 Mexico City sits in a valley surrounded on three sides by mountains, which serve to trap contaminants produced by the metropolitan area's 15 million residents.One government study in the late 1980s determined that nearly 5 million tons of contaminants were emitted annually in the atmosphere, a tenfold increase over the previous decade.Carbons and hydrocarbons from the region's more than 3 million vehicles account for approximately 80% of these contaminants, with another 15%, primarily of sulfur and nitrogen, coming from industrial plants.