The Transportation Security Administration is reanalyzing the radiation levels of X-ray body scanners installed in airports nationwide, after testing produced dramatically higher-than-expected results.

The TSA, which has deployed at least 500 body scanners to at least 78 airports, said Tuesday the machines meet all safety standards and would remain in operation despite a “calculation error” in safety studies. The flawed results showed radiation levels 10 times higher than expected.

At least one flier group, the Association for Airline Passenger Rights, is urging the government to stop using the $180,000 machines that produce a virtual-nude image of the body until new tests are concluded in May.

“Airline passengers have enough concerns about flying—including numerous ones about how TSA conducts its haphazard security screenings—so it is TSA’s responsibility to ensure passengers are not being exposed to unhealthy amounts of radiation,” Brandon Macsata, executive director of the group, said in a statement.

The Electronic Privacy Information Center has been a loud voice opposing the machines. Last week, it urged a federal appeals court to stop using them until further health studies were conducted. Marc Rotenberg, EPIC’s executive director, is expected to tell the same thing to a congressional panel Wednesday.

“The agency should have conducted a public rule making so that these risks could have been more carefully assessed,” according to a transcript (PDF) of his expected testimony before the House Committee on Oversight and Government Reform.

Still, the government said the results proved the safety of the devices. “It would appear that the emissions are 10 times higher. We understand it as a calculation error,” TSA spokesman Sarah Horowitz said in a telephone interview.

The snafu involves tests conducted on the roughly 250 backscatter X-ray machines produced by Rapiscan of Los Angeles, which has a contract to deliver another 250 machines at a cost of about $180,000 each. About 250 millimeter-wave technology machines produced by L-3 Communications of New York were not part of the bungled results.

Rapiscan technicians in the field are required to test radiation levels 10 times in a row, and divide by 10 to produce an average radiation measurement. Often, the testers failed to divide results by 10, Horowitz said.

“Certainly, the errors are not acceptable. It’s not every report. We believe the technology is safe,” she said. ”We’ve done extensive, independent testing. It doesn’t raise alarms in terms of safety.”

Rapiscan, in a letter to the TSA, admitted the mistake and is “redesigning the form” used by its “field service engineers” when surveying the Rapiscan Secure 1000 that is deployed to 38 airports.

“Oftentimes, the FSE will bypass the step of dividing by 10. While the resulting entry, at a pragmatic level, is understandable on its face and usable for monitoring purposes, the value, if read literally by persons unfamiliar with our system and the survey process, would imply energy outputs that are unachievable by the Secure 1000 Single Pose,” (PDF) Rapiscan wrote.

A recent Wired.com three-part series examined the constitutionality, effectiveness and health concerns of the scanners, which the TSA mandated as the preferred airport screening method in February 2009. A federal appeals court hearing EPIC’s lawsuit suggested last week it was not likely to halt the scanners’ use.

Among other things, the Wired.com series concluded that there was discord among the scientific community about the scanners’ health risks to humans, and that they were not tested with mice or other biological samples before being deployed.

The government, however, maintains a thousand screenings equal the amount of radiation of one standard medical chest X-ray.