WATCH: TSA Makes Full Body Scanners Mandatory for Some Travelers

Just in time for the busy Christmas travel season, when a record number of Americans plan to get away for a holiday, the Transportation Security Administration has changed its security screening rules at airports nationwide. The provision to opt-out of full body scanners is no longer a guarantee.

As CBS News was first to report, the directive, issued last week, is to be implemented on a case-by-case basis, "as warranted by security considerations."

That means, at the discretion of TSA agents, some transgender and other airline passengers will not be able to request a pat-down, and instead face mandatory scanning using the TSA's Advanced Imaging Technology. Ticketed travelers will be required to step into the tall cylinders (pictured above) where travelers raise their hands, and set their feet apart, as the machine scans each person for potential dangers such as weapons and explosives.

"Generally, passengers undergoing screening will have the option to decline AIT screening in favor of physical screening," a TSA spokesperson tells The Advocate. "However, some passengers will be required to undergo AIT screening if warranted by security considerations in order to safeguard transportation security. This will occur in a very limited number of circumstances where enhanced screening is required. The vast majority of passengers will not be affected."

What travelers don't see is that on the other side of the machine, a TSA agent makes a determination on sight whether each individual is male or female, presses a button and then the scanner follows programming that very closely aligns with that binary view.

This is one reason transgender travelers in particular choose to avoid the scanners: because they can trigger what the TSA now calls an "alarm" if, for example, a penis is detected on a female-presenting passenger, or a penis prosthesis worn by a trans man. Such an alarm can cause suspicion, concern, and in cases that made headlines around the world, lead to detention and delays in some circumstances.

Activists created a hashtag to call attention to the issue: #TravelingWhileTrans.

The only exceptions, according to the TSA spokesperson, are passengers with disabilities, medical conditions or special needs. "The current policy will not change unless warranted by security considerations in order to safeguard transportation security," the spokesperson said.

Click here to read the directive, and here to learn more about current TSA policies. Watch a report from CBS News below.