If you’ve traveled by airplane in the past few years, chances are you’ve walked through a full body scanner at the security checkpoint. Full body scanners are pretty common in airports, as well as in courthouses and jails. A lot of people worry that the scanners aren’t safe. But it actually depends on what type of scanners you’re walking through.

The first generation of full body X-ray scanners rolled out in airports across the country in 2008. Known as backscatter scanners, there was a bit of an uproar from consumers when these came on the scene. Fliers feared that the imaging would visually strip them of their clothes in front of the TSA agents. Some health experts also worried that these models might expose people to unnecessary radiation.

But in 2013, the Transportation Security Administration (TSA) replaced backscatter scanners with new-and-improved scanners, called millimeter wave screeners. Today, these scanners are in 160 airports across the country.

They fixed the “see you naked” issue. These devices create a standard outline of a person, rather than a detailed image of the person’s body. They’re also an improvement for your health. The millimeter wave scanners use low power radio frequency waves, not ionizing radiation, to create the image. The frequency falls between microwaves and infrared radiation, the same frequency used for Wi-Fi devices. If you stood for three seconds in a scanner, it would be no more radiation exposure than standing in front of your computer router at home. In fact, the dose is actually a lot less.

Backscatter scanners, meanwhile, can still be found in other places like courthouses and jails. These devices do expose people to small amounts of X-rays. The Food and Drug Administration (FDA) considers them safe. A person receives more radiation from naturally occurring sources in less than an hour of ordinary living than from one screening with any general-use X-ray security system, says the FDA. Yet others are not so convinced.

The European Union prohibits the backscatters. Some scientists believe that even very low level X- ray exposure could increase the risk of cancer. “I would agree that the individual risk associated with X-ray scanners is likely to be extremely small, so their use in low-volume scenarios seems reasonable to me,” says David Brenner, head of Columbia University's Center for Radiological Research. “The issue of concern is when the X-ray scanners are used in a very high volume setting (just under a billion security screens occur each year in US airports). A very tiny risk multiplied by a billion has the potential to represent a public health issue,” he says. Research has shown that if you expose millions, you’ll cause four or five cancers. In other words, for most people the level of radiation is low enough not to cause a problem. But the scans might create just the right amount of damage in a handful of people to cause a cancer. 

If you’re concerned about any of these scanners, you have options, though none are perfect.

  • Opt out of scanner screenings. You have the right to opt out of airport screenings, but you’ll receive a full body pat down instead. If you choose this, get to the airport early because it will take extra time. Be prepared for the agent to use the front or back or his or her hands and go over most of your body (they typically have same-gender agents doing pat-downs).
  • Inform the TSA agent if you are pregnant. You may be allowed to pass through a metal detector without additional screening.
  • Children are more sensitive to radiation. TSA agents typically allow young children to bypass the scanners. If you encounter one who doesn’t, you may want to opt out. They may let your child through security without a scan. But your child may require a pat down instead before passing through. 

Laurie Tarkan is an award-winning health journalist for the New York Times, national consumer magazines and websites.