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By Alan Boyle

It's bad enough that prolonged periods of weightlessness in outer space can cause muscle loss, bone loss, a weakened immune system and impaired vision. But now researchers say it may cause skin and hair problems as well.

The concern is raised in a study published online Wednesday in the first issue of npj Microgravity, a journal focusing on the effects of the space environment. A research team led by Betty Nusgens, a biologist at the University of Liege in Belgium, focused on how mice fared after spending 91 days in weightlessness for an experiment aboard the International Space Station in 2009.

The researchers zeroed in on how skin samples from the zero-G mice compared with skin samples from a different set of mice on Earth. They found that the skin of the astro-mice was thinner, and that there were higher levels of degraded collagen molecules. It also looked as if the hair growth cycle might have been disrupted — with the result that more hair follicles were active when they should have been resting.

NASA astronaut Nicole Stott works with the Mice Drawer System in the International Space Station's Kibo laboratory during her flight in 2009. The mice inside the habitat were the focus of multiple studies on the health effects of prolonged weightlessness.NASA file

Scientists are interested in the effects of zero-G on skin health because of a history of complaints about skin dryness and itching from spacefliers during long-duration flights. Minor skin problems, such as irritation and scratches, were the medical incidents reported most frequently by 19 crew members who spent time on Russia's Mir space station in the mid-1990s.

When German astronaut Thomas Reiter returned from the International Space Station after a six-month orbital tour of duty in 2006, researchers found that his skin was thinner and had decreased elasticity. "These observations were, however, limited to one test subject," Nusgens and her colleagues wrote. The mouse study was aimed at exploring the issue more deeply.

Unfortunately, the researchers say the study didn't turn out to be as definitive as they hoped. Three of the six astro-mice died during the flight, and the fact that only three remained "is a limiting factor to the power of statistical analysis," they said. Nevertheless, they found enough of an effect to raise concern.

Based on their postmortem analysis of the samples, the researchers speculate that prolonged weightlessness might disrupt the way proteins are processed in skin cells. "A lack of mechanical stimulus from the muscles might also be involved," Nusgens told NBC News in an email.

Although skin and hair problems probably aren't a deal-breaker for trips to Mars, Nusgens said "our results can be considered a warning signal to space program policymakers to perform clinical investigations on the astronauts' skin to evaluate potential thinning and fragilization." One experiment, known as Skin-B, is already monitoring crew members on the space station for signs of premature skin aging.

Can't this problem be solved with a little extra body lotion? Nusgens doubts it. "A body lotion is only 'cosmetic,' and could only improve skin dryness," she said.

Which just goes to show that the challenges of long-duration spaceflight are more than skin-deep.

In addition to Nusgens, the authors of "Skin Physiology in Microgravity: A 3-Month Stay Aboard ISS Induces Dermal Atrophy and Affects Cutaneous Muscle and Hair Follicles Cycling in Space" include Thibaut Neutelings, Yi Liu, Sara Tavella, Alessandra Ruggiu, Ranieri Cancedda, Maude Gabriel, Alain Colige and Charles Lambert.