The Transportation Security Administration is planning to roll out body scanners at nearly every airport security lane in the country by 2014. Right now, it has deployed more than 500, split about evenly between two technologies -- one using X-rays and another using radio frequency waves.

Several prominent radiation safety experts have raised concerns about exposing millions of airline passengers to X-rays. While the dose is extremely low, research studies have estimated that six or 100 cancer cases may result among the 100 million passengers who fly every year. In 1998, a Food and Drug Administration advisory committee recommended a federal safety standard for the X-ray scanners. But the agency decided to go with a voluntary standard set by an industry panel made up mostly of manufacturers and government agencies that wanted to use the machine.

The TSA says tests show the machines are safe, but several scientists have pointed to flaws in those tests. Facing doubts, TSA administrator John Pistole agreed to conduct a new independent study of the health effects but backed off after receiving a draft report on the machines by the Department of Homeland Security’s inspector general. Meanwhile, the European Union prohibited the use of X-ray body scanners “in order not to risk jeopardizing citizens' health and safety."

The body scanners can only detect explosives hidden on the body; they cannot detect any that might be implanted inside the body.