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.

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