By Rob Goodier

(Reuters Health) - Space tourists may soon be plunking down six figures and buying passage to a low-earth orbit – but they should know there are likely to be health risks, experts say.

Both coming and going, spaceflight can threaten tourists' health, with potential dangers from higher gravitational forces during acceleration, and space motion sickness that strikes some people in low and zero-gravity.

Outside of the Earth's protective magnetosphere, space radiation might also pose a risk, possibly to implanted medical devices.

And a hidden threat might be the unpredictable ways people act while confined in a ship in this new situation.

But the experts’ bottom-line message? There’s too little information now to definitively answer the question of who is fit for this kind of travel.

“We don’t have a specific list of conditions that would be disqualifying, but certainly uncontrolled medical problems (whether it’s hypertension or heart disease or lung disease, or many other conditions), would most likely cause concern and result in disqualification,” Dr. Tarah Castleberry, an assistant professor of aerospace medicine at the University of Texas Medical Branch in Galveston, told Reuters Health by email.

Castleberry and her colleagues are a part of an academic network created by the U.S. Federal Aviation Administration to assess challenges in commercial spaceflight. Recently in the quarterly journal New Space they published an outline of what remains to be done to understand the health risks (see http://bit.ly/1rNwwjF).

So far, most data on the risk of spaceflight comes from professional astronauts. But space tourists will likely be a much more diverse group, with a broader range of health conditions.

Earlier this year, researchers from Castleberry's group ran 335 volunteers through a centrifuge that simulates the forces of acceleration in spaceflight. Most had one of five medical conditions: hypertension, diabetes, back or neck problems, cardiovascular or lung disease.

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No one suffered significant damage or setbacks from the experience, the researchers reported last July in the journal Aviation, Space and Environmental Medicine (http://bit.ly/12kOLa9). The most common complaint was grayout, the blurred vision that is a precursor to blackout, which occurred in more than two-thirds of the volunteers. Twenty percent had nausea, and six percent had chest discomfort.

With historical data so limited, it's difficult to make predictions, the researchers say.

They have drawn up a shopping list of tools that could answer some of these questions and offset risks. Their to-do list includes setting medical standards for spaceflight crew and developing training and risk management systems. They also want to create a health database to collect information from space tourists.

Fliers could potentially wear monitoring devices, the researchers say – but most off-the-shelf products are not up to the task. New devices may have to be developed to track heart rate, breathing, skin temperature and other useful measurements.

Those measures may start to fill the gap in medical knowledge about this new endeavor, but more study and more scientific publication is needed.

“The aerospace medical profession is one of the smallest and least published fields in scientific literature,” Drs. Natacha Chough and Rebecca Blue, also at the University of Texas Medical Branch, wrote in a New Space editorial.

The problem with the lack of data may be compounded by the public's high expectations for safety.

“The only real reference point right now is commercial air travel, and we are a victim of our own excellent safety measures,” says Dr. Clay Cowl, chair of the Division of Preventive, Occupational and Aerospace Medicine at Mayo Clinic in Rochester Minnesota, who was not involved in Castleberry's research.

“My fear is that public tolerance for a bad outcome is not like it was when we introduced the public to regular flight in fixed-wing aircraft. There were a lot of health risks at that time before we came up with things like pressurized cabins, before we came up with ways to mitigate the problems with traveling at altitude,” Cowl told Reuters Health by phone.

Something that's a nuisance on the ground can become an emergency during the flight. Indigestion, for example, could become a medical concern as pockets of gas trapped in the body expand at high altitudes, Cowl says. And unexpected behavior could become a problem if the flight triggers a phobia or a passenger gets angry.

“In credit to the groups thinking about space flight, my opinion is that we're going to have to figure out ways to mitigate potential side effects of this travel as much as possible because I don't know what the public appetite is going to be to tolerate in-flight emergencies and catastrophes,” Cowl said.