Manufacturers began mixing triclosan into soap, toothpaste and other products in the 1990s; some are now phasing it out. (BIGSTOCK)

Though you may not realize it, you have probably been in contact with the controversial chemical triclosan. Triclosan can be found in soaps and mouthwashes. It’s used to coat earplugs and sporting equipment. It’s mixed into paint.

Some experts believe that triclosan is harmful to humans and animals, and that it promotes the development of antibiotic-resistant bacteria. Last month, Minnesota banned triclosan from consumer hand and body sanitizers, a restriction that will take effect in 2017. It was a bold move, because the federal Food and Drug Administration is currently reviewing the safety and efficacy of triclosan. By acting now, Minnesota lawmakers effectively preempted the FDA. Manufacturers are also trying to get in front of public sentiment: Both Johnson & Johnson and Procter & Gamble are phasing triclosan out of their products.

People who use antimicrobial soap may be worried that their hygienic standards have put them at risk of hormonal malfunction and drug-resistant superbugs. A tour through the scientific literature on triclosan can offer guidance on how much — or whether — you should worry about this chemical.

How does triclosan work?

Triclosan was developed in the 1960s by the Swiss chemical manufacturer Ciba (now part of BASF). At high concentrations, such as those used in hospitals to sterilize surgical instruments, triclosan can permeate the cell wall of a bacterium and kill it.

At lower concentrations, such as those found in hand soaps, the chemical behaves slightly differently. It binds to an enzyme that helps bacteria grow, preventing them from reproducing. Manufacturers began mixing small amounts of the chemical into soap, toothpaste and other consumer products in the 1990s, saying that it would inhibit bacterial growth on the skin and inside the mouth. (Not all antimicrobial products contain triclosan: Sanitizers that you leave on your hands rather than rinse off kill bacteria with alcohol.)

Is triclosan effective?

High-concentration triclosan works in the hospital setting. No one disputes that. There is, however, a lively debate over whether antimicrobial soaps containing a lower concentration of triclosan benefit the average consumer.

In a sense, all soaps are antibacterial. Washing your hands with ordinary soap kills many bacteria through a combination of the mechanical rubbing action and the alkalinity of the soap. For antimicrobial soaps to be considered effective, they must kill a higher proportion of bacteria than plain soap.

The most common way to study this question is to compare bacteria populations on the hands of people who use ordinary soap with those of people who use antimicrobial soap. It sounds straightforward, but differences in methodology have led to mixed results.

Some researchers send subjects home with soap, then count up their hand bacteria a few times over the course of weeks. These studies typically show little or no benefit to antimicrobials. Other researchers inoculate volunteers with bacteria in a laboratory, then see how much of that mass of bugs is transferred when the volunteers handle food after washing their hands. Triclosan performs well in these studies.

Neither approach is perfect. The long-term studies have the benefit of real-world conditions, but they can be affected by myriad extraneous factors, such as where the participants had been sticking their hands. The laboratory studies are better controlled but may not reflect how people actually wash their hands. In addition, many of the lab studies are funded by companies whose products contain triclosan.

Another approach is to investigate whether people who use antimicrobial soaps get sick less often than those who use regular soap. In these studies, participants report the same amount of congestion, coughing and diarrhea, no matter which soap they use. Although these results are bad news for triclosan, the method is very problematic. You can contract illness just about anywhere, even if your hands are squeaky-clean. In addition, many respiratory and gastrointestinal illness are caused by viruses, which antimicrobial soaps don’t pretend to control.

The FDA is agnostic on the efficacy issue, noting that it “does not have evidence that triclosan added to antibacterial soaps provides extra health benefits over soap and water.” Taken as a whole, the data suggest that antimicrobial hand soaps may be more effective than ordinary soap at killing bacteria but that your hand-washing technique is probably at least as important as your choice of soap.

Is triclosan harmful to humans?

Triclosan doesn’t stop at your skin. A 2008 study suggested that 75 percent of Americans have remnants of triclosan in their urine. The chemical has also appeared in human blood and breast milk samples. Having a synthetic antimicrobial circulating through your system sounds scary, but it’s not clear whether it’s actually dangerous.

Triclosan’s critics say the chemical is an endocrine disrupter. Exposing some fish and rats to high levels of triclosan seems to affect their organ development in laboratory studies, possibly because the chemical prevents hormones from properly regulating growth. This line of research has problems, though. There’s a famous saying among toxicologists that “the dose makes the poison,” and it applies here.

“These studies are plagued by the fact that we’re dosing these animals at levels 10,000 to 20,000 times higher than human exposure,” says Richard Sedlak of the American Cleaning Institute, which represents manufacturers of products containing triclosan. The group also points out that 1970s studies on animal behavior, development, reproduction and morphology, which form the basis of governmental approval of triclosan, found the chemical to be safe.

Sedlak says that the new rat studies challenging triclosan’s safety are inapplicable to humans and fail to tilt the balance of evidence against triclosan. He believes the safety data for triclosan is “totally conclusive.”

Nneka Leiba, deputy director of research for the Environmental Working Group, disagrees.

“No one is saying that these effects are definitely going to happen in humans, but the number of studies that show effects leads us to be concerned,” she says. “These studies show there is a need for more research.”

What about antibiotic resistance?

The proliferation of drug-resistant bacteria is an enormous public health issue. When people take antibiotics unnecessarily and don’t complete the course of medication, bacteria get a chance to evolve defenses against the drugs. Feeding antibiotics to livestock also contributes to the problem.

Can antimicrobial soaps likewise spawn resistant bacteria? Theoretically, yes. By exposing bacteria to low levels of triclosan in a laboratory, researchers have trained the bugs to tolerate the chemical. But there is very little evidence that soaps containing triclosan promote resistance to even closely related medications, such as the anti-tuberculosis drug isoniazid, in real-world settings.

More research on this question is needed, but the existing evidence strongly indicates that using antimicrobial hand soap doesn’t make you complicit in the global problem of drug-resistant bacteria.

What now?

Unless you have a compromised immune system or you are a food service worker hoping to keep bacteria on your hands from infecting other people, you probably have little to gain from using antimicrobial soaps. At the same time, the risks are pretty minor. (Triclosan may have negative impacts on aquatic life, but that discussion will have to wait for another article.) For now, give the scientists time to work, let the regulators duke this one out and wash your hands of the debate — with the soap of your choice.

Palmer writes the How and Why and EcoLOGIC columns for The Post and is the chief explainer for Slate.