La Oroya, Peru

Number of people potentially affected: 35,000
Type of pollutant: Lead, copper, zinc and sulfur dioxide
Source of pollution: Heavy metal mining and processing

Lead is the contaminant that shows up most frequently on Blacksmith's list because the toll it takes on children can be so devastating. In La Oroya, a mining town in the Peruvian Andes, 99% of children have blood levels that exceed acceptable limits, thanks to an American-owned smelter that has been polluting the city since 1922. The average lead level, according to a 1999 survey, was triple the WHO limit. Even after active emissions from the smelter are reduced, the expended lead will remain in La Oroya's soil for centuries — and there's currently no plan to clean it up.

— by Bryan Walsh