Contaminated drinking water—most of which currently meets legal quality standards—could cause an estimated 100,000 cancer cases in the U.S., according to a new report.

The study—which was published today in the journal Heliyon and looked at 22 cancer-causing compounds found in 48,363 U.S. municipal water systems— is the first to estimate what the cumulative cancer risk is from the mixture of chemical contaminants in tap water (the study did not include the roughly 13.5 million people on private wells).

"We're seeing cancer risk estimated at about 100,000 cases for the U.S.– due to drinking water contaminants at levels that currently meet requirements," lead author, Sydney Evans, a science analyst at the Environmental Working Group, told EHN.

The "vast majority" of water systems studied were in compliance with U.S. drinking water standards.

"Water at the legal limit may still hold health risks," Evans said.

She said, rather than look at one chemical contaminant at a time, they looked at all 22 that could occur simultaneously on public water systems.

"Unlike air [pollutants], water is still assessed one by one," Evans said. "That's not how real-life exposures work."

The 100,000 cancer cases estimate is the calculated risk of people drinking water over a lifetime (considered about 70 years). "For approximately 279 million people served by community water systems, or 86 percent of the U.S. population, this number of cases represents an overall cumulative lifetime risk of approximately … 4 lifetime cancer cases per 10,000," the authors wrote.

Most of the cancer risk is due to arsenic and disinfection byproducts, senior author Olga Naidenko, vice president of science investigations for EWG, told EHN. "Drinking water disinfection is essential and saves lives," Naidenko and colleagues wrote, but the disinfectants used to kill bacteria can turn into carcinogenic byproducts when they react with organic matter.

Other major contaminants included radioactive elements such as uranium and radium.

Cancer risk was higher, in general, in Western states that have greater water scarcity, including Nevada, New Mexico and Arizona. Arsenic, a naturally occurring compound, is found more frequently in the Western U.S.

The researchers said, if anything, their estimate is conservative due to insufficient monitoring and carcinogenic compounds not currently tested.

"Numerous other contaminants, such as nitrosamines, unregulated disinfection byproducts, per- and polyfluoroalkyl substances (PFAS) and a variety of industrial and agricultural chemicals, are not monitored as frequently or are not monitored at all, precluding their inclusion in our study," they wrote.

Both Evans and Naidenko said they hope the study provides a framework for communities to better protect people from harmful water by looking at a suite of contaminants — and potential toxic mixtures—rather than individual pollutants.

"We want to open the discussion, where we should we invest infrastructure dollars?" Naidenko said. "How much cancer risk are we as a society willing to accept?"

See the full study here.