A recent article published by the scientific journal PLoS ONE looked at natural arsenic levels in drinking water:
Chronic exposure to arsenic (As), by contamination of drinking water from natural geological sources, is a significant worldwide environmental health concern , . As many as 25 million people in the United States are exposed to As at levels above the current EPA standard from private, unregulated wells, and worldwide the estimated exposure is several hundreds of millions of people. Chronic exposure to such elevated levels has been associated with a variety of adverse health impacts in human epidemiology studies, including various cancers, cardiovascular disease, diabetes and developmental/reproductive effectsThe article was based on studies on mice, but Wired.com helped put it in perspective in an interview with the articles lead author, Jack Hamilton, who said, "Arsenic is the number one environmental chemical for human health."
[Right, arsenic levels in drinking water. Credit, USGS]