TSA is installing full-body scanners at Seattle-Tacoma airport and other airports around the United States. Many have expressed concerns about privacy and the health risks of exposure to low-level radiation. Here are some answers to frequently asked questions.

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The Transportation Security Administration’s plan to install new full-body scanners at U.S. airport security checkpoints is raising lots of questions about privacy and health issues.

Add to those the potential for hassles and confusion, especially during the upcoming holidays, when record numbers of families will be flying.

TSA plans to have 450 scanners installed in 50 U.S. airports, both big and small, this year, and 500 more next year, performing what it calls advanced imaging technology. The screening allows security agents to see through clothes and find hidden objects, such as plastic explosives, that metal detectors can’t identify.

Not all airports will get scanners this year, and no airport will have enough to replace all the metal detectors, so it will be a while before the screening becomes routine for most passengers.

At Sea-Tac, TSA is installing 14 scanners alongside 35 metal detectors already in place at the airport’s four checkpoints. Checkpoint No. 1 at the south end of the terminal, closest to the international ticket counters, has five of the 14. Checkpoint No. 4 at the north end, used mainly by Alaska, Horizon and United passengers, will have two.

Here are answers to some frequently asked questions.

Q: How does the technology work?

A: Travelers will see one of two types of scanners, depending on the airport.

Twenty-seven airports are getting millimeter wave machines that use electromagnetic waves to produce a 3-D image.

Thirty-three airports, including Sea-Tac, are getting the more controversial “backscatter”‘ machines that use low-dose X-rays to produce nude images resembling a chalky drawing with facial features blotted out.

A traveler using the backscatter machines walks between what looks like two large boxes, and stands 5 to 7 seconds with hands overhead.

An inspector in another room views the picture on a monitor. If a screener sees something suspect — the scanners can detect shapes but can’t tell what the objects actually are — he or she radios an agent on the other side to make a closer inspection. The scanners can’t see inside body cavities. Critics contend that’s a major flaw.

Q: Why are airports getting these scanners now?

A: TSA and the U.S. Department of Homeland Security have been testing advanced imaging technology since 2007. Congress approved the program for nationwide rollout after a man attempted to bring down an Amsterdam-to-Detroit flight last Christmas by detonating explosives hidden in his underwear.

TSA says the scanners will provide a needed extra layer of security, but some experts question whether they would have caught the material.

Testimony by ex-Homeland Security Chief Michael Chertoff held sway with lawmakers but was later questioned after CNN reported that Rapiscan, the California company that makes the backscatter machines as well as cargo and baggage screening systems, was a client of Chertoff ‘s private security company.

Rapiscan’s parent company, OSI Systems, owns Spacelabs Healthcare in Issaquah.

Q: How much will this cost?

A: The machines cost $130,000-$170,000 each, paid for with federal stimulus funds. Estimates are the government will spend $234 million to $300 million overall on as many as 1,800 scanners by 2014.

Q: What are the health risks?

A: TSA and Rapiscan say the radiation exposure is equal to what passengers get in a plane for two minutes at 30,000 feet.

However, scientists, including a group of researchers at the University of California in San Francisco, have raised red flags and have questioned the quality of the safety guidelines TSA followed. Those guidelines were established by the American National Standards Institute, an organization whose members include companies that make the machines and the government agencies promoting them.

“While the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high,” the scientists wrote in an April 6 letter calling for a review by a panel to include medical physicists and radiation biologists.

“The risk of radiation emission to children and adolescents does not appear to have been fully evaluated,” they said, and the “policy toward pregnant women needs to be defined.”

Q: What about privacy?

A: TSA says its scanners don’t store or save images, and emphasize that the screeners looking at the images never see passengers in-person. But the Electronic Privacy Information Center in Washington D.C. equates the technology to a “digital strip search,” and has filed a suit to block use of the scanners.

Q: So what are the alternatives? Do I have to submit to a full-body scan the next time I fly?

A: No. For now, with the metal detectors still in place, it’s voluntary. Agents may steer some travelers toward a scanner depending on wait times in various lines, but for the most part, “You’ll be able to look around and choose the lane you want to go through,” says TSA’s Dwayne Baird.

If an agent does insist you join a line with a scanner, you can refuse, but you’ll get a physical pat-down. Screeners might also use explosive-tracing devices (cotton swabs) or hand wands to check for suspect items.

Q: Right now, all I have to take off are my shoes, jacket and anything that might set off metal detector, like a belt buckle or heavy watch. Will that change?

A: Yes. If you go through a scanner, TSA asks that you take everything out of your pockets, including wallets, pens and papers, and remove belts and chunky jewelry.

Q: One of the complaints about this system is that agents in different airports require passengers to remove items such as earrings and rings, and some don’t. Are the rules spelled out?

A: Not yet. TSA’s website (www.tsa.gov) includes a section called “How to get through the line faster,” but it only includes guidelines for walking through metal detectors.

Q: I don’t want to be separated from my wallet while I’m standing inside a scanner. What should I do?

A: TSA recommends putting valuables in your carry-on rather than in the plastic bins.

I recommend locking your carry-on. Police at Newark’s Liberty airport recently charged a security-screening supervisor with stealing as much as $700 a day from travelers’ bags as they passed through his checkpoint.

Carol Pucci’s Travel Wise column runs Sundays in The Seattle Times travel section and online at www.seattletimes.com/travel. Contact her at cpucci@seattletimes.com. Twitter updates https://twitter.com/carolpucci