The claim [that you shouldn't drink hot water from the tap] has the ring of a myth. But environmental scientists say it is real.
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. ... 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.
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.
--Anahad O'Connor, NYT, on urban legends that happen to be true