JUDY WOODRUFF: Millions of Americans are taking to the skies during this holiday travel season. But some experts continue to raise concerns about those full-body scanners being used to screen passengers.
NewsHour science correspondent Miles O'Brien looks into the science behind the machines.
His story was done in partnership with the online news site ProPublica.
MILES O'BRIEN: OK. So, it's a .38, right, snub nose revolver. OK. But it's heavy.
I am getting outfitted with some tools of the terrorist trade.
And this is pretty much the same consistency as C-4, which is plastic explosives, right? Arms up or not?
This is a full-body scanner with Superman vision. Steve Smith is its inventor. It uses X-rays to see right through clothing and identify all kinds of potential threats. Critics call it an electronic strip search.
Let's go back to the backroom, see how I did.
STEVE SMITH, Backscatter Body Scanner inventor: Here, we have a metal object in your right front pocket.
MILES O'BRIEN: That's the wireless, the transmitter for the microphone.
STEVE SMITH: Here we have some small metal objects in your left front pocket.
MILES O'BRIEN: Keys. Yes.
STEVE SMITH: Metal watchband. We have the handgun.
MILES O'BRIEN: They are called backscatter body scanners. They were widely deployed by the Transportation Security Administration at U.S. airports last year, amid an uproar over the perceived privacy invasion.
At the University of California, San Francisco, biochemist and imaging expert John Sedat was and is concerned about the small amount of ionizing radiation from the X-rays. This is the sort of radiation that damages our DNA and can cause cancer. Sedat reached out to some colleagues.
JOHN SEDAT, University of California, San Francisco: We all became kind of concerned as to what exactly were the intensities of these X-rays. And the more questions we asked, the -- it was clear there were fewer and fewer answers.
MILES O'BRIEN: Even though this was the biggest change in airport security since the FAA first mandated baggage screening and metal detectors in the early '70s, the TSA conducted only limited tests on the backscatter machines, and no independent safety tests have been done at all.
So the scientists wrote a letter to president's science adviser raising numerous red flags about the safety and the scientific rigor used to test the technology. Eventually, they received a four-page response from the Food and Drug Administration.
JOHN SEDAT: Which was, in reality, nothing more than a restatement of the -- the facts that are out there, primarily coming from the company that makes these machines.
MILES O'BRIEN: The company is called Rapiscan, based in Torrance, Calif. The TSA has purchased 250 of the machines so far at $180,000 a piece.
Rapiscan hopes to sell the government many more machines, assuming the TSA approves a new model, which replaces the privacy-stripping images with cartoonish graphics.
Peter Kant is the vice president for government affairs.
All right, so walk me through this. What's going on in here?
PETER KANT, Rapiscan Systems: So, here on the left are the computers that are operating the system and its power sources.
MILES O'BRIEN: Kant gave me an under-the-hood look at the device. This rectangular box generates an X-ray beam through a narrow slot. As it moves, a disc spins in front of the slot, creating a so-called raster scan. The X-ray beam is about the diameter of a sharpened pencil point.
PETER KANT: So, it's never on one part of the body for over one two-thousandths of a second as it's moving through. If there are any changes in how fast that X-ray is moving, either faster or slower, the system automatically shuts down.
MILES O'BRIEN: Kant claims the system is failsafe. The X-rays penetrate only a quarter-inch into the skin. Like radar, they reflect back to detectors which capture the images.
X-rays used in the backscatter, do they raise the risk level for those who are subjected to them?
PETER KANT: They use as small amount of X-rays as possible that we can generate, because that actually helps in the detection, having the lower-power X-ray. And, so, it's about equivalent to eating -- the same level of radiation you get from eating about half-a-banana. The potassium in a banana is slightly radioactive.
MILES O'BRIEN: Everyone agrees the backscatter machine is designed to emit a tiny amount of ionizing radiation. In fact, we get a much bigger dose while we are in the air.
Our atmosphere is what protects us from radiation from space. The higher you go, the less protection you get. And so when you are flying at 35,000 feet, you are exposed to a lot more radiation than you are at sea level.
On a typical cross-country flight, we absorb about half the radiation we would receive in a chest X-ray. The advertised radiation emitted from a backscatter scan, about 20 to 50 microsieverts, is equivalent to the dose we get flying at 35,000 feet for about two or three minutes.
STEVE SMITH: But it's also true that, you have more radiation, it is worse for you; if you have less radiation, it is less bad. And if you have less and less and less and less and less, you finally get to a point where any risk of that radiation is just trivial, and you shouldn't be making decisions on the basis of it. And that's literally the place that body scanners operate at.
DAVID BRENNER, Center for Radiological Research, Columbia University Medical Center: So, the question is, what is the cancer risk for this individual?
