kind of x-ray photons as are produced in medical x-ray machines but expose the subject at a considerably lower dose, so it is possible that the results from medical radiology may be relevant, at least until a study is done of any effects specific to backscatter x-ray machines. Fathers exposed to medical diagnostic x-rays are more likely to have infants who contract leukemia, especially if exposure is closer to conception or includes two or more X-rays of the lower gastrointestinal (GI) tract or lower abdomen. In medical radiography the x-ray beam is adjusted to expose only the area of which an image is required, so that generally shielding is applied to the patient to avoid exposing the gonads, whereas in an airport backscatter scan, the testicles of men and boys will be deliberately subjected to the direct beam in order to check for weapons in the underpants, and some radiation will also reach the ovaries of female subjects. A linear dose-response relationship has been observed between x-ray dose and double-strand breaks in DNA in human sperm. Extrapolations of cancer risk from minuscule exposures to radiation across large populations, however, are not supported by analysis by the National Council on Radiation Protection (NCRP). On May 26, 2010 NCRP issued a press release to address such comments about full body scanners that are compliant with ANSI N43.17. In Commentary No.16 issued on May 26, 2010, it reads as follows: According to NCRP, the use of statistical extrapolations that predict 1 death for every 200 million persons scanned for example (as above) is an unrealistic over-estimation. Other scientists at Columbia University have made the following statements in support of the safety of body scanners: Furthermore, other scientists claim the health effects of backscatter are well understood whereas those from millimeter wave scanners are not: Experts evaluating backscatter x-ray machine technology have also argued that defects in the machines, damage from normal wear-and-tear, or software errors could focus an intense dose of radiation on just one spot of the body. For example, Dr. Peter Rez, a professor of physics at Arizona State University, has said, "The thing that worries me the most, is not what happens if the machine works as advertised, but what happens if it doesn't", adding that a potential malfunction of the machine could increase the radiation dose. The designers and manufacturers of backscatter X-ray scanners claim that the scanners are