Millions of fliers pass through them, but scientific experts are still at odds about the safety of full-body airport scanners that use an X-ray technology called backscatter. The machines use low-level beams to create an image of the body, revealing weapons or other concealed items beneath a passenger’s clothing.

The scanners emit very small doses of ionizing radiation, which is known to cause cellular changes in larger doses. Rapiscan, the company that manufactures the backscatter machines used in airports, says that the radiation dose of one scan is equivalent to two minutes of air travel at 30,000 feet.

Critics of the backscatter scanners agree that the radiation exposure of a single scan is extremely low. Their concern, rather, has to do with lingering uncertainty about how that low-dose radiation is measured, as well as how to determine cancer risk as a result of radiation at such low levels.

These concerns are magnified by the fact that the Transportation Security Administration also uses full-body scanners that don’t emit ionizing radiation. Known as millimeter-wave scanners, the machines use electromagnetic waves to produce an image similar to the type created by backscatter technology, and exposure to their waves have not been linked to health risks.

Of the 488 full-body scanners in airports nationwide, 241 are millimeter-wave and 247 are backscatter. Pending funding from the federal government, the TSA plans to install 775 more full-body scanners in the next two years; the agency has yet to decide how many will be backscatter or millimeter-wave technology.