Reverse osmosis filter use and high arsenic levels in private well water.

Inorganic arsenic causes cancer, and millions of people worldwide are exposed to arsenic-contaminated water. Regulatory standards for arsenic levels in drinking water generally do not apply to private domestic wells. Reverse osmosis (RO) units commonly are used by well owners to reduce arsenic concentrations, but may not always be effective. In a survey of 102 homes in Nevada, 19 used RO devices. Pre- and post-RO filtration arsenic concentrations averaged 443 microg/l and 87 microg/l, respectively. The average absolute and percent reductions in arsenic concentrations after filtration were 356 microg/l and 79%, respectively. Postfiltration concentrations were higher than 10 microg/l in 10 homes and higher than 100 microg/l in 4 homes. These findings provide evidence that RO filters do not guarantee safe drinking water and, despite regulatory standards, some people continue to be exposed to very high arsenic concentrations.