would not be concerned about it from a radiation dose standpoint – the issues of personal privacy are a different thing". The health effects of the more common millimeter wave scanner are largely unknown, and at least one expert believes a safety study is warranted. "I am very interested in performing a National Council on Radiation Protection and Measurements study on the use of millimeter-wave security screening systems", said Thomas S. Tenforde, council president. However, no long-term studies have been done on the health effects of millimeter wave scanners. Perhaps the most notable and debated professional opinion in regard to the safety of scanners is the so-called "Holdren Letter" from a number of world-renowned biochemists and biophysics researchers from the University of California to the Assistant to the US President for Science and Technology, Dr. John P. Holdren. The opening paragraph of their letter of concern reads: "We, a number of University of California, San Francisco faculty, are writing—see attached memo—to call your attention to our concerns about the potential serious health risks of the recently adopted whole body back scatter X-ray airport security scanners. This is an urgent situation as these X-ray scanners are rapidly being implemented as a primary screening step for all air travel passengers." Critics of backscatter X-ray scanners, including the head of the Center for Radiological Research at Columbia University, say that the radiation emitted by some full-body scanners is as much as 20 times stronger than officially reported and is not safe to use on large numbers of persons because of an increased risk of cancer to children and at-risk populations. Researchers at the University of California, San Francisco, (UCSF) have argued that the amount of radiation is higher than claimed by the TSA and body scanner manufacturers because the doses were calculated as if distributed throughout the whole body, but the radiation from backscatter X-ray scanners is focused on just the skin and surrounding tissues: The majority of [the scanners'] energy is delivered to the skin and the underlying tissue. Thus, while the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high. The X-ray dose from these devices has often been compared in the media to the cosmic ray exposure inherent to airplane travel or that of a chest X-ray. However, this comparison is very misleading: both the