How Safe is that Airport Body Scan?

When you’re standing in line at the airport, putting your shoes in bins and taking loose change out of your pockets, here’s a conversation starter: How safe is that radiation exposure you’ll get walking through the whole-body security scanner?

That’s the issue discussed in a pair of articles to be released in April’s print issue of Radiology. Are airport whole-body backscatter X-ray units safe?

“In brief, yes and no,” said David J. Brenner, PhD, DSc, director of the Center for Radiological Research at Columbia University Medical Center in New York in his article.

Yes, the whole-body advanced imaging technology (AIT) units are the controversial machines that provide holographic renderings of the body underneath that pesky clothing. They use either backscatter or millimeter wave technology to capture the image.

Backscatter technology involves scanning the body with a pencil beam of 50 or 120-kVp X-rays (depending on the manufacturer). Millimeter wave units use low-power (non-ionizing) millimeter waves, and the reflected signals are analyzed with a holographic imaging algorithm. Most machines in the U.S. currently use backscatter technology, though the assumption is that both types of machines produce similar results.

In his article, Brenner focused on the radiation safety of the backscatter scanners. He noted that individual risks from the scan are sufficiently low enough - “at most, an extremely small cancer risk” - to call it “safe” for the majority of those who only fly a few times a year. Risk would be higher for frequent flyers and airline employees, who potentially go through the scanners hundreds of times a year.

As for that cancer risk, it’s a tough one to pin down because the radiation amount from a single scan is so low. Brenner said that according to some estimations, the fatal cancer risk from going through a backscatter scanner twice a year is one in 10 million. Using the same set of assumptions, he wrote that getting scanned before 200 or more flights per year could result in a one in 100,000 fatal cancer risk. Frequent flyers might want to opt for a pat-down instead.

In a companion Radiology article, David A. Schauer, ScD, CHP, executive director of the National Council on Radiation Protection and Measurements (NCRP), estimated that someone could go through a backscatter scan 2,500 times a year without exceeding the NCRP-recommended exposure for that type of scan.

The NCRP recommends that the backscatter units use an effective dose of no more than 0.1 microsieverts of ionizing radiation per scan, the same amount of radiation a passenger gets from flying at 30,000 feet for about two minutes. TSA’s website lists this is the dosing standard they use. The NCRP also notes that the average person in the U.S. is exposed to about 3.0 millisieverts yearly from naturally-occurring radioactive materials and cosmic radiation.

So both authors argue that the airport scanning risk to passengers is low. Even with a low risk, though, Brenner cautioned that the long-term consequences of the collective population getting scanned up to one billion times per year is unknown. By that, he means we should look at the scanning risks from a societal framework. A small individual risk could still mean that cancers could develop in the overall population as a result of the airport scanning. And he pointed out that there are other reasonable screening options that don’t use ionizing radiation. Like the millimeter wave technology.

Schauer dismissed collective risk to the population. He argued that converting the small individual risk into a public health concern is distorting, especially compared to risks we voluntarily and involuntarily accept daily. He noted that converting the individual risks based on the AIT radiation dose into a collective risk would be taking the statistics out of context, could be inaccurate, and were not intended for use that way.

Whole-body X-ray backscatter scanners have been used in U.S. airports since 2007, though in more limited capacity. After the Christmas 2009 attempted airline bombing, AIT scanner use was shifted from secondary to primary screening status. Currently 486 AIT units are used at 78 U.S. airports, according to TSA’s Web site.