 U.S. opponents of fluoridation were heartened by a 2006 National Research Council report about hazards of water naturally fluoridated to high levels; the report recommended lowering the U.S. maximum limit of 4 mg/L for fluoride in drinking water.Water was fluoridated in large parts of the Netherlands from 1960 to 1973, at which point the Supreme Court of the Netherlands declared fluoridation of drinking water unauthorized.The Dutch Court decided that authorities had no legal basis for adding chemicals to drinking water if they did not also improve safety.