Using some antibacterial soaps may promote tumor growth, according to a study just published in Proceedings of the National Academy of Sciences. The findings add to a body of concerns about triclosan, one of the most common antimicrobial chemicals in consumer products from detergents to cosmetics, including links to allergy development in children, and potentially to breast cancer via disruption of hormone signals that may also cause thyroid dysfunction and weight gain.

"Our interest in this was that triclosan is just so abundant," said lead researcher Robert Tukey, a professor at University of California San Diego School of Medicine. "It's really everywhere in the environment."

Today's study found that mice who were exposed to triclosan regularly for six months showed abnormal cell proliferation, liver fibrosis, and inflammatory responses—all of which, the researchers write, "resemble the environment within which human liver cancer forms." The researchers expect that the same triclosan-induced formation of liver tumors "would occur in humans as it occurs in mice."

Triclosan is regulated in many countries, but the U.S. isn't among them. In 1974 the Food and Drug Administration (FDA) proposed a ruling on the safety of triclosan; but, four years later, the agency said that was not possible due to insufficient evidence. In 2010, still with no FDA ruling, the National Resources Defense Council sued the FDA over the matter. Still today there's no ruling, but the FDA has said that it will commit to something by 2016. The chemical is in an estimated 75 percent of antimicrobial soaps and body washes, though some companies have begun voluntarily phasing it out due to health concerns. Products like Johnson's baby shampoo and Palmolive no longer contain triclosan.

Still a study in August from the University of California, San Francisco, found that about three-fourths of doctors and nurses had triclosan in their urine, and another study earlier this year found triclosan in the urine of 100 percent of pregnant women tested in Brooklyn. Because triclosan-infused products have been so widely used for many years, exposure to the chemical entirely is unavoidable. It is among the most common chemicals to be detected in streams.

"The result that it led to liver fibrosis was startling to us," Tukey said. The researchers also noted a similar effect in kidneys. Their findings suggest that triclosan does not cause liver tumors by itself, in that it does not mutate DNA. But it does promote tumor formation once a mutation has occurred. Liver cancer (hepatocellular carcinoma, specifically) is the world's number-three cause of cancer death.

“If non-triclosan-containing soaps are available, use the alternative," said Paul Blanc, a professor of medicine at UCSF, earlier this year in a press statement. "This is based on the precautionary principle–that is, if you don’t know for certain that something is unsafe, it’s better to err on the side of caution.”

Pair that with findings from the FDA last December that "there is currently no evidence that [antibacterial soaps] are any more effective at preventing illness than washing with plain soap and water," and the case against them is pretty clear.

So is the point that this sort of chemical should be tested for safety before it's introduced so widely?

"Well," Tukey said, "that would be a good scenario."

To know the effects in humans would require clinical trials, he noted, which have not happened. "We aren't saying that triclosan causes cancer," Tukey said. "We're just saying that with constant exposure, this environmental agent, which is extremely ubiquitous, can promote development of tumors." Because, you know, in the right amount, everything is toxic. "If one can avoid it," he said. "I would avoid it."

So once again, in attempt to become extremely healthy by ridding ourselves of bacteria, we may have actually become less so.