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A recent study looked at potential carcinogens in drinking water. Getty Images
  • Even community water systems that meet national drinking water standards may carry some health risks due to contaminants.
  • A study from the Environmental Working Group looked at lifetime risk of cancer from drinking water.
  • They estimate 100,000 cancer cases may be related to tap water.

When you turn on your tap, you trust that the water that comes out is safe to drink.

But as seen with recent cases of lead in public water systems in Flint, Michigan, and Newark, New Jersey, that trust can be easily broken.

But it’s not just community water systems with high levels of contaminants that put people’s health at risk, suggests a new study.

Even community water systems that meet national drinking water standards can carry health risks, especially when you add up the health effects of the individual contaminants in the water.

The Environmental Protection Agency (EPA) currently has drinking water regulations for more than 90 contaminants.

These rules set the maximum amount of each contaminant that’s allowed in public water systems, what’s known as the maximum contaminant level (MCL).

For contaminants that are difficult to monitor, the rules specify how the water should be treated.

The rules, which are set for individual contaminants, don’t take into account that people are often exposed to multiple contaminants at the same time.

To get a better sense of the true health risks, researchers from the nonprofit Environmental Working Group (EWG) conducted a “cumulative assessment” — basically looking at the combined health effects of contaminants in the water.

This is the same type of assessment the EPA uses to determine the health risks of air pollution.

The study was published Sept. 19 in the journal Heliyon.

The authors of the study focused on 22 cancer-causing contaminants found in varying levels in 48,363 community water systems in the United States.

Using the cumulative approach, they estimated these carcinogens could result in more than 100,000 cases of cancer over the lifetimes of the people using the water systems.

Most of these cancer cases would be due to arsenic and the byproducts of chemicals used to disinfect the water.

So, even though the “vast majority” of the water systems met national drinking water standards for these carcinogens, they still carried some risk.

These water systems supply water to around 279 million people. The analysis didn’t include the 13.5 million households that obtain drinking water from private wells.

Sydney Evans, lead author of the study and a science analyst at the EWG, says the goal of the study was “to see what the overall impact [of these contaminants] might be as people are experiencing these real-life exposures.”

In their analysis, the authors used benchmark contaminant concentrations that are established by health agencies like the EPA and the California Office of Environmental Health Hazard Assessment.

These benchmarks show what level of contaminant in the water would be expected to cause one case of cancer over the lifetimes of a million people drinking the water. This takes into account how carcinogenic the contaminants are.

They combined these benchmarks with real-world data on the amounts of contaminants found in the water systems to estimate the overall health risks.

Kelly Reynolds, PhD, a researcher and public health educator in environmental science at the University of Arizona in Tucson, says this type of cumulative assessment provides a better picture of what happens in the real world.

“Exposure to one [contaminant] might be harmless by itself, or below an acceptable risk level,” said Reynolds, who wasn’t involved in the study. “But in combination with something else, it might put you into a level of unacceptable risk.”

Evans points out that although this is a good first step toward using this kind of cumulative assessment for drinking water, “our approach may underestimate some of the risks.”

The analysis assumes that contaminants act independently to affect human health. In reality, some contaminants may interact with each other to become more dangerous — even across contaminant categories.

For example, says Reynolds, some research has found that “exposure to arsenic makes a population more vulnerable to microbial infections.”

Evans says their analysis can inform how the government regulates water contaminants in drinking water. But it can also help educate people about their exposure risks.

“This research is not just for regulators and policymakers,” Evans said. “We want individuals to know that the legal [contaminant] levels aren’t necessarily safe.”

Gerald J. Kauffman, PhD, project director of the Water Resources Center at the University of Delaware in Newark, said, “This is another step forward in trying to understand how toxic some of these substances are.”

He also thinks even more needs to be done to protect our drinking water, especially since the EPA has water contaminant regulations for only a small percentage of the tens of thousands of chemicals in society.

“When in doubt [about the health risks], if there is a substance that’s in our drinking water supply, it ought to be monitored for,” said Kauffman, who wasn’t involved in the study. “And if the science is there, there should be an MCL established for it.”

He says a good example of unregulated chemicals that deserve some caution are per- and polyfluoroalkyl chemicals, which are used in manufacturing and show up in drinking water.

The EPA issued a health advisory earlier this year that set a lifetime exposure limit for these chemicals. But the agency hasn’t yet passed an enforceable drinking water standard that covers them.

Reynolds points out that other countries take a precautionary approach to regulating water contaminants, rather than the United States’ “innocent until proven guilty” stance.

This allows those other countries to protect public health, even as scientists carry out research on the health risks of the contaminants.

“I think it’s important to look at how other countries are protecting populations before they fully know whether there’s an absolute risk,” Reynolds said.