Medical/biological study (experimental study)

Human exposure to 2450 MHz CW energy at levels outside the IEEE C95.1 standard does not increase core temperature med./bio.

By: Adair ER, Mylacraine KS, Cobb BL

Published in: Bioelectromagnetics 2001; 22 (6): 429-439

Aim of study (acc. to author)

To study human thermoregulatory effciency in radiofrequency environments.
Permission was received to exceed the peak power density (35 mW/cm²) the authors had previously studied during partial body exposure of human volunteers at 2450 MHz (publication 2287).

Background/further details

Two additional peak power densities were tested (50 and 70 mW/cm²) at each of three ambient temperatures (24, 28, and 31°C). The thermophysiological data were combined with comparable data at power densities of 0, 27, and 35 mW/cm² from the previous study to generate response functions across power densities.

Endpoint

thermoregulation: thermophysiological responses (local skin blood flow; heart rate; deep body core temperature; skin temperature; metabolic heat production; local sweating rate)

Exposure

- 2.45 GHz
- microwaves

Exposure	Parameters
Exposure 1: 2,450 MHz Modulation type: CW Exposure duration: continuous for 45 min	power density: 50 mW/cm² maximum SAR: 11 W/kg maximum (at the surface) power density: 70 mW/cm² maximum SAR: 15.4 W/kg maximum (at the surface) SAR: 1 W/kg average over mass (whole body)

General information

Details of the exposure system, field measurements, and dosimetry appear in two earlier publications [Adair et al., 1999].

Exposure 1

Main characteristics

Frequency	2,450 MHz
Type	electromagnetic field
Charakteristic	far field
Exposure duration	continuous for 45 min

Modulation

Modulation type	CW

Exposure setup

Exposure source	horn antenna anechoic chamber klystron amplifier, E-polarization
Chamber	The subject's immediate environment was climate controlled for temperature, relative humidity, and air movement.
Setup	Subjects wearing a bathing suit sat on a light plastic chair, facing the rear chamber wall.
Sham exposure	A sham exposure was conducted.
Additional info	After 30 min of equilibration to the prevailing temperature (24, 28, or 31 °C), the subjects were exposed or sham exposed for 45 min.

Parameters

Measurand	Value	Type	Method	Mass	Remarks
power density	50 mW/cm²	maximum	measured	-	-
SAR	11 W/kg	maximum	measured	-	at the surface
power density	70 mW/cm²	maximum	measured	-	-
SAR	15.4 W/kg	maximum	measured	-	at the surface
SAR	1 W/kg	average over mass	estimated	whole body	-

Additional parameter details

Power densities (PD) specified above were determined with the E-field probe placed on the antenna boresight at the location of the subject's center back. These PDs were verified every 2-3 weeks as the study progressed.

Measurement and calculation details

The PD was mapped extensively in the vicinity of the subject's chair [Adair et al., 1999]. A central 25 x 45 cm portion of a 65 x 80 cm plane at the location of the subject's back showed a 13% nonuniformity, with the chair absent. Dosimetric measurements were conducted on a 70-kg soft plastic mannequin filled with muscle equivalent material. The normalized maximum surface SAR of 0.22 W/kg per mW/cm² was calculated from the greatest temperature rise occurring at the lower back [Chou et al., 1996, publication 1466].

Reference articles

Adair ER et al. (1999): Human exposure at two radio frequencies (450 and 2450 MHz): similarities and differences in physiological response

Exposed system:

human
partial body: back

Methods Endpoint/measurement parameters/methodology

effects on the cardiovascular system: local skin blood flow at four sites (left forearm, right thigh, upper back, right chest); heart rate
thermoregulation: deep body (esophageal) temperature; skin temperature at six sites (ventral thigh, upper chest, forearm, upper and lower back, forehead); metabolic heat production (fractions of oxygen and carbon dioxide in the expired air); local sweating rate

Investigated system:

investigation on living organism

Investigated organ system:

cardiovascular system

Time of investigation:

during exposure
after exposure

Main outcome of study (acc. to author)

No change in deep body core temperature or metabolic heat production was recorded at any power density in any ambient temperature.
At a power density of 70 mW/cm², skin temperature on the upper back (irradiated directly) increased 4.0°C in ambient temperature of 24°C, 2.6°C in ambient temperature of 28°C, and 1.8°C in ambient temperature of 31°C. These differences were primarily due to the increase in local sweat rate, which was greatest in ambient temperature of 31°C.
Also at a power density of 70 mW/cm², local skin blood flow on the back increased 65% over baseline levels in ambient temperature of 31°C, but only 40% in ambient temperature of 24°C.
Although ambient temperature becomes an important variable when radiofrequency exposure exceeds the IEEE C95.1 guideline partial body exposure limits, vigorous heat loss responses of blood flow and sweating maintain thermal homeostasis efficiently.

Study character:

medical/biological study
experimental study
full/main study

Study funded by

not stated/no funding

Hirata A et al. (2010): Acute dosimetry and estimation of threshold-inducing behavioral signs of thermal stress in rabbits at 2.45-GHz microwave exposure
Adair ER et al. (2001): Partial-body exposure of human volunteers to 2450 MHz pulsed or CW fields provokes similar thermoregulatory responses
Adair ER et al. (1999): Human exposure at two radio frequencies (450 and 2450 MHz): similarities and differences in physiological response