BioEd Online

A preservative commonly found in cosmetics such as shampoo and moisturizers harms developing nerve cells, according to a controversial study.

But claims that the compound may therefore pose a risk to unborn babies have provoked concern from other scientists, who are worried that such assertions may create unnecessary panic.

Methylisothiazolinone (MIT) is widely used in hand creams, shampoos and other cosmetics. It kills bacteria, making it easier to store the lotions for longer periods of time without colonies of microbes developing.

Safety tests have previously found that the chemical may cause slight skin irritation in susceptible people¹. But Elias Aizenman, a neurobiologist at the University of Pittsburgh, Pennsylvania, says that he could not find any information about the chemical's impact on developing nerve cells.

So his research team has been studying how the chemical affects neurons taken from the brains of rat embryos. Preliminary research published in 2002 found that relatively large doses of MIT killed most of the neurons within ten minutes². Many chemicals are toxic in high enough doses, however, so Aizenman's group went on to test much lower doses over longer periods of time. This mimics the occupational exposure of people who work with MIT every day, for example in factories that make cosmetic products.

Fetus exposure

The team exposed developing rat neurons to 1 micromolar concentrations of MIT (equivalent to 1 gram of the compound dissolved in more than 8,000 litres of water) over 18 hours. Growing neurons normally shoot out growths called axons to connect to different cells, but after the exposure to MIT, the amount that each axon grew was almost halved.

"I think the US Environmental Protection Agency should be concerned," says Aizenman. "I would advise a pregnant woman not to work in a factory using this compound, because it might result in some abnormality in the development of the neural system of the fetus."

Aizenman presented the study on 5 December at the annual meeting of the American Society for Cell Biology in Washington, DC.

In vitro results

Aizenman's interpretation has provoked anger and concern from some other toxicologists. "Extrapolating from that sort of tissue culture to human beings is scientifically dangerous," argues Tony Dayan, a retired toxicologist formerly of Queen Mary, University of London, who has also acted as a consultant to pharmaceutical companies.

A change in neuron growth is not particularly surprising, Dayan suggests, because the cells used in in vitro studies like the one carried out by Aizenman are so exposed and vulnerable. "These cells are in completely different circumstances from developing cells in the body," says Dayan.

"It's inherently difficult to go from in vitro studies to hazards in human beings, because we're rather more complicated," agrees Wilson Steele, a toxicologist at the University of East London, UK. But he advocates further tests on the compound.

Human hazard

Aizenman says he believes that the study does reveal a potential risk. Shampoos contain concentrations of MIT between 100 and 200 times greater than in his experiment, he points out. But he accepts that normal shampoo users or hairdressers, who receive much lower exposures than factory workers, may not be affected.

"I just can't say for sure," he says. "But there's very little information around about what these compounds are doing, and I'm concerned the appropriate testing hasn't been done."

Aizenman has already found that MIT seems to target a particular enzyme that is responsible for triggering axon growth. He is now trying to repeat the work in living animals, and to pin down exactly how MIT is affecting the neurons. But he says that epidemiological studies on people with autism and other neurological disorders are also needed, to discover if there is a link with fetal chemical exposure.