Arsenic occurring naturally in groundwater is classified as a human carcinogen (prone to cancer of the bladder, lungs, skin, kidney, liver, and prostate) by the International Agency for Research on Cancer and the National Research Council. In 2002.
The USEPA lowered the maximum contaminant level (MCL) for arsenic in drinking water from 50 g/L to 10 g/L, and the new MCL became effective in January 2006.