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/cm², 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/cm²). 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.