MILES O'BRIEN: David Brenner is director of the Center for Radiological Research at Columbia University Medical Center. He says the odds are one in 10 million people will develop cancer as a result of a backscatter scan. But when fully deployed, a billion travelers could pass through a backscatter machine each year.
DAVID BRENNER: It's a very small risk. So, you and I don't have to worry about walking through these machines. But if 1,000 million people walk through these machines, it's a whole different ball game.
MILES O'BRIEN: Brenner says its like the lottery. The odds may be long, but people still win. Brenner predicts, when fully deployed, backscatter scanners could give 100 travelers cancer every year.
MILES O'BRIEN: So, instead of Powerball, its cancer ball?
DAVID BRENNER: Instead of Powerball, it's cancer ball, indeed. And the issue is, we don't need to have a cancer ball, because we have alternatives which don't use X-rays.
MILES O'BRIEN: The alternative Dr. Brenner refers to uses lower-frequency millimeter waves to do the same job. Unlike X-rays, millimeter waves do not emit ionizing radiation. The TSA says the devices have the same detection capabilities. So far, the agency has deployed equal numbers of each machine.
ROBIN KANE, Transportation Security Administration assistant administrator for security technology: I think they're both very, very safe.
MILES O'BRIEN: Robin Kane is the TSA's assistant administrator for security technology.
ROBIN KANE: Keeping multiple technologies in play is very worthwhile for the U.S. in getting that cost-effective solution and being able to increase the capabilities of technology, because you keep everyone trying to get the better mousetrap and spiral up their capability.
MILES O'BRIEN: But the U.S. stands nearly alone in its embrace of backscatter. European Union regulators recently banned any body scanner that uses X-rays, "in order not to risk jeopardizing citizens' health and safety."
In the U.S., the Food and Drug Administration is responsible for approving electronic devices that emit radiation. But the agency relies solely on information provided by the manufacturers, and generally does not physically test or inspect the devices.
The TSA tested the devices behind closed doors, without scrutiny from independent scientists. The Johns Hopkins Applied Physics Laboratory and the Army Public Health Command measured the radiation dose, but they didn't address the potential impact to human health. The TSA says it is reluctant to allow a more open scientific vetting of these machines because it might undermine security.
ROBIN KANE: The balance between the sharing of information vs. what detection capabilities are is very sensitive to us. And to be able to understand what the machine can do is either classified information or sensitive security information.
So there is less knowledge out there. I don't think that that means we haven't done an honest assessment of what the safety is, because we have had all those independent bodies looking at it.
DAVID BRENNER: These are what we call anthropomorphic phantoms. They are basically plastic versions of human beings.
MILES O'BRIEN: David Brenner says the TSA owes it to the public to be more forthcoming.
So, in theory, this would react to a radiation source much as a human body would?
DAVID BRENNER: Very much as a human body would.
MILES O'BRIEN: He says a phantom test is the most accurate way to measure the radiation dose emitted.
DAVID BRENNER: Well, these are not easy measurements to make at all. In fact, they are really hard measurements to make. And that's why, really, you need the whole scientific community contributing to try and make sure that we understand what's going on inside these machines. There is a lot of uncertainty, I think, right now, and that's because very few people have had access to doing these measurements.
PETER REZ, Arizona State University: I would like to, in collaboration with Dr. Brenner, figure out how we're actually going to measure the dose. It's not trivial.
MILES O'BRIEN: Arizona State University physicist Peter Rez worries about the uncertainty as well. And after carefully scrutinizing backscatter images, he is also concerned about the efficacy of the machines.
PETER REZ: So, what machines detect is edges. And that becomes the anomaly that they go after. What they cannot do is to differentiate between a high explosive and human tissue.
MILES O'BRIEN: The TSA won't comment on the capabilities or vulnerabilities of its screening devices. But the so-called underwear bomb nearly detonated on a flight to Detroit by Umar Farouk Abdulmutallab on Christmas Day 2009 would very likely have gone undetected by backscatter X-rays.
Ironically, it was that incident that hastened the decision to use backscatter scanners at U.S. airports.
STEVE SMITH: Yes, that is certainly our goal, is to be able to develop a system where you can leave your shoes completely on.
MILES O'BRIEN: By that time, Steve Smith had already sold his invention to Rapiscan. So now he is trying to get back in the game.
He runs a small company working on a new generation of backscatter scanners that are smaller and, he says, more accurate. He says his critics are in dark.
So they're not making an informed argument, you think?
STEVE SMITH: No, they are clearly not making an informed argument.
MILES O'BRIEN: Should they be given that information, or is there no way to do that?
STEVE SMITH: The issue of what information you can release to third parties regarding security equipment is -- of course, is a very touchy situation.
MILES O'BRIEN: The scientists who are critical of backscatter scanners wouldn't disagree they are uninformed. In fact, they say that is precisely the point.