Instead of an increase in mortality, as with many other cancer sites, the study found that breast cancer deaths were cut in half during the period that coincided with high arsenic exposure. The effect was more pronounced among women under age 60, with mortality in these women reduced by 70 percent.
"What we found was astonishing," said study lead author Dr Allan Smith, University of California Berkeley professor of epidemiology and director of the Arsenic Health Effects Research Programme. "We've been studying the long-term effects of arsenic in this population for many years, focusing on increased disease and mortality attributed to the historical exposure to arsenic in this population," Smith said.
In 1958, the northern Chilean city of Antofagasta switched to a geothermal water source originating in the Andes Mountains. Years later, it was discovered that the water sources contained more than 800 micrograms per litre of arsenic, 80 times higher than the levels recommended by the World Health Organisation.
An arsenic removal plant was installed in 1970 after toxic effects from arsenic exposure became apparent in some residents. As part of the study, researchers at the Stanford Cancer Institute found that human breast cancer cells grown in lab cultures are killed by arsenic, and normal breast cells are more resistant to arsenic.
The medicinal use of arsenic is not entirely new. Arsenic trioxide was approved in 2000 by the US Food and Drug Administration (FDA) as an effective treatment for a rare type of leukemia, researchers said. Smith and collaborators in Chile are proceeding to design clinical trials in which some advanced breast cancer patients would be given arsenic treatment.