ProPublica reports that the controversial “backscatter” scanners have been replaced with non-radiation-emitting “millimeter-wave scanners” at airports in  Chicago, Boston, Los Angeles, New York and Orlando.  Speed — not safety — was the reason cited by the Transportation Safety Administration for the switch.

The X-ray scanners have faced a barrage of criticism since the TSA began rolling them out nationwide after the failed underwear bombing on Christmas Day 2009. One reason is that they emit a small dose of ionizing radiation, which at higher levels has been linked to cancer.

In addition, privacy advocates decried that the machines produce images, albeit heavily blurred, of passengers’ naked bodies. Each image must be reviewed by a TSA officer, slowing security lines.

The replacement machines, known as millimeter-wave scanners, rely on low-energy radio waves similar to those used in cell phones. The machines detect potential threats automatically and quickly using a computer program. They display a generic cartoon image of a person’s body, mitigating privacy concerns.

It is unclear how many of the scanners will be replaced or where. Some of the X-ray scanners will be  redeployed in smaller airports. 

To compare the two types of scanners, check out this side-by-side summary from ProPublica.