Arsenic Risks

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In 1999, the National Academy of Sciences published a study of distinguished experts in the field that concluded that 50 ppb (parts per billion) of arsenic in drinking water was equivalent to a one percent risk of cancer. This study was the basis of the Clinton Administration's decision to reduce the drinking water standard to 10 ppb. That level was chosen as a compromise to keep costs to community drinking water systems down. It still represented a 0.2 percent chance of getting cancer, twenty times higher than EPA's usual "acceptable risk". The Bush administration is still reviewing this and other studies, and has stated that the new standard may be even lower than 10 ppb, and will not be higher than 10 ppb. A few weeks ago, the National Academy of Sciences published a report updating the 1999 report, and it reported that 3 ppb of arsenic in drinking water corresponded to a one-in-a-thousand risk of bladder and lung cancer combined. This is about three times as bad as the previous estimate of arsenic carcinogenicity.

EPA routinely defines acceptable cancer risk as a one-in-10,000 risk. What they mean by this is that the lifetime exposure to a single carcinogen should not cause more than one in 10,000 people to have cancer. We eat, drink and breathe carcinogens every day, and are exposed to millions of carcinogens over our lifetimes. The average risk of cancer is about 40 percent.

Some people get more than one kind of cancer during their lifetimes, but about a third of all Americans get cancer at least once during their lives, usually late in life because cancers have long latency periods: there are many years between exposure and actually getting cancer. For the lung and bladder cancers associated with arsenic, the average age at diagnosis is about 70 years.

Most times, EPA uses data from laboratory experiments with rats and mice to estimate cancer risk. They extrapolate lab data to human exposure, based on the dose per weight. EPA adds safety factors to these estimates just in case a particular carcinogen is more potent in humans than in rodents. These safety factors range from a ten-fold to a thousand fold. In the case of arsenic, however, lab studies don't work. Only humans (and possibly other primates) actually get cancer from arsenic. So scientists turn to epidemiological data where groundwater is naturally contaminated by arsenic. This is the case in Taiwan, Bangladesh, Argentina and Chile. There are also places in the US where arsenic is relatively high, but not nearly as high as in these overseas locations. Because estimates for arsenic carcinogenicity are based on studies with humans, no safety factors are built into the risk assessments for arsenic.

Most risk assessments assume that people are exposed to the same conditions over their entire lives, usually estimated to be 70 years. This is what was done in the National Research Council's study mentioned above. The authors assumed that people drank one liter of water every day for 70 years. They also assumed that people weigh 70 kg (154 lbs) on average. These figures can be used in the Washington State Model Toxics Control Act (MTCA) standard risk assessment model, which is used to calculate cleanup levels, based on childhood exposure through accidentally consuming soil. Studies have shown that when consumed in small amounts, essentially all the arsenic from the soils that are eaten is absorbed through the gut. In contrast, except in industrial situations, almost no arsenic will enter the body through the skin, and very little that is breathed in will be absorbed. Eating and drinking are the only significant ways to get arsenic into the body.

Using the MTCA risk assessment model and the latest arsenic potency figures from the National Academy of Sciences, the is one-in-10,000 risk level for soil arsenic is 4 parts per million (ppm). This level is about the level of naturally-occurring background arsenic on the island. At 32 ppm, the average surface soil concentration at child use areas on the island, the risk is 0.07 percent. At 100 ppm (the Department of Ecology's suggested interim action level) the risk is 0.23 percent.

How much dirt do we actually eat over the course of a day? EPA suggests that we eat 100 to 200 milligrams per day. Probably adults consume less-on the order of 50 to 100 milligrams per day. Children, with their hand-to-mouth and toy-to-mouth behavior probably eat more than adults. In fact between 5and 20 percent of children actually eat soil in a wholesale manner, by the mouthful. This is called pica behavior, and may lead to about 10 grams of soil consumption per day, and as much as 50 grams in a single day.

If a child has pica behavior, he or she accumulates about a one per cent risk of cancer.

Overall, exposure to soil arsenic on Vashon-Maury Island (with its population of 10,000) leads to about one case of bladder or lung cancer every six years. For an individual living on Vashon Island for the full 70 years it takes to develop one of these cancers, the cumulative risk is 0.09 percent unless he or she had pica behavior as a child-when the risk is one percent.

These exposures and risks should be seen against the background of arsenic we live with. The extensive sampling of soils on Vashon-Maury Island shows that the background at depth in soils and on beaches is about two to three parts per million. That means that about 6 % of the soil related risks are naturally occurring. The rest come from the Tacoma Smelter Plume, which stretches over hundreds of square miles of Western Washington-all of which area is potentially exposed to similar levels of risk from contaminated soils. The risk of cancer is higher for those who are living closer to the former smelter, because the concentration of arsenic in the soils is higher.

The risks of cancer posed by exposure to soil arsenic are significant, but are lower than the overall lifetime risk of cancer, which is 6.8 percent for lung and 2.3 percent for bladder cancer. The 0.07 to 1 percent risks posed by soil arsenic will be difficult to see against these background risks.

This analysis has not evaluated the potential for arsenic in drinking water exposure. We do know that arsenic is quite high in the groundwater in much of the Western United States, including Washington State. Besides lung and bladder cancer, arsenic is associated with prostate cancer and liver cancer. It appears to directly affect the immune system, with unknown consequences. Arsenic is associated with cardiovascular disease and diabetes. These risks are not included in these calculations because there is not enough known to provide numerical estimates of risk. Cadmium is associated with lung, prostate and other cancers as well, and cadmium is one of the heavy metals found at relatively high levels in the Tacoma Smelter Plume. Cadmium also affects the reproductive systems of both men and women.

This and all other risk assessments contain some uncertainty. For example, we never really know how much soil someone is consuming. I have used the average arsenic concentration on Vashon Maury Island as a starting point, but there is a great deal of variability between different areas on the island, and that difference will affect the total risks. Furthermore, as we learn more and more about the toxicity of arsenic and other heavy metals, we change our estimates of how dangerous they are. For example, as noted above, the National Research Council in its 2001 publication estimated cancer risks of arsenic as being about three times as high as they did only two years ago. Body weight probably has an effect on toxicity, and so does the different genetic makeup of individuals. We can be certain that these issues will be revisited again and again.

You can get more information by checking out our website http://www.iere.org/vashon-metals.html, or call us at 206-463-7430. Government information is available at King county's site http://www.metrokc.gov/health/hazard/tsp/ and the Department of Ecology's site http://www.ecy.wa.gov/news/2001news/2001-042.html