Carcinogens in drinking water could be the cause of more than 100,000 cases of cancer in the United States, according to a recent study.

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Is tap water safe? New research investigates.

Researchers from the Environmental Working Group (EWG) in Washington, DC, used a new approach to analyze cumulative cancer risk due to cancer-causing chemicals in tap water across the U.S.

They report the findings in a recent Helyion paper.

They note that the study is the first to apply a “cumulative cancer risk framework” to the analysis of tap water contaminants for the whole of the U.S.

The analysis drew on water quality data from 48,363 community water systems across the country.

The dataset did not include private wells, which supply drinking water to around 14% of the U.S. population, or about 13.5 million households.

The analysis revealed that the most significant impact on cancer risk came from arsenic, followed by byproducts of disinfection.

“Drinking water contains complex mixtures of contaminants, yet government agencies currently assess the health hazards of tap water pollutants one by one,” says first and corresponding study author Sydney Evans, an EWG science analyst.

“In the real world,” she adds, “people are exposed to combinations of chemicals, so it is important that we start to assess health impacts by looking at the combined effects of multiple pollutants.”

In their study paper, the authors explain that since the 1990s, scientists and health organizations in the U.S. and elsewhere have been urging that risk assessments should focus on cumulative impact rather than just the effect of one contaminant at a time.

Indeed, in the U.S., it is already standard practice to use a cumulative approach to assess the risk of developing both “cancer and other serious health effects” from exposure to toxic air pollutants.

The new study builds on a model that the state of California used to assess the cumulative cancer risk of drinking water contaminants.

The team obtained cancer risk benchmarks for each of the 22 contaminants that they analyzed from the U.S. Environmental Protection Agency and the California Office of Environmental Health Hazard Assessment (California OEHHA).

These benchmarks give the level of a contaminant that corresponds to the risk that 1 person per million of the population exposed to the contaminant could develop cancer during their lifetime. Government agencies use 70 years as the approximate statistical lifespan in the U.S.

From the average results of water quality tests that each community had conducted on their water systems during 2010-2017, the researchers could see which had exceeded the benchmark threshold for each of the contaminants.

Therefore, for each community in which the contaminant had exceeded the threshold, the estimated number of cases would be the lifetime cancer risk multiplied by the size of the population.

At a national level, the estimated number of lifetime cancer cases due to a given contaminant would then be the sum of estimated cases in communities that exceeded the threshold.

In the case of arsenic, for example, the California OEHHA defines the level of contaminant that corresponds to a lifetime cancer risk of 1 case in 1 million as 0.004 micrograms per liter (mcg/l).

From the water quality and population data for each community, the researchers calculated that 141 million people across the U.S. live in areas in which levels of arsenic are above this threshold.

The calculation yields a national figure of 45,300 estimated number of lifetime cancer cases due to arsenic in drinking water.

The cumulative lifetime cancer risk is, therefore, the sum of the estimated number of lifetime cancer cases due to each contaminant.

“Overall,” conclude the authors, “tap water exposure to the carcinogenic contaminants analyzed in this study corresponds to 105,887 estimated lifetime cancer cases.”

They point out that this magnitude of cumulative cancer risk is on a par with that of cancer-causing air pollutants.

The highest risk came from water systems that rely on groundwater and supply mostly smaller communities.

However, larger surface water systems also account for a sizeable share of the overall risk, note the researchers. This is because they have a persistent presence of disinfection byproducts in them and serve more people.

“We need to prioritize source water protection,” says senior study author Olga Naidenko, Ph.D., vice president for science investigations at EWG, “to make sure that these contaminants don’t get into the drinking water supplies to begin with.”

Jim Smith, professor of environmental science at the University of Portsmouth in the United Kingdom, was not involved in the study.

He acknowledges that while the study tries to “model cancer risks from drinking water,” it fails to explain them in their proper context. For example, it does not make any comparisons with other environmental risk factors.

“The comparison,” he notes, “of carcinogenic risk from drinking water and those from air pollution fails to mention that carcinogenic risk from organic contaminants is only a very small factor in total air pollution risk.”

Such a comparison could mislead people who are not familiar with these types of studies into believing that the health risks of drinking tap water are similar to those of exposure to air pollutants.

Without appropriate context, scientific results can lead to an “understandable public overreaction to risks and to wrong policy decisions,” he concludes.