Underestimating Arsenic's Risk: The Latest Science Supports Tighter Standards. (the Arsenic Controversy)
Wilson, Richard, Regulation
ARSENIC HAS LONG BEEN KNOWN TO BE acutely poisonous at high doses. However, if individuals ingest it at subacute doses, they become partially tolerant to the chemical. That makes arsenic the darling of detective story writers: A villain can take subacute doses for a week, and then share a meal containing arsenic with his victim. The villain lives but the victim dies, and the investigators have no reason to suspect that the meal served as the murder weapon.
But toxicity is only one hazard of arsenic; another is its carcinogenic properties. Scientific evidence of a link between cancer and arsenic dates back to the late nineteenth century when researchers found a connection between regular use of the medicinal Fowler's Solution (one percent arsenate) and skin cancer. In the following decades, similar connections were found among people who had regular exposure to arsenic-based pesticides and to the fumes produced during metal smelting.
More recently, researchers have given considerable attention to incidence of cancer among users of water supplies with high concentrations of arsenic. Earlier this year, the New York Times reported on a five-year study in Chile that showed some 700 people in excess of the background rate died from cancer that was linked to arsenic in drinking water at concentrations of 500 [micro]g/L. In Bangladesh, 30 million people are exposed to similar levels of arsenic in their drinking water, and thousands of Bangladeshis have died from secondary effects of cancerous skin lesions.
Those studies involved arsenic levels that are significantly higher than what the U.S. Environmental Protection Agency (EPA) allows. But, concerning low-level exposure, scientists traditionally have believed that arsenic has few adverse effects. Standard experiments supported that belief: Test animals, particularly rats and mice, did not show any unusual incidence of cancer when they ingested arsenic concentrations proportional to concentrations ingested by the average American. Therefore, researchers have long believed that people do not develop cancer from low-level arsenic exposure.
However, that belief is now in question. A number of recent studies have indicated a potential link between low-level arsenic ingestion by humans and cancers of the bladder, kidney, and lung. What is more, Australian researchers have announced that they had induced mice to develop cancer using small doses of arsenic. Though there is still debate within the scientific community over those studies, evidence is now accruing that such a link exists.
That leads us to a crucial question: Given the recent scientific findings, should the United States lower the allowable limit for arsenic in drinking water below the 50 [micro]g/L level? As often happens, the whole world is watching us as we discuss what to do.
ESTIMATING LOW-DOSE RISK
The fundamental issue in trying to answer that question is how to estimate arsenic risk at lower levels from the generally accepted measurements at higher exposure levels.
Some scientists and physicians argue that a linear dose-response model is appropriate. Under such a model, researchers would assume that the rate of cancer resulting from exposure to 50 [micro]g/L of arsenic would be approximately 10 percent of the rate that results from 500 [micro]g/L. But other scientists believe such a linear projection is inappropriate; they claim that there exists an exposure threshold below which no effect will be seen. According to their model, even if exposure to 500 [micro]g/L of arsenic produces a certain incidence of cancer, it is possible that a 50-[micro]g/L exposure would have no effect at all on cancer rates.
Linearity A half-century ago, Sir Richard Doll presented his multi-stage theory of cancer. Doll noted that most cancers caused by an external agent could not be distinguished from those that occur naturally and may be fundamentally indistinguishable. …