Hand-washing has solved many of the world’s sanitation problems, but there may be a dark side to the common hygiene practice. An agent in some antibacterial hand soaps has been linked to tumor growth in mice, according to a study published Monday in the journal Proceedings of the National Academy of Sciences.

Researchers studying the effects of the chemical triclosan found that mice exposed to the ingredient had more and bigger liver tumors than mice that were not in contact with it. While researchers said the triclosan was “not a direct carcinogen” -- in other words, it does not mutate DNA -- study author Robert Tukey, a professor of chemistry, biochemistry and pharmacology at the University of California, San Diego, called it a “tumor promoter” that causes tumors to form once a mutation has occurred, according to ABC News.

Study authors exposed the mice to high concentrations of triclosan for six months -- much higher amounts than the average human is exposed to. "The result that it led to liver fibrosis was startling to us," Tukey told the Atlantic. However, research showing effects in animals doesn’t always predict responses in humans, the Food and Drug Administration noted.

The study adds to a growing body of evidence that triclosan is potentially harmful. The FDA is considering banning the chemical following previous research that has shown a correlation between it and negative health effects. Several other countries regulate triclosan, which is an ingredient in many household products besides hand soap, including toothpaste and body washes.

Companies that use the chemical have until next winter to prove that products containing triclosan “[provide] an extra benefit to health.” However, some scientists are convinced the agent is unnecessary in the first place. "To me it looks like the risks outweigh any benefit associated with these products right now," Allison Aiello, a professor at the University of Michigan's School of Public Health, told CBS last year. "At this point, it's just looking like a superfluous chemical."

Triclosan is so ubiquitous in modern products that it appears in the urine of 75 percent of people and in the breast milk of 97 percent of women who are lactating, researchers noted.

“We could reduce most human and environmental exposures by eliminating uses of triclosan that are high volume, but of low benefit, such as inclusion in liquid hand soaps,” Bruce D. Hammock, a professor at University of California, Davis, said in a statement. “Yet we could also for now retain uses shown to have health value -- as in toothpaste, where the amount used is small.”