Roni Caryn Rabin / New York Times News Service

Even before she was pregnant, Yolanda Marin-Czachor tried to avoid the full-body X-ray scanners that security officers use to screen airport passengers. Now she’s adamant about it: She’ll take a radiation-free pat-down instead any day.

“I had two miscarriages before this pregnancy,” Marin-Czachor, a 34-year-old mother and teacher from Green Bay, Wis., recalled, “and one of the first things my doctor said was: ‘Do not go through one of those machines. There have not been any long-term studies. I would prefer you stay away from it.’ ”

There are 244 full-body “backscatter” X-ray scanners in use at 36 airports in the United States. They operate almost nonstop, according to the Transportation Security Administration. Other airports use millimeter wave scanners, which look like glass telephone booths and do not use radiation, or metal detectors.

Most experts agree that as long as the X-ray backscatter machines are functioning properly, they expose passengers to only extremely low doses of ionizing radiation.

But some experts are less sanguine, and questions persist about the safety of using X-ray machines on such a large scale. A recent study reported that radiation from the machines can reach organs through the skin. In another report, researchers estimated that 1 billion X-ray backscatter scans per year would lead to perhaps 100 radiation-induced cancers in the future. The European Union has banned body scanners that use radiation; it is against the law in several European countries to X-ray people without a medical reason.

The machines move a narrowly focused beam of high-intensity radiation very quickly across the body, and David Brenner, director of the Center for Radiological Research at Columbia University Medical Center, says he worries about mechanical malfunctions that could cause the beam to stop in one place for even a few seconds, resulting in greater radiation exposure.

For security reasons, much about how the machines work has been kept secret. The TSA says the full-body scanners have been assessed by the Food and Drug Administration, the U.S. Army Public Health Command and the Johns Hopkins University Applied Physics Laboratory.

But researchers at these institutions have not always had direct access to the scanners in use, and some of the published reports about them have been heavily redacted, with the authors’ names removed. Independent scientists say limited access has hampered their ability to evaluate the systems.

John Sedat, emeritus professor of biochemistry and biophysics at the University of California, San Francisco, believes that the effective dose could be 45 times as high as the TSA has estimated, equivalent to about 10 percent of a single chest X-ray.

TSA officials scoff at scientists’ statements that measuring the effective radiation dose received by passengers is very complex, saying that it is not difficult, that the machines are inspected for problems at least once a year, and that they are equipped with fail-safe shutoff systems.

The machines, though, have had mechanical problems. A recent TSA report said that between May 2010 and May 2011, there were 3,778 service calls concerning mechanical problems in backscatter X-ray machines. Radiation safety surveys were conducted after only 2 percent of the calls.

Avoiding X-ray exposure

While the risk to the average passenger may be low, here are some suggestions for those who wish to reduce their exposure.

• Get to the airport early. That gives you extra time to opt for a pat-down if you want.

• If you are pregnant or think you may be pregnant, tell a TSA agent. You may be allowed to pass through a metal detector without additional screening.

• The younger that children are, the more sensitive to radiation. TSA employees have been known not to require children younger than 13 to go through an X-ray machine, although the agency denies there is any policy on this.

• If you have any concerns about medical conditions, you have the right to opt for a pat-down by a TSA employee.