Millimeter wavelength radiation is a subset of the microwave radio frequency spectrum. Even at its high-energy end, it is still more than 3 orders of magnitude lower in energy than its nearest radiotoxic neighbour (ultraviolet) in the electromagnetic spectrum. As such, millimeter wave radiation is non-ionizing and incapable of causing cancers by radiolytic DNA bond cleavage. Yet, the WHO (World Health Organization) IARC (International Agency for Research in Cancer) in 2011 classified non-ionizing radiation such as millimeter waves or microwaves, as 2B, which means possibly carcinogenic to humans. Due to the shallow penetration depth of millimeter waves into tissue (typically less than 1 mm), acute biological effects of irradiation are localized in epidermal and dermal layers and manifest primarily as thermal effects. There is no clear evidence to date of harmful effects other than those caused by localised heating and ensuing chemical changes (expression of heat shock proteins, denaturation, proteolysis, and inflammatory response, see also mobile phone radiation and health). It should be noted however that the energy density required to produce thermal injury in skin is much higher than that typically delivered in an active millimeter wave scanner.
The fragmented or misfolded molecules resulting from thermal injury may be delivered to neighbouring cells through diffusion and into the systemic circulation through perfusion. Increased skin permeability under irradiation exacerbates this possibility. It is therefore plausible that the molecular products of thermal injury (and their distribution to areas remote from the site of irradiation) could cause secondary injury. Note that this would be no different from the effects of a thermal injury sustained in a more conventional fashion. Due to the increasing ubiquity of millimeter wave radiation (see IEEE 802.11ad), research into its potential biological effects is ongoing.
Independent of thermal injury, a 2009 study funded by National Institute of Health, conducted by U.S. Department of Energy’s Los Alamos National Laboratories Theoretical Division and Center for Nonlinear Studies and Harvard University Medical School found that terahertz range radiation creates changes in DNA breathing dynamics, creating apparent interference with the naturally occurring local strand separation dynamics of double-stranded DNA and consequently, with DNA function.The same article was referenced by MIT Technology Journal article on October 30, 2009.
Millimeter wave scanners should not be confused with backscatter X-ray scanners, a completely different technology used for similar purposes at airports. X-rays are ionizing radiation, more energetic than millimeter waves by more than five orders of magnitude, and raise concerns about possible mutagenic potential.