Heating of tissues by microwaves: a model analysis.

We consider the thermal response times for heating of tissue subject to nonionizing (microwave or infrared) radiation. The analysis is based on a dimensionless form of the bioheat equation. The thermal response is governed by two time constants: one (tau1) pertains to heat convection by blood flow, and is of the order of 20-30 min for physiologically normal perfusion rates; the second (tau2) characterizes heat conduction and varies as the square of a distance that characterizes the spatial extent of the heating. Two idealized cases are examined. The first is a tissue block with an insulated surface, subject to irradiation with an exponentially decreasing specific absorption rate, which models a large surface area of tissue exposed to microwaves. The second is a hemispherical region of tissue exposed at a spatially uniform specific absorption rate, which models localized exposure. In both cases, the steady-state temperature increase can be written as the product of the incident power density and an effective time constant tau(eff), which is defined for each geometry as an appropriate function of tau1 and tau2. In appropriate limits of the ratio of these time constants, the local temperature rise is dominated by conductive or convective heat transport. Predictions of the block model agree well with recent data for the thresholds for perception of warmth or pain from exposure to microwave energy. Using these concepts, we developed a thermal averaging time that might be used in standards for human exposure to microwave radiation, to limit the temperature rise in tissue from radiation by pulsed sources. We compare the ANSI exposure standards for microwaves and infrared laser radiation with respect to the maximal increase in tissue temperature that would be allowed at the maximal permissible exposures. A historical appendix presents the origin of the 6-min averaging time used in the microwave standard.

Evaluation of developmental toxicity data: a discussion of some pertinent factors and a proposal.

There is currently no well-accepted standard method for evaluation of developmental toxicity data. This paper presents one approach to the evaluation of developmental toxicity data. We initially identify some pertinent factors that influence the interpretation of animal data and summarize the literature pertaining to these factors. Such factors include the quality and quantity of data and the relationship between maternal and developmental toxicity. We proceed with a discussion of quantitative assessment of data and propose schemes for qualitative and quantitative developmental hazard assessments.

Potential ocular hazards from xenon flashlamps.

High radiance optical sources used in fast photocopiers represent a potential hazard to the human retina. Primates (macaque monkeys) were exposed to a prototype flashtube assembly similar to that used in fast photocopiers. One animal trained in a restraint chair to perform a visual task for a major portion of his food was exposed to multiple pulses (1000 pulses daily) from two xenon flashtubes placed 22 cm from the eyes. After sixty daily exposures over a period of three months with pupils dilated to 8 mm or greater, no anomalies were detectable in the retina. Examination for defects with the fundus camera was negative even a year after the exposures. Four additional monkeys (eight eyes) were exposed under anesthesia to insure that repetitive exposures on the retina were coincident. Exposure to two xenon flashtubes, each with 160-J input, did not produce a retinal lesion after 4200 flashes spaced 1.7 s apart. Exposure to a single flashtube with 540-J input produced negative results even after fifty flashes focused on the same retinal site. It was concluded that neither of these optical sources was capable of producing a thermal lesion in the monkey retina. Calculations predict that a photochemical-type retinal lesion is possible but only in extraordinary conditions of exposure which would be extremely unlikely, if not impossible, while viewing a photocopier.