Lead pipes like the ones that led to contamination of the tap water in Flint, Michigan, can be found in older neighborhoods across the nation.

Estimates vary on how many of these pipes are still in use. One recent industry survey puts the number at 6.5 million. Most U.S. cities stopped installing such pipes in the 1930s.

To prevent lead from getting into drinking water, cities often add chemicals that protect these pipes from corrosion, or they adjust the chemistry of the water.

A few cities, including Madison, Wisconsin, and Lansing, Michigan, have taken steps to remove all of their lead pipes. But such projects can cost tens of millions of dollars.