What risk is there with airport full-body screening?

A paper just published in the Archives of Internal Medicine puts the matter in perspective with a review of the science involved in analyzing risk associated with airport full-body scanners. The authors state:

"There are 2 types of full-body scanners in use. Each generates a detailed outline of the human body for the purpose of identifying contraband hidden under clothing. The millimeter-wave scanners emit extremely low-energy waves—each scan delivers a small fraction of the energy of a cell phone—and the scanners capture the reflected energy. The backscatter x-ray scanner, the type used more commonly in the United States, uses very low dose x-rays, similar to those used in medical imaging."

The backscatter x-ray scanner is the type that has elicited the most concern. However...

"In contrast to x-rays used for medical imaging in which variation in the transmission of x-rays through the body is used to generate an image, backscatter scanners detect radiation that reflects off of the person imaged. When radiation passes through air, it deposits energy into the tissue that absorbs it, and with the backscatter technology, all of the energy of the scan is absorbed by the most superficial tissues of the body, such as the skin."

In respect to the two linked but factors of dose and risk there is relevant evidence to consider:

"According to a recent report from the National Council on Radiation Protection and Measurements, individuals in the United States are exposed to an average of 6.2 millisieverts of ionizing radiation annually, or approximately 0.01 microsievert (µSv)/min. The 2 most common sources of exposure are medical procedures and ubiquitous background radiation, sometimes described as natural sources of radiation, including radiation from the sun and cosmic rays, and radiation from radon that is released from the earth. The backscatter x-ray scanners expose individuals to 0.03 to 0.1 µSv per scan or the equivalent to 3 to 9 minutes of radiation received from sources naturally occurring as part of daily living...The radiation associated with a flight will vary with altitude and latitude, but overall, air travel is associated with an exposure of approximately 0.04 µSv/min of flight time. The backscatter x-ray scans deliver radiation equivalent to around 1 to 3 minutes of flight time."

Moreover...

"Put into context of the entire flight, if a woman embarks on a 6-hour flight, she will be exposed to approximately 14.3 µSv of radiation from the flight and 0.03 to 0.1 µSv from passing through the scanner at the airport. Thus, the scan will increase her exposure by less than 1%."

To put this in further perspective:

"An individual would have to undergo more than 50 airport scans to equal the exposure of a single dental radiograph, 1000 airport scans to equal the exposure of a chest radiograph, 4000 airport scans to equal the exposure of a mammogram, and 200 000 airport scans to equal the exposure of a single abdominal and pelvic computed tomographic scan."

Regarding the cancer risk posed by the use of airport full-body scanners...

"Estimating the risk associated with these extremely low dose exposures is more difficult than quantifying the exposure. Published studies that have demonstrated an association between radiation exposure and cancer risk have been performed at doses that are much higher than the levels emitted by the scans. To estimate the risk of these scans we must rely on extrapolation from these higher-dose studies, yet extrapolation of cancer risks from high doses to the exceedingly small doses of these scans is questionable and may be inappropriate."

The authors selected 5 year old girls as a cohort to focus on for their analysis. They were chosen because...

"......children are more sensitive than adults to the effects of radiation, and we can use existing models to accurately estimate the risk of breast cancer from these scans...Several groups have developed models to allow estimation of the risk of cancer following exposure to ionizing radiation, and we used their estimate of an increase of approximately 0.08 cancers per sievert of exposure..."

When the data on 5 year old girls is entered into the equation here's how it comes out:

"The breast dose for the backscatter scans is 0.049 µSv per scan, and the risk of breast cancer increases by 9140 cases per 100 000 five-year-old girls exposed to a sievert of radiation. We estimate that for every 2 million girls who travel 1 round trip per week, 1 additional breast cancer could occur from these scans over their lifetime. This increase of 1 cancer per 2 million young girls needs to be put in the context of the 250 000 breast cancers that will occur in these girls over the course of their lifetimes owing to the 12% lifetime incidence of breast cancer."

The authors thus conclude:

"Based on what is known about the scanners, passengers should not fear going through the scans for health reasons, as the risks are truly trivial. If individuals feel vulnerable and are worried about the radiation emitted by the scans, they might reconsider flying altogether since most of the small, but real, radiation risk they will receive will come from the flight and not from the exceedingly small exposures from the scans."

There is, however, a caveat associated with maintenance of the equipment:

"...if the machines do not function as provided, or if the settings are changed by employees after the machines are installed, or if they undergo software or mechanical errors or malfunctions, unknown effects could result...It would seem prudent for the TSA to permit additional testing to verify the safety of the devices."