The Dirty 25

"Unless you're in the oil business, there's little reason to brave the choking pollution of Baku, Azerbaijan. Fetid water, oil ponds and life-threatening levels of air pollution emitted from drilling and shipping land the former Soviet manufacturing center at the bottom of this year's list as the world's dirtiest city.

Baku is bad, but far from alone. For residents of the 25 cities on this year's list, black plumes of smoke, acid rain and free-flowing sewage are part of everyday life. Not as immediately visible: the impact on the population's health and life expectancy.

To see which cities in the world were dirtiest, we turned to Mercer Human Resource Consulting's 2007 Health and Sanitation Rankings. As part of their 2007 Quality of Life Report, they ranked 215 cities worldwide based on levels of air pollution, waste management, water potability, hospital services, medical supplies and the presence of infectious disease.

All cities are positioned against New York, the base city with an index score of 100. For the Health and Sanitation Rankings, the index scores range from the worst on the list--Baku, Azerbaijan, with a score of 27.6--to the best on the list--Calgary, Canada, with a score of 131.7.

Nos. 3 and 4 on this year's list are the capital cities of Madagascar and Haiti, respectively. Antananarivo, Madagascar and Port au Prince, Haiti, both face the challenge of a rapidly growing urban population and the ever-growing need for efficient water and waste management.

Mexico City, Mexico, ranks No. 5 on this year's list. Residents can thank industrial and automobile emissions for air quality so bad that city ozone levels fail to meet World Health Organization standards an estimated 300 days of the year. "

Lead-poisoned air lands Dhaka, Bangladesh, the No. 2 spot on the list. Traffic congestion in the capital continues to worsen with vehicles emitting fatal amounts of air pollutants daily, including lead. The World Bank-funded Air Quality Management Project aims to help.