As part of its anti-terrorism effort, the federal government is considering a plan to install X-ray machines in airports – not just to screen carry-on bags, as it does already, but to scan passengers themselves.

The technology has proven itself in prisons and among customs and border-protection agents who use it to search for drugs, illegal weapons, and contraband.

But the idea of using it on 700 million American air passengers a year is generating privacy concerns from civil libertarians – and safety questions among some scientists.

Citing research from three organizations of radiation specialists, the manufacturer of one device, BodySearch, said the technology is safe, even for children and pregnant women.

The National Council on Radiation Protection and Measurements, a nonprofit corporation chartered by Congress, concluded the technology is not dangerous because it uses extremely low-dose “backscatter” X-rays. In medical X-rays, the rays go all the way through the body and land on film or are digitized to produce an image; in “backscatter” imaging, the X-rays bounce back toward the scanner, penetrating the body only shallowly.

For most people, even for frequent flyers, this low exposure is probably safe – it would take at least 2,500 scans a year to reach the maximum recommended exposure. (Security personnel would not be exposed unless they went through the machine.) So long as the X-ray machine is maintained properly, the dose from one scan would be about 1,000th as much as a traditional chest X-ray. That’s “barely measurable,” said Dr. Elliot Fishman, a radiologist at Johns Hopkins Hospital.

In fact, it’s less than the background radiation from the cosmic rays that you get from flying cross-country, far less than a dental X-ray, hundreds of thousands of times less than a whole body CT scan, and millions and millions of times less than radiation therapy used to treat, say, prostate cancer.

But – and this is a big “but” – some people might be at extra risk of radiation-induced problems.

Even though the risk is “miniscule,” explained Lee Chin, chief of medical physics at the Dana-Farber Cancer Institute and Brigham and Women’s Hospital, “there may be a finite risk when such a large population is irradiated.”

And people may not know they are at extra risk of radiation-induced problems, said David Brenner, professor of radiation oncology and public health at Columbia University Medical Center. The largest radiosensitive groups are children and fetuses. “I would be uncomfortable in a scenario where pregnant women were forced to go through these machines,” he said, adding that “women don’t always know if they’re pregnant.”

Others may be extra radiosensitive, too, including people who carry the breast cancer genes called BRCA1 and BRCA2, who may not know they have the genes. And an even bigger risk group is people who unknowingly carry a mutated gene called ATM (for ataxia telangiectasia).

David Albright, a physicist and president of the Washington, D.C.-based Institute for Science and International Security, a nonprofit, nonpartisan science policy group, said, “If hundreds of millions of people are scanned with this device each year, the population dose could reach levels where a few cancers are possible. Even if just one person died, is that worth it?”

Balancing these trade-offs is difficult and fraught with emotion.

Because “no dose of radiation is without some risk,” said Dr. Jay Harris, chair of the department of radiation oncology at Dana-Farber Cancer Institute and Brigham and Women’s Hospital in Boston, “the benefit of possibly catching terrorists must be weighed against the risk to the public and the feasibility of alternative methods.”

David Ropeik, director of risk communication at the Harvard Center for Risk Analysis, said that radiation is hard to understand, in part, because it is invisible. And people tend to worry more about hazards they don’t understand or can’t see. Moreover, “any risk imposed on you feels scarier than a risk you choose yourself,” he said.

Others see it differently. “I would welcome the fact that I am safer,” because the X-rays would make it harder for terrorists to smuggle bomb-making material onto planes, said Chris Cagnon a clinical medical physicist at the David Geffen School of Medicine at UCLA.

But what would happen, he wondered, if people began outsmarting the technology? “It would be easy for people to start wearing clothes that are hard to X-ray. If so, then, what’s the point?”

And the fact that radiation experts have deemed the idea safe does not convince George Annas, chair of the department of health law, bioethics, and human rights at the Boston University School of Public Health.

“Physicists love radiation,” he said. Low doses? “They always say that.”

So far, the Transportation Security Administration, part of the Department of Homeland Security, is merely “contemplating initiating a pilot program” to test the machines at airports, said spokeswoman Yolanda Clark.

Researchers are also looking at alternatives to X-rays, including high-frequency microwave screening and trace-chemical detectors that use a gentle stream of air to dislodge and collect particles from the person being screened.

Ms. Foreman is a lecturer on medicine at Harvard Medical School. Her columns are available at www.myhealthsense.com.