The next time you go through security at the airport, you might be told to empty your pockets, put your hands over your head and stand still while an X-ray machine looks for anything hidden under your clothing.

If this body scanning option sounds unappealing, you have another choice: an “enhanced pat down” conducted by a Transportation Security Administration employee, which some travelers have described as too much information.

The new screening measures have been hotly debated, but mostly in theory. Now that there are nearly 200 body scanning machines in about 50 domestic airports, with 800 more on the way, passengers are facing real-life decisions about what to do. Here’s some information to help you choose.

How do they work?

If you somehow missed the hoopla, there are two types of machines being installed. Both have raised concerns about privacy, health risks and even their effectiveness. The more controversial “backscatter” devices project an X-ray beam onto the body, creating an image displayed on a monitor viewed by a TSA employee in another room. The “millimeter wave” machines, which are considered less risky because they do not use X-rays, bounce electromagnetic waves off the body to produce a similar image.

Unlike metal detectors, these machines can detect objects made with other materials, like plastic and ceramic. But they can’t see anything hidden inside your body, or detect certain explosives.

So why bother?

That’s a question a federal oversight body and members of Congress are asking, especially since the TSA plans to spend billions to buy the devices and hire more screeners to operate them — using the $5 to $10 security fee included in the price of any airline ticket

“This is all done to keep the traveling public safe,” said Nicholas Kimball, a TSA spokesman.

What can screeners see?

What these images reveal is unclear. Kimball said that the TSA uses filters to blur the images, and the agency has posted sampleimages and a video of the screening process on its website, www.tsa.gov. But critics say these samples aren’t detailed enough for travelers to judge how explicit they are, especially if a screener zooms in on a specific area.

Another concern is whether the images can be saved or transmitted. The TSA first said this wasn’t possible, then later admitted the machines can save photos, but that the feature had been disabled.

How safe are they?

The main concerns are how much radiation the scanners give off (the manufacturers say the amount is very low), whether the scanners might malfunction and emit more radiation than they are supposed to and what the health effects may be for travelers. Since there is no precedent for routinely screening so many people with X-rays, there are a lot of unknowns.

Another issue is that the devices haven’t been thoroughly tested. The TSA claims that the machines have been evaluated by the Food and Drug Administration’s Center for Devices and Radiological Health, the Commerce Department’s National Institute for Standards and Technology and the Johns Hopkins University Applied Physics Laboratory. When I called these organizations to ask about their evaluations, though, I learned that tested only one whether the amount of radiation emitted meets guidelines established by the American National Standards Institute, a membership organization of companies and government agencies.

But guess who was on the committee that developed the guidelines for the scanners? Representatives from the companies that make the machines and the Department of Homeland Security.

That’s one reason Peter Rez, a physics professor at Arizona State University, has been pushing for more data.

Other medical experts are worried that the government has not adequately evaluated the health risks of such extensive X-ray screening, particularly for children, pregnant women, cancer patients and people who are sensitive to radiation. One concern is that the data the government is relying on underestimates the amount of radiation absorbed by the skin, potentially raising the risk of skin cancer.

“It’s premature to put a whole population through this thing, not without much more due diligence and much more independent testing,” said John Sedat, a biochemistry professor at the University of California, San Francisco. Sedat, along with several colleagues, sent a letter to the Obama administration calling for independent evaluations of the X-ray scanners.

So far, the TSA and government regulators have disputed their concerns. “If there is any risk, it’s very, very small,” said Daniel Kassiday, an FDA radiation official who was co-chairman of the committee that created the standard for the machines.

Kassiday said an individual could receive up to 1,000 screenings a year before reaching recommended annual limits for this type of radiation exposure.

Can you opt out?

Kimball said passengers can choose not to go through the scanner and opt for the metal detector and a pat down instead. But the message travelers are getting isn’t that clear.

“It definitely didn’t feel optional at all,” said Drew Hjelm, an Army veteran who recently encountered the X-ray machine at O’Hare Airport in Chicago.

“The officer said, either you go through the body scanner or you leave the airport or we’re going to call the police and they’re going to come and arrest you,” Hjelm said.

Since other passengers have said they weren’t given a choice, or were subjected to an aggressive pat down, the Electronic Privacy Information Center has created an online form for travelers to report problems.