Initiated by the Acid Precipitation Act of 1980, the NAPAP studied acid deposition throughout the United States. After 10 years, the researchers reported to Congress that, while acid rain was a problem, the danger was not quite as severe as initially feared. About four percent of the lakes in the United States were deemed unacceptably acidic, and 25 percent of these lakes had acidic water due to natural causes (for example, decaying vegetation debris can raise water acidity levels).