The Transportation Security Administration says its full-body X-ray scanners are safe and that radiation from a scan is equivalent to what's received in about two minutes of flying. The company that makes them says it's safer than eating a banana.

But some scientists with expertise in imaging and cancer say the evidence made public to support those claims is unreliable. And in a new letter sent to White House science adviser John Holdren, they question why the TSA won't make the scanners available for independent testing by outside scientists.

The machines, which are designed to reveal objects hidden under clothing, have the potential to close a significant security gap for the TSA because metal detectors can't find explosives or ceramic knives, which can be just as sharp as the box cutters that hijackers used on 9/11.

They are also important for TSA's public relations battle over the alternative, the "enhanced pat-down," which has bred an epidemic of viral videos: A 6-year-old girl is touched from head to toe. A former Miss USA says she was violated. A software programmer warns a screener, "If you touch my junk, I'm going to have you arrested."

After the underwear bomber tried to blow up a Northwest Airlines plane on Christmas Day 2009, the TSA ramped up deployment of full-body scanners and plans to have them at nearly every security line by 2014.

There are two types of body scanners. Millimeter wave machines emit a radio frequency similar to cellphones. Backscatters work like a fast-moving X-ray. In the latter, the rays bounce off the skin and create a fuzzy white image of the passenger's body. Because the beam doesn't go through the body, most of its radiation is received by the skin.

The TSA says the backscatter technology has been evaluated by the Food and Drug Administration, the National Institute for Standards and Technology and the Johns Hopkins University Applied Physics Laboratory. Survey teams are using radiation-detecting dosimeters to check the machines at airports. The TSA says the results have all confirmed that the scanners don't pose a significant risk to public health.

According to the agency and many radiation experts, the dose is so low, even for children or cancer patients, that someone would have to pass through the machines more than a thousand times before approaching the annual limit set by radiation safety organizations.

But the letter to the White House science adviser, signed by five professors at University of California, San Francisco, and one at Arizona State University, points out several flaws in the tests. Studies published in scientific journals in the last few months have also cast doubt on the radiation dose and the machines' ability to find explosives.

A number of scientists, including some who believe the radiation is trivial, say more testing should be done given the government's plans to put millions of passengers through the machines. And they have been disturbed by the TSA's reluctance to do so.

"There's no real data on these machines, and in fact, the best guess of the dose is much, much higher than certainly what the public thinks," said John Sedat, a professor emeritus in biochemistry and biophysics at UCSF and the primary author of the letter.

The same group stirred controversy last year when it sent a letter to Holdren arguing that while the overall dose to the body may be low, the TSA hadn't quantified the dose to the skin. Last fall, FDA and TSA officials released a study that estimated the dose to the skin to be twice the dose to the body, though still extremely low.

In the most recent letter sent to Holdren on April 28, the professors note that the Johns Hopkins lab didn't test an actual airport machine. Instead, the tests were done on a model built by the manufacturer, Rapiscan, and configured to resemble a system previously tested by the TSA.

The researchers' names have been kept secret, and the report on the tests is so "heavily redacted" that "there is no way to repeat any of these measurements," they wrote.

The physics and medical professors also took issue with the device used to measure the radiation. Although the device, known as an ion chamber, is commonly used to test medical equipment, they argue that the detector gets overwhelmed by the amount of radiation the backscatter deposits in a short time and might not provide accurate readings.

Helen Worth, a spokeswoman for the Johns Hopkins lab, referred questions to the TSA.

Part of the trouble is that there is no ideal device for measuring the radiation dose given by backscatter X-rays, said David Brenner, director of the Columbia University Center for Radiological Research. The machines emit a pencil beam that rapidly moves across and up and down the body, he said.

"We are one of the oldest and biggest radiological research centers in the country, and we find this to be a very hard technical problem," said Brenner, who was not involved with the letter.

Another issue is that there is a lot of uncertainty with the model used to estimate cancer risk from radiation exposure to the skin, said Rebecca Smith-Bindman, a UCSF radiologist who also was not involved in the letter.

Smith-Bindman, who has testified before Congress about excessive radiation from medical scans, studied the TSA reports and said she wasn't concerned about the airport X-rays.

The risks are "truly trivial," she wrote in an article for the Archives of Internal Medicine. A passenger would have to undergo 50 airport scans to reach the level of a dental X-ray, 1,000 for a chest X-ray, and 4,000 for a mammogram.

Though imperfect, the available models predict that the backscatters would lead to only six cancers over the course of a lifetime among the approximately 100 million people who fly every year, Smith-Bindman concluded.

"There's really unnecessary fear related to these scans," she said. "What I'm not as comfortable with is that there has not been access to these machines. They are not being tested on the same regulatory basis that we see on medical equipment."

After her article was published, Smith-Bindman was contacted by a TSA public affairs officer. During the conversation, she suggested that she or other outside scientists be allowed to test the machine. The official was shocked by the suggestion and said such access could tip off people who want to avoid detection, Smith-Bindman said.

"It was not appreciating that there's legitimate scientific questions that have to be balanced against the security questions," she said.

The TSA did not respond to ProPublica's questions about why it wouldn't allow outside testing. But at a congressional hearing in March, Robin Kane, assistant administrator for security technology, said doing so would expose a lot of sensitive information the agency wouldn't normally share publicly. The machines had already been tested several times, he said, and if set up securely, the agency would allow more testing.

The available information leaves scientists with little to work with. Peter Rez, the Arizona State physics professor who signed the letter to Holdren, has tried to calculate the radiation by examining the handful of backscatter images that have been released publicly.

The Electronic Privacy Information Center, a civil liberties group, sued the Department of Homeland Security, TSA's parent agency, in federal court seeking release of 2,000 backscatter images used in testing. But it has not been successful.

The few images that have been made public do not reveal faces or detailed private features. The TSA says the images Rez used are out of date, but Rez says the current image on TSA's website is unusable.

Using the earlier images, Rez concluded in the Radiation Protection Dosimetry journal that it was highly unlikely the machines could have produced such high-quality images with doses of radiation as low as those described by TSA. He estimated the dose, while still very small, is 45 times higher than the results measured by Johns Hopkins.

Applying Rez's numbers, Brenner wrote a paper for the journal Radiology, estimating that 100 additional cancers would develop for every 1 billion scans.

For Rez, the real danger occurs if the machine stops in the middle of a scan, allowing the beam to focus on a tiny area for several seconds. Given that the backscatter works with a wheel rotating at a high speed, and that the agency plans to use the scanners continuously 365 days a year, mechanical failures are likely, he said.

The TSA says that the scanners have safety systems, such as automatic shutoffs and emergency stop buttons, that will kill the beam in the event of any problem that could result in abnormal radiation. How those fail-safe systems work isn't entirely clear.

When Johns Hopkins researchers visited the Rapiscan facility, the automatic termination appeared to work. But the full results of the shutoff tests are redacted.

What's more, the test system didn't have an emergency stop button.