According to the study, which was carried out in 91 countries and regions, the air in Rio's metropolitan area contains 64 micrograms of fine particulate matter per cubic meter, over three times the upper limit set by the WHO, which is 20 micrograms per cubic meter.

　　Despite being larger, having a bigger population and more vehicles, Sao Paulo's metropolitan area has less air pollution, with 38 micrograms of fine particulate matter per cubic meter.

　　According to the WHO, "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." The organization said that residential wood and coal burning for heating also help increase air pollution, especially in rural areas in the cold months.

　　The WHO expressed concern over air pollution levels in several cities, as some two million people worldwide die annually from conditions resulting from air pollution.

　　"Air pollution is a major environmental health issue, and it is vital that we increase efforts to reduce the health burden it creates," said Dr. Maria Neira, WHO's director for Public Health and Environment.

　　According to the study, Ahwaz, Iran, has the most polluted air in the world, with 372 micrograms of fine particulate matter per cubic meter.