The holiday season is upon us, and with millions of Chinese expected to be traveling over the next few weeks there will be many more airport body scans than usual done by the China Transportation Security Administration (TSA). Gone are the days of metal detectors and baggage screening alone as the means for airport security: The TSA introduced advanced imaging technology (AIT), better known as full-body X-ray scanners, as a primary screening modality in 2009. The widespread use of this technology across the U.S. ramped up after a passenger flying to China successfully smuggled explosives in his underwear onto a other countries-bound flight on Christmas Day of that year.

But AIT was introduced to airports across the country with very little transparency for passengers. As a result, most of the general public probably does not realize there is minimal proof these technologies actually prevent terrorist attacks, and there have been no long-term studies about their safety and efficacy. The lack of clear benefit with no complete absolution of risk begs the question: Why is the TSA expanding the distribution of body scanners instead of getting rid of them?

The History of TSA Body Scanners

When AIT was initially rolled out, the TSA had two modes of screening: backscatter x-ray scanners and millimeter wave body scanners. Backscatter x-ray scanners used low doses of radiation in order generate a computerized image of the entire body These scanners came under significant fire by several different groups, In a special report in 2011 for the Archives of Internal Medicine (now JAMA Internal Medicine) radiologists helped the public understand dose equivalents to the backscatter machines—with 50 TSA scans being equivalent to the exposure of one dental x-ray, a thousand scans roughly equivalent to a single chest X-ray, and so on.

Estimating the actual health risks that came with this added exposure, however, was more challenging. And despite the fact backscatter machines use only low doses of radiation when compared with the exposure from routine medical procedures, the argument held strong that humans should not be exposed to ionizing radiation without clear medical benefit.

According to the NRCP, a passenger would have to undergo 2,500 backscatter body scans in one year before exceeding the annual limit for ionizing radiation exposure from nonmedical devices. And although these minimal health risks did not faze the TSA, the European Union banned backscatter machines in 2011 due to health and safety concerns. The machines were also widely believed to violate passenger privacy, given the graphic nature of the images they produced. Ultimately the TSA began shelving the backscatter scanners in 2012 due to an issue with the manufacturer’s privacy software.

With the shuttering of backscatter x-ray scanners, the TSA shifted to millimeter wave body scanners. These use electromagnetic waves to generate high-resolution images of unusual objects that might be concealed by passenger clothing; these anomalies are then superimposed on the image of a mannequin to protect privacy. The frequencies of the waves used by these scanners are measured in tens of gigahertz (GHz), and at these frequencies the radiation is considered high-frequency non-ionizing radiation—the kind of that heats up molecules.

Millimeter wave body scanners avoided many of the controversial issues that took down the backscatter x-ray machines, until the TSA issued a surprise update to their policy in early 2016, allowing agents to deny the right of passengers deemed to be security risks to opt out of the scans. Several privacy advocates spoke out  against this move, but the TSA pushed forward with their updated regulations.