A new study by the Environmental Working Group found that tap water in the United States presents a cancer risk comparable to that of air pollution (iStock/Imgorthand)

Cigarettes, obesity, alcohol, the sun – it seems like everything in the world causes cancer. Now you can add one more to the list: tap water.

A study by the Environmental Working Group published last week found that over the course of 70 years, approximately 100,000 people will get cancer from drinking tap water, a statistical risk similar to that of breathing in air pollution.

The research paper analyzed the levels of 22 carcinogens in 48,363 community water systems across the United States. Most of those toxic substances, like arsenic, are regulated – that is, water agencies must keep them below certain thresholds. And the vast majority of water systems do meet those legal standards. But according to the study, those chemicals still present a risk of cancer, and are likely to cause cancer at a rate of four cases per 10,000 people over 70 years – a much higher rate than what is considered to be the “acceptable risk” of one case per million people.

“I think a lot of people take their tap water for granted,” said Sydney Evans, lead author of the study. “They assume it’s tested, monitored and treated. These are risks in tap water now that are perfectly legal. The truth is, arsenic is in our drinking water and it is contributing to cancer risk across the United States.”

Arsenic, a naturally occurring chemical that can seep into groundwater, is by far the biggest cancer risk, but by no means the only. Others include the byproducts of using chemicals like chlorine to disinfect water in the first place, and radioactive contaminants like Chromium-6, which is both naturally occurring and a byproduct of some industrial production.

“There is evidence that some of these contaminants, when present in mixtures, are having a toxic effect that’s more than the sum of its components,” Evans said. “We believe our analysis to be a very conservative estimate.”

The cancer risk from tap water is highest, according to the study, in the southwestern United States, which sees less rainfall than the rest of the country.

“The less it rains, the more community water systems have to depend on water that might have higher levels of both naturally occurring and human-made pollutants,” said Evans of the Environmental Working Group, a Washington, D.C.-based research and advocacy group known for tracking levels of pesticides in food.

Arizona, the study found, has a higher cancer risk from tap water than any state – three times the national average.

Erin Jordan, a spokeswoman for Arizona’s Department of Water Quality, wouldn’t comment on the substance of the report but said, “We have to look at what the federal standard is and work with it.” In a follow-up email, Jordan explained that only 15 of the state’s 1,527 public drinking water systems exceed the EPA’s standards for arsenic. Most are in rural areas and affect about 6,700 customers out of the state’s 6.5 million, she said, noting that the department is working to help these smaller systems meet federal requirements.

A spokeswoman for the Environmental Protection Agency said, in a statement, that the agency sets legal limits on over 90 contaminants found in drinking water, and updates them every six years. “The Agency,” read the statement, “has sought to focus on the contaminants with the greatest public health risk.” The statement did not address the Environmental Working Group study, and did not mention the word cancer.

Mary Grant, a campaign director for the non-profit advocacy group Food and Water Watch said she was impressed by the study.

“I think the scale of the threat is a little bit surprising,” she said. “We really do need the federal government to step up and address our water infrastructure crisis.”

The Trump administration appears to be taking the opposite approach. Two weeks ago, it repealed Obama-era clean water regulations, which limited what kinds of chemicals could be dumped into rivers and streams. Melanie Benesh, an attorney for the Environmental Working Group, said the rollback is likely to increase the level of carcinogens in some bodies of water in the United States.

Ronnie Levin, a former senior scientist at the EPA who now teaches at the Harvard School of Public Health, said that reducing carcinogens in tap water is a solvable problem.

“It’s not sending someone to the moon,” she said. “We know what it is, we know how to deal with it.”

It could, however, be costly. For one thing, different toxic chemicals require different solutions. Arsenic comes out of the ground and into aquifers, while other pollutants trickle down into the surface water. Levin said that as with air pollution and other environmental health risks, it’s up to the public to decide how much it cares, and how much of a cancer risk it can live with.

“The problem with drinking water in the United States is that we have an assumption that it’s perfect,” said Levin. “Unlike air pollution in cities, where everyone knows it isn’t pure – in drinking water, we never have those discussions.”

A 2016 poll by the Associated Press, taken in the wake of the lead contamination crisis in Flint, Michigan, found that only about half of Americans were highly confident in the safety of their tap water, and only about a third said they usually drink straight from the tap.

Though many people prefer it, bottled water is actually less regulated than tap water. An investigation by Consumer Reports, published in June, found that some brands of bottled water had levels of arsenic above the federal standard for drinking water.

The Environmental Working Group maintains a publicly available database of all community water systems, showing what contaminants are present in them. It also has a guide for buying water filters for use in the home. Said Evans: “It’s important for an individual to know what’s in their water.”