(Michael Grabell, ProPublica) This is part of our year-end series, looking at where things stand in each of our major investigations.
It has become routine for airline passengers across the country: Instead of walking through a metal detector, they now step into a body scanner, hold their arms over their heads and wait until a machine peers through their clothing to make sure they're not hiding explosives.
The Transportation Security Administration has deployed more than 500 of the body scanners, which they call "advanced imaging technology." And the agency plans to install them at nearly every security lane by 2014.
The TSA has insisted that the new scanners present "no health or safety concerns for any passenger." The agency has said they have been used around the world. And it has reiterated that the machines were evaluated by the Food and Drug Administration, leading many to believe that one of the government's top safety regulators approved the technology.
But a ProPublica/PBS NewsHour investigation this year detailed how the TSA had glossed over cancer concerns about one kind of scanner that uses X-rays. In independent, peer-reviewed studies, radiation experts concluded that the X-ray scanner could cause six to 100 airline passengers each year to develop cancer. Outside the United States, few countries use X-ray imaging machines, also known as backscatters, in their airports. And the FDA has no authority to approve body scanners before they are sold because they are electronic products, not medical devices.
In 1998, an FDA advisory panel recommended a federal safety standard for the X-ray scanners. But the agency decided to go with a voluntary standard set by an industry group made up mostly of manufacturers and government agencies that wanted to use the machine.
In November, the European Union decided to prohibit X-ray body scanners in European airports. In the United States, members of Congress have pushed the TSA to conduct a new, independent safety review. And in Florida earlier this month, Broward County commissioners voted  to demand the TSA prove that the X-ray imagers at Fort Lauderdale-Hollywood International Airport are safe.
The TSA uses two types of body scanners:
The X-ray scanner emits extremely low levels of ionizing radiation, a form of energy that strips electrons from atoms and damages DNA, potential leading to cancer. That risk, although small, has led some prominent scientists to ask why the TSA doesn't use just the millimeter-wave scanner, which uses low-powered electromagnetic waves that have not been linked to adverse health effects.
The TSA has said that keeping both technologies in play encourages the manufacturers to improve detection capability while lowering the cost for the taxpayer. The agency says the X-ray machine is safe because the radiation is equivalent to the amount passengers receive in two minutes of flying at altitude.
But ProPublica found some potential problems with the millimeter-wave scanner. Several other countries have reported a high rate of false alarms caused by innocuous things, such as folds in clothing, buttons and even sweat.
Other studies and a congressman briefed on classified tests have suggested the scanners could miss carefully concealed plastic explosives like the weapon used by the underwear bomber on Christmas Day 2009.
As Congress continues to debate the safety and quality of the body scanners, government investigators are set to release two important reports in the new year. The inspector general of the Department of Homeland Security is evaluating how well the TSA is monitoring the radiation of the backscatters. Meanwhile, the Government Accountability Office is wrapping up an investigation of the machines' detection capability, the results of which are likely to be classified.