"Hopefully this represents the beginning of a phase-out of the X-ray-type scanners, which are more privacy intrusive and continue to be surrounded by health questions," said Jay Stanley, a privacy expert at the American Civil Liberties Union.
The machines will not be retired. They are being moved to smaller airports while Congress presses the TSA to put in place stronger privacy safeguards on all of its imaging equipment.
In the two years since they first appeared at the nation's busiest airports, the "backscatter" model of scanner has been the focus of protests and lawsuits because it uses X-rays to peer beneath travelers' clothing.
The machines are being pulled out of New York's LaGuardia and Kennedy airports, Chicago's O'Hare, Los Angeles International and Boston Logan, as well as airports in Charlotte, N.C., and Orlando, Fla.
The TSA would not comment on whether it planned to remove machines from any other locations.
Some of the backscatter scanners have gone to airports in Mesa, Ariz., Key West, Fla., and San Juan, Puerto Rico. The TSA is still deciding where to send others.
The switch is being made as the TSA is under pressure from Congress. Legislation approved in February gave the agency until June to get rid of the X-ray scanners or upgrade them with software that produces only a generic outline of the human form, not a blurry naked image. The agency, however, has the authority to grant itself extensions, and the current deadline is now May 31, 2013.
So far, only the other type of scanner that TSA uses has been upgraded. Called millimeter-wave scanners, they resemble a large glass phone booth and use radio frequencies instead of X-rays to detect objects concealed beneath clothing.
The scan is processed by software instead of an airport security worker. If the software identifies a potential threat, a mannequin-like image is presented to the operator showing yellow boxes over areas requiring further inspection, by a pat-down for example.
Besides eliminating privacy concerns, the machine requires fewer people to operate, takes up less space in crowded security zones and completes a scan in less than two seconds, allowing screening lines to move faster.
"It's all done automatically to look for threats, so you don't have anybody in a back room that has to look at the imaging," said Doug McMakin, who led the team that developed the millimeter-wave technology at the Pacific Northwest National Laboratory.
The TSA did not announce the change until after news reports revealed it last week. In a statement, officials said speed was the reason for the switch to the millimeter-wave machines.
In addition to speed and space advantages, the millimeter-wave technology does not produce the ionizing radiation that has led to safety concerns with the X-ray machines, which required passengers to stand between two refrigerator-sized boxes.
The TSA and other experts have said the amount of radiation is less than what passengers get on the flight itself.
A TSA spokesman would not say whether the change was the beginning of a phase-out for the X-ray scanners. The agency said in the statement that it was confident both types of machine could ensure passenger safety.
The government began deploying both types to airports in 2010 after a foiled al-Qaida plot to bomb a U.S.-bound jet using explosives that can be missed by traditional metal detectors.
The scanners can cost as much as $170,000 each. There are currently about 800 of them at 200 U.S. airports. About two-thirds of them are the millimeter-wave machines.
The TSA has spent nearly $8 million developing the upgraded privacy software and plans to spend more as it works to develop software for the backscatter machines, according to a September report by the House Homeland Security Subcommittee on Transportation Security.
The committee's Republican chairman, Rep. Mike Rogers of Alabama, said the TSA needs to be more forthcoming about when it will have that upgrade "rather than simply shuffling" the machines from one airport to another.
"Travelers deserve to see a concrete timeline for implementing privacy software on all (scanning) machines and a commitment from TSA to sponsor an independent analysis of their potential health impact," he said.
Aviation expert Robert Poole of the Reason Foundation said it made sense to switch to the millimeter-wave scanners at busier airports, noting that "the faster processing time is a huge advantage."
"But it still seems like a very poor decision to still be foisting those flawed machines — or certainly less good machines — on people in the smaller airports," he said.