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At the Denver International Airport, a passenger goes through a millimeter-wave scanner. (Craig F. Walker/The Denver Post)

Detecting Danger

New high-tech scanners are screening for possible threats to travelers

By Karina Hamalainen | May 9, 2011

This summer, if you took a flight for a vacation, you might have gone through a new full-body scanner at an airport security checkpoint. These high-tech scanners can detect concealed metal weapons, like knives and guns, as well as nonmetal threats, like plastic explosives.

The scanners are the latest devices in the terrorist-fighting arsenal managed by the Transportation Security Administration (TSA), the government agency charged with ensuring the safety of transportation in the United States. “The TSA is faced with a constantly evolving threat and must remain flexible as an agency to adapt quickly to stay ahead of our adversaries,” says TSA spokesperson Michael McCarthy. But some people question the science behind the new scanners and whether their use violates personal privacy.


After the Wright brothers made their first flight in 1903, terrorists quickly realized that airplanes could be hijacked, or stolen. The first airplane hijacking was in 1931 in Peru, and many more followed until they peaked in 1969. That year had a record 82 hijackings worldwide. This prompted the introduction of metal detectors in airports in the early 1970s. Changes in airport security often occur after a new terrorist plot unfolds.

The tragic events of September 11, 2001, prompted officials to revolutionize airport security. On that day, terrorists hijacked four commercial airplanes. Two of the planes struck the World Trade Center in New York City and caused the Twin Towers to collapse. A third plane crashed into the Pentagon just outside Washington, D.C. Passengers and crew members on the fourth plane fought back, but the plane crashed in a field in Shanksville, Pennsylvania. None of the passengers on any of the flights survived, and many more people died in the wreckages. Osama bin Laden, the leader of the militant Islamic group called Al Qaeda, claimed to have organized the plot that took so many innocent lives. U.S. forces killed bin Laden this May in Pakistan.

The terrorist attacks of 9/11 exposed holes in airport security and led to the creation of the Transportation Security Administration in November 2001. Since then, the TSA has been ramping up security in U.S. airports in an attempt to stay at least one step ahead of potential terrorists.


Over the past two years, airports worldwide have been adding full-body scanners to their security checkpoints. The high-tech devices use electromagnetic waves to peer through a person’s clothing to detect hidden objects. The TSA is using two different types of scanners: backscatter and millimeter-wave scanners. Each uses a different region of the electromagnetic spectrum to produce detailed images of passengers.

Backscatter scanners use high-energy X-rays similar to those used by doctors. But there’s one big difference: A doctor’s X-rays transmit through your skin and muscle tissue; whereas the airport’s backscatter X-rays bounce off your skin and any concealed items. The way in which the X-rays return to the scanner’s detectors provides information about where a hidden object is and of what type of material it is made.

The other type of scanner uses millimeter waves, which have wavelengths that fall between infrared and microwave. Much like the X-rays in backscatter scanners, the millimeter waves bounce off your skin and any hidden objects. “We detect those signals and form a three-dimensional image,” says Doug McMackin, an engineer at Pacific Northwest Laboratory in Richland, Washington, whose team developed the millimeter-wave technology.


Although backscatter scanners are effective at detecting hidden objects, many people worry about adverse health effects that could result from exposure to extra high-energy rays, which can damage cells. (Millimeter wave energy is much lower than that of X-rays, so it poses fewer health concerns.) Kelly Classic, a health physicist at the Mayo Clinic in Rochester, Minnesota, explains that this fear is unfounded for the average traveler. That’s because flying at 30,000 feet exposes you to X-rays from the sun anyhow, so the amount of X-rays beamed at you during the scan is the same as tacking a few extra minutes onto a flight from New York to Los Angeles.

Some people, like Kate Hanni, founder of the advocacy group, also think that the ultra detailed images generated by the scanners are an invasion of personal privacy. Although the checkpoint guards don’t actually see the scanned images, a TSA employee in a separate, secure room does.

McMackin believes that the scanners are actually less of an invasion of privacy than older technology. “The nice thing about this system is that you can identify exactly where the threat is, as opposed to metal detectors, where if it goes off, they search your whole body,” he says.

Still, the companies that produce both types of scanners are working to protect air passengers’ privacy further by tweaking their technologies. In future scanners, computer algorithms (mathematical formulas) will interpret the data and flag only hidden objects rather than show an image of the person. McMackin says, “There’s lots of techniques we can use to remove privacy concerns while keeping people safe from advanced threats.”

This article originally appeared in the September 5, 2011, edition of Science World magazine.

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