Airline passengers will face the long lines, interminable delays and frustrating backups that come with holiday travel. Through it all, they’ll also have to decide whether to submit to one of the 500-plus x-ray or radio wave scanners found in airports nationwide and wonder about their safety.

Much of the debate surrounding the increasingly common security scanners revolves around their effectiveness and privacy. But the health implications are coming to the fore as the European Union bans x-ray scanners because of health concerns. Many EU nations will instead use millimeter-wave, lower frequency scanners.

Both types use a beam of electromagnetic energy to create an image of a passenger — sans clothing — in an effort to detect weapons and other contraband. Millimeter wave scanners use a portion of the spectrum close to microwaves, while x-ray scanners, of course, use the higher frequency x-ray portion of the electromagnetic spectrum. Both devices collect the scattered waves that reflect off the body to create an image.

The dose of radiation from the x-ray scanners is very low. But whether it is low enough to be harmless remains a lingering question.

A recent report by ProPublica and PBS uncovered concerns over the level of radiation passengers are exposed to. Although the dose is very low, the scanners still violate “a longstanding fundamental principle of radiation safety — that humans shouldn’t be x-rayed unless there is a medical benefit,” the report states. There also is the concern that repeated exposure to even low doses of radiation could be a problem.

According to the story, research suggests “anywhere from six to 100 U.S. airline passengers each year could get cancer from the [x-ray backscatter] machines,” based on roughly 100 million passengers flying annually. The report also questions why the decision to deploy x-ray scanners was made by the Transportation Security Administration, not the Food and Drug Administration, which regulates drugs and medical devices that can affect public health.

The TSA argues the radiation poses very little threat to human health compared to the security provided by the devices.

“It’s a really, really small amount relative to the security benefit you’re going to get,” Robin Kane, the agency’s assistant administrator for security technology, told ProPublica.

In response to the ProPublica/PBS report, the FDA said the risk of getting cancer is just 1 in 400 million. The agency also clarified several points made in the story.

And as our colleagues at Threat Level noted, Johns Hopkins University’s Applied Physics Laboratory analyzed the Rapiscan 1000 x-ray scanner and published the leading and most often-cited study (.pdf) in October 2010. The 49-page report, released in a redacted form, says the machines leak virtually no radiation to TSA staff and nearby passengers and expose the person being scanned to a fraction of the maximum exposure level deemed medically safe.

“You would have to go through the scanner 1,000 times to equate to one medical x-ray,” said Peter Kant, Rapiscan’s executive vice president, summarizing the study. “You get twice as much radiation when eating a banana than when going through the scanner.”

But critics note the mechanical beam’s intensity level has not been published, making it impossible to evaluate the safety claims. Moreover, medical x-ray machines disperse radiation throughout the body, whereas the airport scanners penetrate to about skin level. That means there is a high concentration of radiation on a single organ — the skin.

Questions remain regarding the safety of the scanners and whether such tests were bungled, the manner in which they were placed into widespread use and just how effective they are. There also have been questions about the connection between Rapiscan, which produces the scanners, and former TSA boss Michael Chertoff. Chertoff’s consulting firm had done work for Rapiscan. Both companies deny anything inappropriate occurred.

Beyond the health concerns and the EU ban on x-ray scanners, France and Germany stopped using millimeter wave radio scanners because of numerous false positive results.

According to a separate story about the effectiveness of the scanners, of all the passengers singled out for closer scrutiny after being scanned by millimeter wave machines, pat-down searches revealed more than half of them posed no threat at all. The most mundane things, like sweat and folds in clothing, were among the things contributing to false positives.

Several tests of both types of scanners have shown they are effective at detecting items like guns and knives, but no more so than much cheaper metal detectors already in use. Other tests have shown explosives can be hidden on the body in a manner unlikely to be detected by those monitoring images generated by the scanners.

Passengers do not have a choice whether they are being scanned in a millimeter wave scanner, which resembles a phone booth with glass walls, or an x-ray scanner in which they stand between two large boxes. Airports often have one or the other, but they typically are not used for every security line.

There are roughly 250 x-ray machines and 260 millimeter wave machines in use nationwide. The TSA plans to deploy a total of 1,800 scanners by 2014.