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.