• Two of the most prevalent and concerning chemicals, arsenic and hexavalent chromium, were found in drinking water in all 50 states.
  • Both chemicals are known carcinogens commonly found in California taps.
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In 2017, the nonprofit Environmental Working Group (EWG) released a database of all the known contaminants lurking in US drinking water. After compiling data from 50,000 public water utilities across the country from 2010 to 2015, the group found 267 chemicals that they dubbed concerning to human health.

On Wednesday, the group announced an update to those findings: After analyzing the same data set from 2012 to 2017, the EWG found 278 contaminants in US drinking water.

The health risk of each contaminant is "going to vary region by region, state by state, utility by utility," Tasha Stoiber, a senior scientist at EWG, told Business Insider.

To enable people to learn about the contaminants in their drinking water, the EWG database allows anyone to look up their ZIP code, find a list of contaminants in the local water supply, and determine what type of filter could help weed out these chemicals.

Two of the most prevalent and concerning chemicals found, Stoiber said, were arsenic and hexavalent chromium. Both chemicals were detected in all 50 US states, though they appear especially often in California groundwater. They're also known to cause cancer.

Arsenic was found in the taps of 107 million Americans

From 2015 to 2017, the EWG detected arsenic in the tap water of 107 million Americans. In California alone, it lurked in the taps of more than 26 million people. In Texas, more than 17 million people had arsenic in their tap water. In Florida, more than 11 million people were exposed.

Arsenic tap water
Arsenic tap water
Ken Lubas/Los Angeles Times/Getty Images

Since arsenic occurs naturally in the Earth's crust, it tends to pollute groundwater as opposed to surface water. In California's drought-prone Central Valley, excessive groundwater pumping has led toxic levels of arsenic to leach into local water supplies. A 2018 study tested wells in the San Joaquin Valley the Central Valley's main agricultural hotspot and found that around 10% had dangerous levels of arsenic over the previous 10 years.

Read more: 11 cities with the worst tap water in the US

Arsenic has no smell, color, or taste , so it's nearly impossible to identify without a test. Exposure to the compound has been linked to brain-development issues in children, including slower motor function, diminished memory, and decreased IQs. It can also lead to birth defects and an increased risk of cancer, heart disease, and diabetes.

Stoiber said arsenic was "definitely a top driver of the cancer risk" from drinking tap water in California and nationwide.

A previous EWG study estimated that 45,000 people who drink arsenic-contaminated water throughout their lifetimes could develop cancer as a result.

Hexavalent chromium is often associated with one California town, but it's in all 50 states

Like arsenic, hexavalent chromium is a naturally occurring element found in rocks, plants, and soil. It was the subject of the lawsuit filed by water advocate Erin Brockovich, who discovered hexavalent chromium in the groundwater in the town of Hinkley in southern California. The chemical was being used to prevent corrosion at the nearby Pacific Gas & Electric plant.

In tests conducted from 2012 to 2017, the EWG detected hexavalent chromium in the tap water of more than 247 million Americans, across every state. The chemical is easier to spot than arsenic, since it can turn clear water yellow . It's known to cause kidney and liver damage, as well as various forms of cancer , such as lung, nasal, and sinus cancer.

hinkley california
hinkley california
David McNew/Getty Images

Stoiber said there's a misconception that hexavalent chromium is solely an industrial pollutant. If a system relies on groundwater, she said, there's a good chance that the chemical could be present. A 2018 study found that industrial uses of chromium lead to more severe cases of groundwater contamination in California than naturally occurring chromium does, but the natural kind affects more people, more wells, and more communities overall.

California had by far the highest concentration of hexavalent chromium in the US, with more than 38 million people in the state exposed to the chemical through their taps, according to the EWG. The chemical was also found in the taps of more than 19 million people in New York and nearly 20 million people in Texas.

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SEE ALSO: Contaminated tap water could lead to 100,000 cases of cancer in the US if people drink it their whole lives. Here's how worried you should be.