Objections to full-body scans at airports have generally centered on privacy concerns. But more serious may be questions about the potential health effects of the radiation emitted by backscatter machines, one of the two types of scanners being used by the U.S. Transportation Security Administration (TSA). The agency’s other type of technology is active millimeter wave.
As of late 2010, the TSA had placed 242 backscatter and 222 millimeter wave units in the field. The agency plans to purchase 500 additional machines in 2011, but has not yet determined how procurement will break down.
Both the active millimeter wave and backscatter machines operate on the same physical principle: the waves emitted by the machines pass through the wearer’s clothing and are absorbed by the skin but are reflected by potential threat objects the person may be carrying, such as ceramic knives and plastic-based explosives, which might get past metal detectors.
Millimeter wave machines employ radiation in the radio band, which does not pose any known ill-health risks. Backscatter machines, however, use small amounts of ionizing radiation, which has been determined to be a contributing factor in causing cells to become cancerous.
Increased exposure brings added risk, and is one critical issue. The TSA asserts that the exposure per single backscatter scan is low. Two leading independent scientific organizations have backed that assertion, quantifying the TSA backscatter scans’ exposure in context.
The National Council on Radiation Protection and Measurement reports that to meet its own suggested limit for annual radiation exposure in security screening would require 2,500 backscatter scans. The American College of Radiology, meanwhile, reported that it would take 1,000 scans to reach the exposure of one medical chest x-ray. In the aviation context, a scan is equal to two minutes in an airliner at 30,000 feet, where exposure to natural cosmic radiation is elevated.
Critics, however, have raised other concerns, including the lack of research into the health effects of cumulative low-dose radiation exposure and full absorption of the machines’ radiation by the skin, as opposed to the body’s entire mass.
Speaking to the Congressional Biomedical Research Caucus on the topic of backscatter screening technology, researcher David J. Brenner of the Center for Radiological Research at Columbia University Medical Center explained that the scans might not pose any notable statistical risk to any one individual, but could pose a public health risk across society. Brenner noted particular concern for basal cell carcinoma. This skin cancer is highly common, with roughly 800,000 cases per year.
Due to sun exposure, 85 percent of cases occur on the neck and head. Further, exposure to ionizing radiation has been shown to elevate skin cancer risk, especially when the exposure occurs during youth. The latter phenomenon was observed in people exposed to the atomic bomb explosions in Hiroshima and Nagasaki as children, he noted.
Brenner proposes that travelers’ necks and heads should not be exposed to backscatter waves, given the difficulty of concealing threats in those areas. He also says that only millimeter wave machines should be used.
Responding to a query from U.S. Senator Susan Collins (R-ME), senior Republican on the Senate Homeland Security and Governmental Affairs Committee, TSA Administrator John Pistole argued that the security risk mitigation provided by the machines far outweighs the risks to the health of screened travelers.
Administratively, skeptics voice two more concerns: the TSA’s poor track record of properly monitoring potential radiation exposure from screening machines, specifically carry-on baggage x-ray machines; and the government’s overall lack of diligence and candor about other health and environmental affects, such as from Agent Orange.
More recently, U.S. Rep. Edward Markey (D-MA) requested an explanation of the agency’s plans to monitor the safety of the full-body scanners for travelers and for TSA screeners. The TSA rebuffed an earlier request for machine inspection records submitted by USA Today.