The reason is that hot water dissolves contaminants more quickly than cold water, and many pipes in homes contain lead that can leach into water. And lead can damage the brain and nervous system, especially in young children.
Lead is rarely found in source water, but can enter it through corroded plumbing. The Environmental Protection Agency says that older homes are more likely to have lead pipes and fixtures, but that even newer plumbing advertised as “lead-free” can still contain as much as 8 percent lead. A study published in The Journal of Environmental Health in 2002 found that tap water represented 14 to 20 percent of total lead exposure.
Scientists emphasize that the risk is small. But to minimize it, the E.P.A. says cold tap water should always be used for preparing baby formula, cooking and drinking. It also warns that boiling water does not remove lead but can actually increase its concentration. More information is at www.epa.gov/lead or (800) 424-5323 (LEAD).