According to U.S EPA, the 15 ppb Action Level is used to indicate whether corrosion-control efforts are effective and to measure progress in reducing lead levels. All large water systems are required to maintain corrosion control. If a large water system detects lead above 15 ppb in the tap water in more than 10% of a sample set of homes, then the water system further informs the public about the health effects so that consumers can make decisions about the sources of lead in their homes.