Drinking water for more than 170 million Americans in all 50 states contains radioactive elements that may increase the risk of cancer, according to an Environmental Working Group (EWG) investigation released Thursday.

Radiation in tap water is a serious health threat, especially during pregnancy, and the U.S. Environmental Protection Agency's legal limits for the most widespread radioactive elements are more than 40 years old. But President Trump's nominee to be the White House environment czar rejects the need for water systems to comply even with those inadequate standards.

The most common radioactive element in American tap water is radium. EWG's analysis of test data from almost 50,000 public water systems found that from 2010 to 2015, more than 22,000 utilities in all 50 states reported radium in the treated water delivered to customers' taps. EWG's interactive map shows the utilities with radium contamination and how many people were affected.

Only a small percentage of those systems exceeded the EPA's legal limits for radium, set in 1976. But almost all exceeded California state scientists' public health goals for two separate radium isotopes, set in 2006, which are hundreds of times more stringent than the EPA's standard for the two isotopes combined. The elevated risk of cancer, as well as potential harm to fetal growth and brain development, decreases with lower doses of radiation but does not go away.

"Most radioactive elements in tap water come from natural sources, but that doesn't take away the need to protect people through stronger standards and better water treatment," said Olga Naidenko, Ph.D., EWG's senior science advisor for children's environmental health. "Millions of Americans are drinking water with potentially harmful levels of radioactive elements, but the outdated federal standards mean many people don't know about the risk they face when they turn on the tap."

California has the most residents affected by radiation in drinking water. From 2010 to 2015, about 64 percent of the state's residents were served by public water systems that reported detectable levels of the two radium isotopes. In Texas, which has a smaller population, about 80 percent of the population was served by utilities reporting detectable levels of those elements.

But while President Trump's nominee to head the White House Council on Environmental Quality, or CEQ, was Texas's top environmental regulator, the state regularly and deliberately lowered the levels of radiation in tap water it reported to the EPA.

The nominee, Kathleen Hartnett White, admitted in a 2011 investigation by Houston's KHOU-TV that if utility tests found radiation levels over the EPA limit, the Texas Commission on Environmental Quality would subtract the test's margin of error to make it appear the water met federal standards. In some cases, this meant that Texans whose tap water posed the extraordinarily elevated lifetime cancer risk of 1 in 400 were not informed of the danger.

Hartnett White told KHOU she did not trust the science behind the EPA's standard. When pressed by a reporter—"What if you're wrong and EPA's right?"—she said: "It would be regrettable."

Last month, after Hartnett White acknowledged to the Senate Environment and Public Works Committee that she knew the commission was cooking the books, her nomination was sent back to the White House. But on Monday the White House stubbornly renominated her.

"Putting someone in charge of CEQ who deliberately falsified data to get around federal regulations is outrageous, and the fact that her deception left people at serious risk of cancer is even more alarming," said Scott Faber, EWG's vice president of government affairs. "The Senate should reject this radioactive nominee."

"With the renomination of Kathleen Hartnett White to the White House Council of Environmental Quality, President Trump is failing these 170 million at-risk Americans who are drinking contaminated water," said Christy Goldfuss, vice president for energy and environmental policy at the Center for American Progress. "When Hartnett White was tasked with addressing radiation in Texas's drinking water as a government official, she regularly and deliberately underreported the amount of radiation in drinking water, keeping families in the dark about their health and safety. Putting her in charge of the Council of Environmental Quality could do untold damage to even more Americans. With [Thursday's] data revealed, it is even more vital that our leaders and representatives prioritize clean and safe drinking water for all."