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Many investigators have reported that microwave radiation can interact with the basic genetic apparatus of cells and cause abnormal biologic expression.1-6 These reports have left varying degrees of doubt as to whether the absorbed radiation energy was enough to cause generalized heating and, if so, whether this heating caused a temperature rise in the system which thus produced the observed e f f e ~ t . ~ . ~ In 1968, Webb and Doddsa described a dramatic, although apparently transient, inhibition of cell growth in response to microwave exposure. This paper is very significant for two reasons. First, the irradiation temperature was 12°C below the optimum growth temperature for the test organism, Escherichia coli. For generalized heating to produce the observed inhibition, the sample temperature would have to increase by more than 14"C, but their temperature monitoring indicated a rise of less than 1°C. Second, the forward power, after passing through a frequency doubler to produce 136-GHz radiation, was much too low (estimated at 7 pW) to produce generalized heating of the sample. Because our experiments have been greatly influenced by Webb and Dodds' report, their results should be reviewed. Cells of E. coli B were irradiated in two growth stages, lag and log phases, and samples were obtained every 30 min and assayed for colony forming units (CFU), that is, for the ability to form colonies on nutrient broth. In FIGURE 1, these data are plotted as the equivalent number of cell doublings that occurred during the irradiation period, expressed as N , / N t = 2,, where N o and N, represent the CFU concentration a t zero and various irradiation times, respectively, and n denotes the number of cell doublings. The control cultures respond as expected for the two growth stages, whereas the irradiated cultures in lag phase do not divide and those in log phase divide only once. Because the assay required the cells to grow, these data can be interpreted as demonstrating the effects of a blockage of cell division or growth, which is removed when the cells are withdrawn from the microwave field. There are other interpretations of these data, but they will be discussed later. Our objective was to use Webb and Dodds' system to determine which microwave frequencies and wave patterns in the environment might affect biologic systems in a nonthermal way and thus merit further detailed study.

A versatile temperature controlled microwave exposure chamber has been designed especially for use in microwave bio-effects research. The device uses a thermoelectric heat pump to cool the air in the chamber below ambient and a resistive heater for above ambient operation. A unique feature of the exposure chamber is that the radiating antenna can be either inside or outside of the chamber.