In the early 1970s, the chemical triclosan began to appear in a multitude of products. From soaps and toothpastes to cleaning supplies and pesticides, triclosan was reportedly an effective antibacterial and antifungal compound. Pretty much immediately after it started appearing in household products, the FDA commenced investigating the chemical's safety profile, but it wasn't until 2016 that it established a final, clear regulation.

This 2016 FDA regulation effectively banned triclosan, and 18 other compounds, from consumer antiseptic wash products such as hand soap. The strangely limited regulation still allowed the chemical to be used in hand wipes or sanitizers and toothpastes with the only substantial comment at the time coming from Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research, saying "Consumers may think antibacterial washes are more effective at preventing the spread of germs, but we have no scientific evidence that they are any better than plain soap and water."

Over the years, a growing body of evidence has come to suggest that triclosan is not only harmful to humans, but also to the broader environment. Initial studies pointed to concerns over the chemical acting as an endocrine disrupter in humans and also increasing the prominence of antibiotic-resistant bacteria.

The chemical's direct effect on human health has been controversially debated. A review by the government of Canada, for example, recently concluded that when used in low concentrations triclosan is not harmful to human health. This includes current levels found in cosmetics, shampoos and toothpaste. The Canadian review does note, however, that the chemical can pose a hazard to the environment, and it has been seen to be highly toxic and disruptive when found in river ecosystems.

It has been noted as one of the top 10 pollutants found in rivers in the United States. A more concerning result from a National Health and Nutrition Examination Survey found traces of triclosan in 75 percent of urine samples from participants in the United States.

This new study, led by a team at the University of Massachusetts Amherst, set out to explore if triclosan had any relationship with gut inflammation, an increasingly diagnosed problem. The study fed mice amounts of the chemical relative to concentrations that have been identified in human blood samples. After three weeks of administration in healthy mice, a low-grade of colonic inflammation was identified.

The study then administered the same regime to mice that were genetically engineered to develop inflammatory bowel disease. After three weeks, the development of symptoms relating to irritable bowel syndrome had significantly accelerated, as had the development of colitis-associated colon cancer cell growth. One group of mice in particular displayed significantly reduced lifespan.

The researchers hypothesized that triclosan could be affecting the gut microbiome and this could be the fundamental mechanism resulting in inflammatory effects. To home in on this mechanism the study subsequently found that when fed to germ-free mice (those with no gut microbiome), the harmful effects of triclosan disappeared. It was also found that triclosan did indeed lower the diversity of an animal's gut flora.

Of course, it is worth noting that this study is only demonstrated in a small mouse cohort and so far has not been validated in humans. But the researchers do suggest the results indicate that the effects of this chemical on human health needs urgent close inspection.

"Little is known about the impact of this chemical on human health or other species," writes Haixia Yang, one of the researchers on the project. "Our study indicates there is an urgent need to further evaluate the impact of triclosan exposure and update the potential regulatory polices."

While regulators may be dragging their heels on working out whether this chemical is harmful or not, a statement released last year and co-signed by over 200 scientists and medical professionals suggests we should probably just avoid it altogether. Called the Florida Statement, this report found that, despite the lack of definitive evidence of its harmful effects, there is no conclusive evidence of its efficacy as an anti-microbial agent.

"People think antimicrobial hand soaps offer better protection against illness," says Barbara Sattler, an environmental health professor at the University of San Francisco. "But generally, antimicrobial soaps perform no better than plain soap and water."

On top of that, an earlier study from the University of Massachusetts Amherst team found that the chemical can build up to concerning levels of toxicity in the bristles of a toothbrush. And worse, when a non-triclosan-containing toothpaste was applied to the same brush, the accumulated triclosan could be released in potentially harmful amounts.

All this ultimately leads us to a scenario where we have a chemical compound in a variety of household products that is at best relatively ineffective, and at worst cancer-causing and environmentally toxic. Further work certainly needs to be done to definitively establish an inflammatory connection in humans, but at this point triclosan can be easily avoided by simply paying attention to the ingredient list of products bought.

The new study was published in the journal Science Translational Medicine.

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