The TSA installed backscatter technology, or body imaging X-ray, at airports across the United States only to remove them a few years later in favor of machines that are less intrusive. Currently, you will encounter a different type of scanner that does not use X-ray technology.
The body imaging X-ray machines used to scan a passenger on all sides and transmit the image of the passenger's body, without clothing, to a TSA agent who was seated 50-100 feet away from the TSA scanner. The object of the scan was to identify concealed (purposely or not) metal, plastics, ceramics, chemical materials, and explosives via millimeter wave technology.
The TSA scanner images produced by the body scanning were not saved or printed, according to the TSA. They said this about privacy and your body parts:
"For additional privacy, the officer viewing the image is in a separate room and will never see the passenger and the officer attending to the passenger will never see the image. The officers have 2-way radios to communicate with others in case a threat object is identified."
People complained about their privacy being violated despite these reassurances and so the backscatter machines have since been replaced by Advanced Imaging Technology (AIT) machines. These provide the TSA officer with a generic outline of a body in a cartoon style, with any suspicious objects colored in yellow to indicate where they are on the person's body. They can then either let you pass through and collect your things if nothing is detected or give you a pat down if something shows up. You can see an example of what the officer will see on their screen here.
Are The New Machines Safe?
Yes. The Advanced Image Technology (AIT) machines are millimeter wave scanners, just like you'd find in your cell phones. If you're happy to use a cell phone, you shouldn't have a problem passing through these scanners. The millimeter wave scanner is a whole-body device which scans with a form of electromagnetic radiation.
And in terms of security, the AIT machines are just as accurate as the backscatter machines, if not more so. The AIT scanners use an algorithm to automatically detect metals and other suspicious objects, removing the likelihood of human error. They don't show your body details, just the objects the TSA might be interested in.
Do You Have to Use Them?
You can opt out if you don't want to be scanned but you still must be checked by TSA.
You can choose to opt out of the full-body scan, but bear in mind that you'll be treated with suspicion if you do so—especially if you're not opting out for health reasons. You'll be given a pat down by a TSA officer instead, and it's likely to be very thorough. Given that there's no health risk by going through these scanners and the TSA can't see you naked when you pass through the AIT machines, there's no real reason not to use them.
Do All Airports Have Full Body Scanners?
Across the United States, 172 airports now have full-body scanners at airport security. You can see a full list of them in this article. Suffice to say, if you'll be traveling through a major U.S. city or airport, you can expect to have to pass through scanners at security. Often the TSA will select on a few passengers in line to be scan. This is usually based on a random selection. You never know when you'll be directed to have a scan.
Scanners Outside of the United States
You may go through a scanner in other countries. It depends on the part of the world you'll be traveling through. In Western Europe, for example, scanners are extremely common and you'll likely find them in most major airports. The same goes for Canada, Australia, and New Zealand.
Outside of the Western world, though, they're not as common. In most parts of the world, you'll just have old school metal detectors scanning you.
In a small airport in the Philippines, for example, you might come across an airport with no security scanners. Instead, the security official, may grab your bag, shake it, and ask you what is inside.