The holiday season is upon us, and with millions of Americans expected to be traveling over the next few weeks there will be many more airport body scans than usual done by the U.S. 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 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 Detroit successfully smuggled explosives in his underwear onto a U.S.-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. As a physician, I cannot help but question the risk/benefit balance involved. I have worried about risk ever since their initial implementation, and I have never set foot in a body scanner despite extensive air travel over the years—I always choose to “opt out” instead. 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, including physicians and experts in the field of radiological research, due to their use of ionizing radiation—the kind that can break apart molecules. 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.

This argument formed the basis for a ProPublica and PBS NewsHour story that decried the nonchalance with which the government introduced this new method of security screening without reliable scientific testing of the risks involved. Despite these public concerns, backscatter x-ray scanners were deemed to provide only a “negligible individual dose” (pdf) of ionizing radiation in a special report on radiation protection issues prepared by the National Council on Radiation Protection & Measurements (NCRP) for the U.S. Food and Drug Administration.

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 (pdf).

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 (pdf) against this move, but the TSA pushed forward with their updated regulations.

Has AIT Been Effective?

The TSA blog regularly posts roundups of weapons discovered during TSA screening procedures. More often than not these posts make no mention of items detected with AIT. Body scanners have detected the occasional knife, underwear full of ecstasy, a plastic dagger and a loaded gun—yet everything but ecstasy and the plastic dagger would likely have been picked up by metal detectors, without exposing passengers to radiation.

What the TSA has not publicized is the high false-positive rates of millimeter wave body scanners, with a ProPublica report citing a 54 percent false-positive rate in Germany due to the machine picking up even sweat as a potential cause for concern. And whereas the backscatter x-ray scanners have been shelved, a team of researchers obtained their own backscatter body scanner and demonstrated multiple vulnerabilities in the scanners—from allowing weapons to be smuggled through to the machines’ susceptibility to malware.

To date, there has not been a single report of aviation terrorism that was thwarted thanks to AIT. Even in the immediate aftermath of the underwear bomber’s failed attack, statistical journalist Nate Silver placed the odds of being on any given flight with terrorist activity at less than one in 10 million in the decade preceding that incident. Nevertheless, nearly eight years after that pivotal moment in U.S. aviation security history, we are still scanning passengers with potentially harmful machines every day. A spring 2016 report from the TSA defends AIT—justifying the over $2.1-million cost of the scanners from 2008 to 2017 by arguing the machines “deter would-be attackers.”

But when it comes to what AIT can or cannot actually detect, the TSA claims the information is classified. Even a former TSA agent spoke out against body scanners in an op-ed for TIME, arguing they are expensive and ineffective. The conservative the Heritage Foundation’s compilation of 60 terrorist plots since 9/11 also notes no post-2009 events were foiled by AIT. And still the TSA firmly stands by the nearly 800 machines at over 150 airports across the country. My requests for comments from aviation security experts have gone unanswered.

The Facts about Health Risks of Millimeter Wave Body Scanners

To understand potential health effects from the millimeter wave body scanners, it is critical to understand non-ionizing radiation, which encompasses everything from high-frequency ultraviolet solar radiation to very-low-frequency radiation from electric and magnetic fields. The millimeter wave body scanners emit radiation that falls in the microwave range of the non-ionizing radiation spectrum. Other technologies in this category include cell phones, microwave ovens, radar, wi-fi signals and cordless phones. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) issued a statement (pdf) regarding potential health issues associated with millimeter wave body scanners in 2012, making it clear that higher-frequency waves will lead to more energy absorption. Guidelines for exposure limitations are set for all non-ionizing forms of radiation to prevent problems from localized heating and, per the ICNIRP, the TSA millimeter wave body scanners provide only a tenth of the radiation limit for the general public.

So does this mean we are safe? According to the World Health Organization’s current electromagnetic field project, there is really no way to know at this point. Ivan Brezovich, a professor of radiation physics at The University of Alabama at Birmingham’s Department of Radiation Oncology, agrees these millimeter wave body scanners may not be 100 percent risk free and could have a biological effect. Brezovich explains that microwaves such as those from the millimeter wave body scanners can interact with the entire body, individual organs or with large molecules, thus having a potentially measurable effect. And although Brezovich was involved in experiments that demonstrated effects on cancer cells with radio frequency non-ionizing radiation exposure, he deems the risk of the millimeter wave body scanners as acceptable due to the low intensity and low penetration depth of the millimeter waves during the short scan duration.

In a world where we are all exposed to non-ionizing radiation every single day, its amount in our environment is only going to increase as technologies advance. As an example, even our cell phones have been deemed “potentially carcinogenic” by the International Agency for Research on Cancer (IARC). Couple this with our existence in modern society today and we are all living in bubbles of potential carcinogens that we cannot rid ourselves of. So although there is no proof of long-term detrimental health effects from chronic exposure to non-ionizing radiation, we have also not been able to prove there is an absence of risk from these regular exposures. Recent Business Insider analysis demonstrated Americans are millions of times more likely to die from heart disease or cancer than at the hands of terrorist attacks carried out by foreigners. And so, until there is proof that the machines either prevent terror attacks or are 100 percent safe even with long-term chronic exposure, I will continue to opt-out of AIT screenings.

Perhaps you should, too.