While MCLs are risk-based standards, and the code requires the water supplier take various actions based on the MCL, the Lead and Copper Rule regulates the level of contamination at the customers' taps based on "action levels." The rule does not define an MCL for lead or copper. This approach allows some percentage of the customer taps to exceed the "action level" without the water system having to take action. Especially for the larger public water systems, having the supplier change the tendency of the water to dissolve lead in the customer plumbing may be more cost effective than having thousands of customers replace their plumbing.