Microwave irradiation and the blood-brain barrier

The mammalian blood-brain barrier (BBB) is an anatomically and physiologically defined system that is believed to play a role in regulating the fluid environment of the brain. Earlier reports of altered BBB function in small animals after brief exposure to weak microwave fields (30 to 1000 µW/cm
²
, averaged) are reviewed in the light of more recently reported studies that have generally yielded negative findings. The physiological data indicate that the tight junctions of the BBB's capillaries are loosened by microwaves only at high field strengths that markedly elevate brain temperature. Anatomical data reveal that the tight junctions apparently remain intact but that enhanced blood-to-brain vesicular transport of normally excluded tracer molecules occurs reversibly in small animals exposed for 2 to 8 h to cm waves at a moderate field strength (10 mW/cm
²
). Recent conceptual and technical advances in measurement of BBB function support the thesis that the early reports of altered permeability actually reflect microwave-induced changes in cerebral circulation. There is no warrant to conclude that brief exposure to weak microwave fields impairs cerebrovascular function, but the question of effects of long-term exposure, which have not been studied, is moot. Hiatuses of reported studies that need to be repaired are absence of dosimetric and thermometric measurements, near-exclusive use of small albinic species, and confounding of experimental endpoints by anesthetics and other sources of stress.