More than half of Americans have a radioactive element in their drinking water, according to a non-profit environmental group's investigation released Thursday. The report analyzed six radioactive contaminants, including radium, radon and uranium, but focuses on the most prevalent element, radium. Radium is a naturally occurring element that is known to be carcinogenic.

The report, released by the Environmental Working Group (EWG), gathered data from nearly 50,000 public water systems. The data revealed that between 2010 and 2015, more than 22,000 utilities in all 50 states had radium in their treated water, totaling 173 million people. A small percentage of Americans had radium that exceeded the Environmental Protection Agency's legal limit for radium.

Tap water filling a glass in France. In the U.S., more than half of Americans could be drinking water with dangerous levels of carcinogenic radioactive elements. Anne-Christine Pouloulat/AFP/Getty Images

But as the group points out, those federal standards were set in 1976. And around 38 percent of Americans had radium concentrations that exceeded another standard: California state scientists' public health goals from 2006. That goal would be hundreds of times more stringent than the EPA's standards, per the non-profit's investigation.

Water drips from a faucet at the Dublin San Ramon Services District residential recycled water fill station on April 8, 2015, in Pleasanton, California. Justin Sullivan/Getty Images

"What the data show are that many consumers are receiving finished drinking water (tap water) that has radium above the levels set by the federal government," Scott Faber, vice president of government affairs at EWG, told Newsweek. "And many more Americans are receiving finished tap water with radiological elements above the level scientists have deemed safe."

"There's a difference between safe and legal, which is an important distinction," he added.

The report found that 158 public water systems that serve 276,000 Americans in 27 states had radium above the federal legal limit, according to Alexis Temkin, a toxicologist at the non-profit. Around 38 percent of Americans, or 122 million people, had radium concentrations above California's public health goals. Water serving 173 million Americans had traces of radium in their tap water, which was the most prevalent radioactive element the investigation found. California and Texas fared the worst, where 64 and 80 percent of residents had radium in their drinking water, respectively.

"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," Olga Naidenko, the senior science adviser for children's environmental health at EWG, said in a statement.

Exposure to radioactive elements can increase the risk of cancer, and the group says that the EPA has classified all ionizing radiation as carcinogenic. California's public health goals are defined as decreasing the risk of cancer to one in every one million people who drink water in their lifetime, the group says. The lifetime cancer risk at the EPA's current levels for radium are 70 cases per one million people.

Though the Environmental Working Group's investigation is not a peer-reviewed study, "it highlights the importance of really conducting an appropriate assessment of environmental chemicals in drinking water," Dr. Ana Navas-Acien, environmental health sciences professor at Columbia University, told Newsweek.

A City of Flint Sewer Department marker flag waves in the wind on a block where lead water lines have started to be replaced on March 17, 2016, in Flint, Michigan.Brett Carlsen/Getty Images

Navas-Acien also noted that this report focuses on public drinking water, but millions are drinking water from private sources. "We don't really have a good system in place to protect people drinking water from private wells," she said. "This is a serious issue in the United States, as there are millions of people in the country drinking water from private wells."

The permitted level of radium in tap water, however, still remains outdated, according to the group. Navas-Acien echoed that sentiment. "It's surprising to see that the standard for radium has not been updated since 1976," she said. "We have a much larger body of scientific evidence that has been developed since then that should have been able to inform new updates [for the] maximum contaminant level for radium."