X-raying passengers for airline security became a lot less common in 2012.

The use of radiation by security agencies, especially at airport checkpoints, was the subject of a ProPublica series in late 2011 and early 2012.

The investigation found that the Transportation Security Administration had glossed over the small cancer risk posed by even low doses of radiation. The stories also showed that the United States was almost alone in the world in X-raying passengers and that the Food and Drug Administration had gone against its own advisory panel, which recommended the agency set a federal safety standard for security X-rays. In addition, ProPublica reported that, outside airports, other security agencies are exposing people to radiation in more settings and in increasing doses.

Now, many of the TSA's 250 X-ray body scanners worth about $14 million are sitting in a Texas warehouse after being removed from most of the biggest U.S. airports, including Los Angeles, Chicago O'Hare, New York's John F. Kennedy, Boston Logan, Charlotte Douglas and Orlando.

The TSA said it replaced the X-ray machines with scanners that use another technology, millimeter waves, to make the lines move faster, allowing the agency to screen more passengers for explosives. But the result, intended or not, is that far fewer airline passengers are being exposed to radiation during screening. Millimeter waves, a form of high-frequency radio waves like those used in cell phones, have not been shown to cause cancer.

The manufacturer of the X-ray scanners, Rapiscan Systems, has also faced problems in developing its privacy software. Such software produces a generic cartoon image of passengers' bodies, allaying privacy groups' complaints that the scans amount to a "virtual strip search." The TSA faces a June 2013 congressional deadline to install the software on all its body scanners.

In November, the TSA sent Rapiscan a "show cause letter," which is typically issued when the government is considering terminating a contract. The agency hasn't said why. Rapiscan said the letter questioned whether the company changed the machine in a way that didn't conform with the design the TSA approved. Rapiscan says it did conform.

Rep. Mike Rogers, the Republican head of the House transportation security subcommittee, cited an allegation that Rapiscan had falsified a software test, which the company denies.

Following months of congressional pressure, in December, the TSA agreed to contract with the National Academy of Sciences to evaluate the health effects of body scanners. A provision to require such a test was included in the Homeland Security funding bill that passed the Senate appropriations committee in May; the final bill has not yet passed. It is not clear if the proposed study will add much to what is already known about the scanners, because it's unclear if the academy will conduct new tests of the machines or merely review previous studies.

Passengers traveling through Seattle-Tacoma, Phoenix Sky Harbor, Washington Dulles and several other airports still must pass through an X-ray scan or opt out and receive a pat-down search.

The TSA says it hopes to eventually move the scanners from storage to smaller airports after resolving the issue with Rapiscan. In addition, the agency is considering an X-ray machine made by another company under a contract for the next generation of body scanners.

The last X-ray scanners in use in Europe were removed from Manchester Airport in the United Kingdom in September. Israel, which is small but influential in the security world, has installed an X-ray body scanner for testing at Ben Gurion Airport in Tel Aviv.

A side-by-side comparison of the TSA's body scanners, including photographs of them, can be found here. And here are some key points about the two types of scanners:

Safety: The X-ray machine, known as the backscatter, uses ionizing radiation, which has long been linked to cancer. According to many studies, the dose of the machine is very small, equivalent to the cosmic radiation received in a few minutes of the flight. The TSA cites those studies in claiming they're safe. The National Academy of Sciences has concluded that there is no known dose of radiation that does not increase the risk of cancer, and radiation groups recommend that the public limit its exposure as much as reasonably possible.

Although there has been some doubt about the long-term safety of millimeter waves, scientists have not found a mechanism for such waves to mutate genes and cause cancer.

Privacy: The millimeter wave machine contains privacy software that scans a passenger's body for anything unusual that might be hidden under his or her clothes. It then creates a generic image of a body and highlights any potential threat with a yellow box. No human being analyzes the image; it is all automated.

The manufacturers of the X-ray scanner are working on similar software. But for now, the machine creates a heavily filtered image of the person's naked body, which is viewed in a separate room by a TSA screener who cannot see the passenger.

Detection: Federal officials have released no information about the detection rates of the two machines. Security experts say that in their original forms, the image of the X-ray machine was clearer than that of the millimeter wave machine. But any difference was made minimal through training and now by the computer algorithms that automatically scan the passenger, they say. Government inspectors have repeatedly found "vulnerabilities" with the machines, but to what degree is not known.

False alarms: Based on reports and interviews with foreign officials, the millimeter wave machine has a much higher false-alarm rate than the X-ray scanner, tripping on innocuous things such as folds in clothing, ties and even sweat. Those false alarms require a quick search of the area where the anomaly was detected, whereas alarms with the X-ray scanner usually require a full-body pat-down.