Matt Wald, New York Times

WASHINGTON — The plan for broad use of X-ray body scanners to detect bombs or weapons under airline passengers’ clothes has rekindled a debate about the safety of delivering small doses of radiation to millions of people — a process some experts say is certain to result in a few additional cancer deaths.

The scanning machines, called “backscatter scanners,” deliver a dose of ionizing radiation equivalent to 1 percent or less of the radiation in a dental X-ray. The amount is so small that the risk to an individual is negligible, according to radiation experts. But collectively, the radiation doses from the scanners incrementally increase the risk of fatal cancers among the thousands or millions of travelers who will be exposed, some radiation experts believe.

Full-body scanners that are already in place in some airports around the country and abroad use a different type of imaging technology, called millimeter wave, that uses less powerful, non-ionizing radiation that does not pose the same risk.

But those machines also produce images that are less clear. And in the wake of the attempted bombing of an airplane traveling to Detroit from Amsterdam on Dec. 25, the United States is turning to backscatter scanners for routine security checks. Congress has appropriated funds for 450 scanners to be placed in American airports. On Thursday, President Obama called for greater use of “imaging technology” to spot weapons and explosives.

Some other countries may follow suit. Britain plans to use whole-body scanners and may test the backscatter system. On Friday, the French government said it would begin testing a few scanners of the millimeter wave type at Charles de Gaulle and Orly airports, for flights bound for the United States. Italy and the Netherlands also plan to use the millimeter-wave scanners.

Most discussion about full-body scanners has focused on privacy issues surrounding the nude images that would result. The American Civil Liberties Union has denounced the practice as a “virtual strip search.”

Some experts argue that the broad use of the scanners raises the same question that pertains to any other routine exposure to small doses of
radiation: Do the benefits outweigh the risks?

“The guiding principle is not whether Mother Nature is going to kill you one day,” said Arjun Makhijani, a nuclear physicist. “It’s whether we can justify doing something to each other based on the benefit you’re going to get.”

Officials at the Transportation Security Administration say they have already tried out a handful of backscatter scanners. They could acquire 450 from the manufacturer, Rapiscan Systems, by the end of September.
The agency has a contract under which it could buy 900 of the scanners.
The machines have been used for years at prisons and other places where the authorities look for weapons, including at nuclear power plants.

In a 2002 report on the safety of backscatter scanners, the National Council on Radiation Protection and Measurements, which is highly influential in setting regulatory standards, said it “cannot exclude the possibility of a fatal cancer attributable to radiation in a very large population of people exposed to very low doses of radiation.”

One author of that report, David J. Brenner, a professor of radiation biophysics at Columbia and director of the university’s Center for Radiological Research, said that risk might be increased as the transportation agency moves from using the scanning machines as a second-round check after metal detectors and hand searches to using them as a first-line screening system.

“When we were looking at these a few years back, it was always going to be as a secondary screening tool,” he said. “In that scenario, I don’t think there’s too much concern.” But, he said, if millions or tens of millions of passengers a year were scanned with the backscatter X-ray, he said, the risk would be higher.

The health effect of small doses of radiation is not observed, but inferred from the visible effects of higher doses. Dr. Makhijani said that if a billion passengers were screened with the dose assumed by the radiation protection council, that would mean 10 more cancer deaths a year.

Those deaths would represent only a tiny increment over the existing cancer rate, he said, just as the extra dose was a tiny fraction of the natural background dose of radiation people get from everyday exposures, but he added that they should still be considered.

Edward Lyman, a nuclear expert at the Union of Concerned Scientists, said that the additional deaths would be indistinguishable from cancers resulting from other causes. But he said, “Just because they can’t be attributed in an epidemiology study to the additional radiation, it doesn’t mean they’re not there.”

Other experts, however, including David A. Schauer, the radiation council’s executive director, disputed the idea that collective doses of radiation increased risks significantly.

Doses divided into tiny portions among millions of people are not the same as those concentrated on a few people, Dr. Schauer said.

“I personally don’t buy it,” he said. “From a public health point of view, it’s a bit of a stretch.”

The radiation council sets standards for doses to radiation workers and to the general public, but does not set a standard for a collective dose.

Robert Barish, a radiation consultant in New York and the author of a
1996 book, “The Invisible Passenger,” said the doses delivered by the scanners were tiny by any standard, and passengers would get the same dose in a few minutes in a high-altitude jet, where most of the earth’s atmosphere is not available to shield people from cosmic rays.

A spokeswoman for the Transportation Security Administration, Kristin Lee, said that even for pregnant women, children and people whose genetic makeup made them more susceptible to X-ray damage, “It would take more than 1,000 screenings per individual per year” to exceed radiation standards.

According to a blog published by the Transportation Security Administration, the radiation dose from the scanner is about the same amount as an average American receives from natural background sources in four minutes on the ground.

But Dr. Lyman, of the Union of Concerned Scientists, noted that at one point the blog had listed a much higher dose for the scanners. When the discrepancy was pointed out, the agency corrected the blog to the lower figure.

Backscatter scanners work by shooting a beam of X-rays at a subject. But rather than making an image from what passes through the body, as a doctor’s diagnostic X-ray machine does, backscatter machines measure what bounces back, producing an image of the passenger without clothing.
The X-rays are a form of ionizing radiation, that is, radiation powerful enough to strip molecules in the body of their electrons, creating charged particles that cause cell damage and are thought to be the mechanism through which radiation causes cancer.

Nicola Clark contributed reporting from Paris.

Copyright 2010 The New York Times Company