Washing your hands or hair are not at the top of the list for activities that may be harmful to health; they are very much the opposite. But a new mouse study suggests that long-term exposure to an antimicrobial commonly found in liquid hand soap, shampoo and other personal hygiene products - triclosan - could cause cancer and liver fibrosis.

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Researchers say long-term exposure to triclosan, an antimicrobial found in personal hygiene products, may be harmful to human health.

The research team - led by Prof. Robert H. Tukey of the University of California-San Diego and Prof. Bruce D. Hammock of the University of California-Davis - publish their findings in the Proceedings of the National Academy of Sciences.

Triclosan is added to an array of consumer products to slow or prevent the growth of bacteria, fungi or mildew. As well as shampoo and liquid hand soap, it can be found in toothpaste, deodorant and various cosmetics. Triclosan is also used as a material preservative in footwear, clothing, sealants and various household products.

Previous research has suggested that triclosan may be harmful to health. Some studies claim the agent promotes bacterial resistance, while other research suggests it alters hormone regulation in mice.

The Food and Drug Administration (FDA) state that there is not enough evidence to suggest triclosan is hazardous to humans, but that studies indicating a possible health risk warrant further review.

Now, the FDA have another study to consider, after Prof. Tukey, Prof. Hammock and their team found that triclosan proved toxic to mice through a mechanism that is also applicable to humans.

Triclosan may disrupt protein responsible for clearing foreign chemicals from the body

The team exposed a group of mice to triclosan for 6 months - the equivalent to 18 years in humans - and compared their health with that of a group of mice not exposed to the agent.

Results of the analysis revealed that mice exposed to triclosan had significantly poorer liver health and function than those not exposed to the agent. What is more, the exposed mice were more susceptible to tumor development in the liver, and their tumors were more frequent and larger than those of unexposed mice.

The researchers believe that triclosan produced these effects in mice by disrupting the function of the constitutive androstane receptor (CAR) - a protein that clears foreign chemicals from the body. This protein is also present in humans.

Disruption to CAR causes the proliferation of liver cells, which leads to fibrosis - the development of excess tissue. If this process is repeated - triggered by long-term triclosan exposure - it can lead to tumor development.

Commenting on their results, the researchers say:

"Although animal studies require higher chemical concentrations than predicted for human exposure, this study demonstrates that triclosan acts as a hepatocellular carcinoma (HCC) tumor promoter and that the mechanism of triclosan-induced mouse liver pathology may be relevant to humans."

The team points out that exposure to triclosan is very high in humans, noting that past research has identified traces of the agent in 97% of breast milk samples of lactating women and 75% of urine samples.

Prof. Hammock says that to be harmful to human health, however, individuals may need to be exposed to high volumes of triclosan over long periods. As such, he suggests that human exposure to the agent could be reduced by "eliminating uses of triclosan that are high volume, but of low benefit, such as inclusion in liquid hand soaps."

He notes, however, that triclosan has been shown to benefit health in some cases. For example, studies have shown that toothpastes containing small amounts of the agent are effective for preventing gingivitis. Prof. Hammock says it may be wise to retain use of the agent in this case, as the low exposure is unlikely to cause harm.

Last year, Medical News Today reported on a proposed rule from the FDA requiring manufacturers of antibacterial soaps and body washes to prove such products are safe for long-term use.