Chip Somodevilla / Getty Images A volunteer stands inside a backscatter X-ray scanner during a demonstration at Ronald Reagan National Airport in Arlington, Va.

As millions of American travelers take to the airways this Thanksgiving, they will increasingly face the new generation of full-body scanners at airport security — including the kind that Europe just banned for reasons of “health and safety.”

In its new airport security policy, the European Commission announced on Nov. 14 that it would ban the controversial “backscatter” X-ray machines, which emit ionized radiation, from all airports in the European Union’s 27 member nations “in order not to risk jeopardizing citizens’ health and safety.”

The U.S. Transportation Security Administration (TSA), meanwhile, has rolled out about 250 backscatter X-ray machines across the country. It has also installed some 260 millimeter-wave scanners, which do the same job but use low-energy radio waves instead of X-rays. Millimeter-wave scanners are allowed at European airports.

MORE: Did Airport Scanners Give Boston TSA Agents Cancer?

Both devices display reasonably accurate images of your body, beneath your clothes, helping airport security workers to spot hidden weapons or explosives that wouldn’t be caught by metal detectors. The difference is that millimeter waves don’t cause cancer, while cumulative, high doses of X-ray exposure are a known carcinogen.

“What makes X-rays different is the fact that they’re more energetic,” says Dr. David J. Brenner, director of the Center for Radiological Research at Columbia University Medical Center. “They have enough energy to knock an electron out of an atom. Basically, if that atom happened to be in DNA, [X-rays] could break strands of DNA in a way that millimeter waves simply can’t because they don’t have enough energy.”

In other words, X-rays have enough energy to cause a DNA mutation that can trigger cancer, while millimeter waves don’t.

The TSA and Department of Homeland Security (DHS) maintain that backscatter X-ray scanners emit such low levels of radiation — equivalent to the radiation you’d get in about two minutes of flying, according to the TSA — that any resulting increase in cancer risk would be negligible. Some scientists say, however, that even if any one individual’s risk of developing cancer from scanner radiation exposure remains low, when you consider the risks accrued over time by an entire population — say, the 100 million Americans who fly each year — the machines pose a potential public-health danger.

VIDEO: Airport Scanners and You: An Odd Todd Cartoon

Brenner further points to the lack of independent and clinical data on backscatter X-ray safety as a reason not to use the devices. “By all accounts both machines are equally effective and the TSA is buying both kinds of machines, which cost about the same and have the same efficiency,” says Brenner. “So it seems to me a strange decision to use the backscatter machine, where you don’t really know what the risks are; the biggest issue is the uncertainty. And it’s going to be 30 years before the cancers start to appear. I’m not sure why you’d want to take that risk.”

The current data on radiation exposure from backscatter machines comes largely from the government and from the scanner’s manufacturer. As Alice Park noted last year at the time of the devices’ public debut, some scientists think the exposure hasn’t been measured correctly:

After studying the degree of detail obtained in the seconds-long scans, the scientists wondered how the [stated] radiation exposure could be so low. The answer, they concluded, lay in how the manufacturer and government officials measured the dose: by averaging the exposure from the beam over the volume of the entire body. This is how scientists measure exposure from medical X-rays, which are designed to zap straight through bone and tissue. But backscatter beams skim the body’s surface. Sedat and his colleagues maintain that if the dose were based only on skin exposure, the result would be 10 to 20 times the manufacturer’s calculations. That’s a huge difference, but the higher amount, TSA and FDA officials maintain, still falls within the limits of safe radiation exposure. Based on measurements conducted by the FDA as well as by technicians at Johns Hopkins University and elsewhere, says the FDA’s Daniel Kassiday, “We are confident that full-body-X-ray security products and practices do not pose a significant risk to the public health.”

In testimony before the Senate earlier this month, TSA administrator John Pistole agreed to a request by Sen. Susan Collins (R-Maine) for an independent analysis of the radiation emitted from the devices. Then, later, he said he had received a draft report on the scanners by the inspector general of DHS that might obviate the need for a new study.

“What I asked for — and what the administrator committed to — was an independent study on the health effects of AIT (advanced imaging technology) machines, not just a study on whether TSA is doing an adequate job of inspecting, maintaining and operating AIT machines, which I understand is the approach” of the inspector general’s report, said Collins in a statement.

MORE: Airport Scanner Safety: What Do Doctors Do?

Such a study could take years. In the meantime, American fliers — especially frequent fliers and airline employees who are exposed to scanner radiation more often than most — might be playing a numbers game in terms of cancer risk. Pilots’ unions, for example, have publicly recommended that members opt for the radiation-free pat-down if the only other option is a backscatter scan.

If you also want to avoid X-ray exposure at the airport, it’s easy to tell one scanning machine from the other, courtesy of ProPublica:

In the backscatter machine, a passenger stands between two large blue boxes and is scanned with a narrow X-ray beam that rapidly moves left to right and up and down the body. In the millimeter-wave machine, a passenger enters a chamber that looks like a round phone booth and is scanned with radio-frequency waves.

Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.