After the "underwear bomber" incident on Christmas Day, President Obama accelerated the deployment of new airport scanners that look beneath travelers' clothes to spot any weapons or explosives.
Fifty-two of these state-of-the-art machines are already scanning passengers at 23 U.S. airports. By the end of 2011, there will be 1,000 machines and two out of every three passengers will be asked to step into one of the new machines for a six-second head-to-toe scan before boarding.
About half of these machines will be so-called X-ray back-scatter scanners. They use low-energy X-rays to peer beneath passengers' clothing. That has some scientists worried.
"Many people will approach this as, 'Oh, it must be safe, the government has thought about this and I'll just submit to it,'" says David Agard, a biochemist and biophysicist at the University of California, San Francisco. "But there really is no threshold of low dose being OK. Any dose of X-rays produces some potential risk."
Agard and several of his UCSF colleagues recently wrote a letter to John Holdren the president's science adviser, asking for a more thorough look at the risks of exposing all those airline passengers to X-rays. The other signers are John Sedat, a molecular biologist and the group's leader; Marc Shuman, a cancer specialist; and Robert Stroud, a biochemist and biophysicist.
"Ionizing radiation such as the X-rays used in these scanners have the potential to induce chromosome damage, and that can lead to cancer," Agard says.
The San Francisco group thinks both the machine's manufacturer, Rapiscan, and government officials have miscalculated the dose that the X-ray scanners deliver to the skin -- where nearly all the radiation is concentrated.
The stated dose -- about .02 microsieverts, a medical unit of radiation -- is averaged over the whole body, members of the UCSF group said in interviews. But they maintain that if the dose is calculated as what gets deposited in the skin, the number would be higher, though how much higher is unclear.
Rapiscan officials declined to comment. Federal officials have prepared a reply, but the UCSF scientists haven't received it yet. Officials at the Transportation Security Administration and the Food and Drug Administration insist that the dose from the scanners is below negligible.
"An individual would need to go through thousands and thousands of times a year to get to the point where it would even possibly reach the equivalent of one chest X-ray," says Maurine Fanguy of the TSA's Office of Security Technology.
Specifically, using the government's dose calculations, it would take 5,000 trips through one of the back-scatter scanners to equal the 100-microsievert dose of a single chest X-ray.
That's indeed a low dose. But the UCSF scientists aren't the only ones who are concerned. David Brenner, head of Columbia University's Center for Radiological Research, recently aired his worries before the Congressional Biomedical Caucus.
"There really is no other technology around where we're planning to X-ray such an enormous number of individuals," Brenner told the caucus and congressional staffers. "It's really unprecedented in the radiation world."
Brenner's name carries some clout, because he served on a small group of experts convened in 2002 by the National Council on Radiation Protection and Measurements to write guidelines for the security scanners. He now says he wouldn't have signed the report if he had known the X-ray scanners were going to be used on virtually every air traveler.
The scientists don't all agree on the nature of the potential risks. For instance, the UCSF scientists, in their letter to Holdren, worry about effects such as melanoma, a dangerous skin cancer; immune-system problems; breast cancer; mutations in sperm cells; and effects on a developing fetus. But Brenner doubts that X-ray doses from airport scanners would cause these problems.
If you're one of those air travelers, understanding your own risk is a tricky exercise.
Brenner says he thinks the danger to most individual travelers is miniscule. But he worries about the unknowns when those very small risks are multiplied times something like 700 million travelers a year.
Recent research, Brenner says, indicates that about 5 percent of the population -- one person in 20 -- is especially sensitive to radiation. These people have gene mutations that make them less able to repair X-ray damage to their DNA. Two examples are the BRCA-1 and BRCA-2 mutations associated with breast and ovarian cancer, but scientists believe many more such defects are unknown.
"I don't know if I'm one of those 5 percent. I don't know if you're one of those 5 percent," Brenner says, "And we don't really have a quick and easy test to find those individuals."
Children are also more vulnerable to radiation damage, because they have more dividing cells at any time. A radiation-induced mutation in their cells can lead to cancer decades later.
Brenner says the most likely risk from the airport scanners is a common type of skin cancer called basal cell carcinoma, which is usually curable. It often occurs on the head and neck.
The Columbia scientist points out that it would be hard to conceal a weapon on one's head or neck, so he proposes a seemingly simple workaround –- avoid scanning the head and neck.
But TSA officials say that's not practical. Think of the long lines, they say, if the machines had to be adjusted for each passenger's height.
In any case, the TSA's Fanguy says the health risk is so tiny that it's not necessary to avoid the head and neck -– or to exempt children.
Screening is optional, Fanguy points out.
"And so parents can choose for children not to undergo screening," she says. "Personally, as a mother of two young children, I want to ensure that all technology that we use is safe. And we would not deploy technology unless we had done very rigorous and thorough health and safety testing."
Fanguy says the new scanners have been found safe by the FDA, the U.S. Army Public Health Command and researchers at the Johns Hopkins University applied physics laboratory.
At the FDA, officials are equally confident.
Daniel Kassiday, a specialist there in radiation hazards, says the radiation put out by an airport scanner is far below what airline passengers get from cosmic rays at 30,000 feet.
"At worst case, flying from New York to L.A., assuming a five-hour flight, it would take 75 screenings to equal the dose you get from that one flight," Kassiday says. "Or more simply, one screening is equivalent to four minutes in the air."
Kassiday also dismisses scientists' concerns that the scanners could malfunction and give passengers too high a dose. The machines have various safety interlocks that would shut the X-ray beam off, he says, if something went wrong.
"Are they going to break? Possibly. Eventually," Kassiday says. "Am I really worried there's going to be a significant dose as a result? Not really."
Brenner says there's an obvious answer to all the questions: "Put more interest in millimeter-wave scanners, which as far as we know don't have any radiation risks associated with them."
Millimeter-wave scanners are a different kind of machine that produces images using radio waves, not X-rays. The images are comparable in quality to the X-ray scanners, the TSA says. The cost is also comparable.
The TSA plans to deploy roughly equal numbers of the X-ray and millimeter-wave machines, Fanguy says. So why doesn't the government just use millimeter-wave machines and sidestep all the issues raised by ionizing radiation?
"Our technology strategy is to have more than one vendor available in any one class of product," Fanguy says. "That allows us to get more competitive pricing, and it makes sure that we don't cut off one avenue of technology that would potentially not allow us to take advantage of innovation later."
In other words, the TSA doesn't want to put all its eggs in one basket.
Meanwhile, it doesn't seem many members of the traveling public are worried about the X-ray exposure they may get from the new scanners. That's striking, given the indoctrination everybody gets that radiation is dangerous and should be avoided if possible.
Jason Snipes, for instance, had no qualms about stepping into the back-scatter X-ray machine at Boston's Logan International Airport recently on his way home to Chapel Hill, N.C.
Snipes might worry if his pregnant wife got scanned, he says, "but for me, personally, no, I don't have any concerns or anything like that. And if it helps, I'm all for it."
Most other passengers in line also seemed unconcerned. But not Ivor Benton, an education consultant on his way to Columbus, Ohio.
"I don't think they're safe," Benton says. "What do you expect them to tell you? You expect them to tell you it's unsafe after they've spent millions of dollars for them?"
Benton says Toyotas were supposed to be safe, too, along with children's Tylenol. But problems have cropped up. So he's learned to be skeptical.