Fresh air can be scarce. Breathing in New Delhi, which one local politician describes as a vast gas chamber, is comparable to smoking between 10 and 20 cigarettes a day. Auto exhaust, in particular, is a suffocating threat in the developing world, where millions of new cars have begun to flood streets and roads not built to hold so many vehicles. Clogged traffic increases smog--since cars in low gear release more pollution than fast-moving autos--and takes a toll on productivity. In Bangkok, where each driver spends the equivalent of 44 days per year in gridlock, delays are estimated to cost the local economy hundreds of millions of dollars a year in lost productivity.

The combination of poor sanitation and air and water pollution has exacted a staggering toll on the health of cities. A 1990 study by the Aga Khan Hospital in Karachi found dangerously elevated lead levels in children's and soldiers' blood. The foul air over Mexico City, which is hemmed in by a ring of mountains, often sends children home from school coughing and elderly citizens to the hospital.

For all their urban nightmares, though, megacities do not have to be unmanageable. While every huge city faces problems, several of the richest megatowns have coped with the most immediate environmental threats. The two largest metropolitan areas in the U.S.--New York City (16 million people) and Los Angeles (12 million)--have both benefited from the booming American economy. Quality of life is rising in New York, where a clean-up campaign has reduced the amount of trash on streets, and fishing has resumed on the Hudson River. Los Angeles residents, sick of the omnipresent smog, are banding together in car pools that decrease the number of vehicles on the road. The developing world has its bright spots as well. The largest urban area in the Western hemisphere--S�o Paulo, Brazil, with 16 million people--has achieved relatively low rates of death from infectious disease by improving sanitation coverage and vaccination programs.