Dar es Salaam, Tanzania is No 12 on The World's 25 Dirtiest Cities Mercer Health and Sanitation Index Score: 40.4 The capital of this east African country continues to grow populationwise, putting a stress on the city's sanitation programs. Solid waste, entering the Msimbazi River, contributes to widely spread infectious diseases among the population. -------------------------------------------------------------- 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. In Pictures: The World's 25 Dirtiest Cities 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. 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. "Addressing air pollution is the easiest way to be able to fix someone's well-being because we're always breathing, and there are all sorts of harmful particulates in the air," says Richard Fuller, founder of the New York-based Blacksmith Institute, a nonprofit dedicated to solving the pollution problems of the developing world. "In fact, the biggest pathway for lead poisoning is particulates in the air. So in areas with a lot of air pollution, shutting down the worst forces of these types of pollution really does make a difference." 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. But things could be worse. "Mexico City has actually seen great improvement recently in terms of air pollution," says Dave Calkins, founder of the Sierra Nevada Air Quality Group and former chief of the Air Planning Branch of the U.S. Environmental Protection Agency in San Francisco. "So much so that the government actually has to campaign to let everyone know that pollution is still a problem." Economies suffer, too. Health care costs and lost productivity drag on business. Companies also face added costs in the form of remuneration packages when relocating employees and their families to some of these cities, noted Slagin Parakatil, senior researcher at Mercer. Cost-benefit analysis certainly suggests making progress toward cleanup. According to a study done by WaterAid, for every $1 spent on improved sanitation, the benefit equals $9 resulting from decreased cost of health care and increased productivity. "If you do the numbers," says Fuller, "to clean up the worst of it doesn't really cost that much. It's the 90/10 rule. To do 90% of the work only costs 10% of the money. It's the last 10% of the cleanup that costs 90% of the money. For relatively little, we can do an awful lot to save a whole lot of lives."