants have been associated to health impacts. More severe impacts affect people who are already ill. Children, the elderly and poor people are more susceptible.

How serious is air pollution?

Air pollution is a major environmental health problem affecting everyone. Air pollution results from exhaust fumes from cars, domestic combustion or factory smoke. It is estimated that 2 million people worldwide - more than half of them in developing economies - die every year from air pollution.

What are the most polluted cities in the world?

Unfortunately, there is no comprehensive, worldwide database allowing precise answers to this question. We can also suspect that no monitoring is conducted in many cities with high pollution levels, making such an assessment impossible. Nevertheless, the available data indicate that air pollution is very high in a number of Asian cities such as Karachi, New Delhi, Kathmandu and Beijing, as well as in the Latin American cities of Lima and Arequipa, and in the Middle East city of Cairo.

It should be stressed that health concerns are not limited to the most polluted cities. Substantial health effects are seen even in the relative cleaner cities of Europe or North America, where PM levels are three to five times lower than in the most polluted cities.

Where are particulate matter concentrations especially high?

As is often the case, the biggest air-pollution-related burden to health is observed in developing economies. The lack of knowledge of the health impact of pollution is a big obstacle in defining the actions and mobilizing local and international resources.

What are the implications for public health?

Public health recognizes air pollution as an important determinant of health, especially in developing economies. There is significant inequality in the exposure to air pollution and related health risks. Air pollution combines with other aspects of the social and physical environment to create a disproportional disease burden in less affluent parts of society.

Exposure to air pollutants is largely beyond the control of individuals and requires action by public authorities at the national, regional and even international levels.

The health sector can play a central role in leading a multisectoral approach to prevention of exposure to air pollution. It can engage and support other sectors, such as transport, housing, energy production and industry, in the development and implementation of long-term policies to reduce the risks of air pollution to health.

(China Daily 01/16/2013 page5)