Modification of tumor promotion in the mouse skin by exposure to an alternating magnetic field.

Some epidemiological studies have suggested that exposure to an alternating magnetic field may increase the incidence of some cancers. Our earlier study of carcinogenesis in mouse skin, indicated that exposure to a magnetic field (MF) alone did not promote the growth of tumors. In the present experiment, the ability of a MF to act as a tumor copromoter was investigated. The dorsal skins of female SENCAR mice (6-7-weeks-old) were treated with 10 nmol of 7,12-dimethylbenzanthracene (DMBA) to initiate the carcinogenic process and then tumor development was promoted, for 23 weeks, by weekly applications of 4.9 nmol (0.3 microgram) of 12-0-tetradecanoylphorbal-13-acetate (TPA). One group of 48 mice were exposed to a 60-Hz magnetic field of 2 mT (20 Gauss) for 6 h/day 5 days/week, while a similar group (48 mice) were sham exposed. After week 12, the percentage of mice with tumors and the mean number of tumors per mouse, were higher for the group exposed to MF. At week 18, for example, where the differences between field and sham groups were statistically significant, the percentage of mice with tumors were, respectively, 25% and 8% (P less than 0.05, Fisher exact) and, the mean yield of tumors 1.9 +/- 0.69 and 0.65 +/- 0.46 (mean +/- S.E.M.) (P less than 0.05, Wilcoxon). At week 23 these differences were no longer statistically significant.

Measurements of contact currents in radiofrequency fields.

Radiofrequency electromagnetic fields can affect human health not only by direct interactions but indirectly through induction of charges on isolated or poorly grounded conductive (metallic) objects located in these fields. A person who touches such an object may perceive a tingling or prickling sensation or heat, or experience pain or electric shock. For sufficiently large objects, these phenomena can occur at field strengths that are relatively low and below the health protection limits based on direct interactions. We describe a method and circuitry developed to evaluate steady-state contact currents that may flow through a person touching conductive objects and give a summary of experimental tests performed. The method is simple and viable for field tests aimed at preventing pain, shock, and burn hazards in radiofrequency electromagnetic fields except those related to spark discharges. The method is applicable up to about 30 MHz.

Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: I. Experimental design and exposure system.

The rationale for selection of an animal model, the experimental design, and the design and evaluation of an exposure system used in studies of 60-Hz magnetic fields are described. The studies were conceived to assay development of cancer and immune responsiveness in mice exposed to magnetic fields. The exposure system utilized a quadrupole-coil configuration to minimize stray magnetic fields. Four square-wound coil provided a uniform field within a volume occupied by 16 animal cages. The magnetic field had a mean flux density of 2 mT that varied less than +/- 10% within the volume occupied by animals' cages. The flux density decreased to less than 0.1 microT at a distance of 2 m from the coils. In each exposure system 32 animals could be housed in plastic cages.

Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: II. Tumor development and immune response.

This paper describes preliminary findings on the influence of 60-Hz (2-mT) magnetic fields on tumor promotion and co-promotion in the skins of mice. The effect of magnetic fields on natural killer (NK) cell activity in spleen and blood was also examined. Groups of 32 juvenile female mice were exposed to the magnetic field as described in part I. The dorsal skin of all animals was treated with a subthreshold dose of the carcinogen 7,12-dimethyl-benz(a)anthracene (DMBA). One week after the treatment, two groups were sham exposed (group A) or field exposed at 2 mT (group B) 6 h/day for 21 weeks, to test whether the field would act as a tumor promoter. No tumors developed in these two groups of mice. To test whether the magnetic field would modify tumor development by directly affecting tumor growth or by suppressing immune surveillance, two additional groups of mice were treated weekly with the tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) and then either sham exposed (group C) or field exposed (group D). The time to appearance of tumors was shorter (but not statistically so) in the group exposed to magnetic fields and TPA. Some differences in NK cell activity and spleen size were observed between the sham- and field-exposed groups.

Exposure to electromagnetic fields in arc welding.

Electric and magnetic fields at the power line frequency (60 Hz) and its harmonics have been measured around 22 arc welders. The electric field strengths in most cases were very low (1 V m-1) and only for one device reached 300 V m-1. Magnetic flux densities ranged from a few microT to a few hundred microT (a few A m-1 to a few hundred of A m-1). Several devices surveyed produced high magnetic flux densities (200-400 microT), which are rarely encountered in the environment and are representative of high occupational exposures.

Teratological assessment of exposure to time-varying magnetic field.

A teratological assessment was performed using rats that were exposed to an alternating magnetic field. The magnetic field had a sawtooth waveform similar to that produced by video display terminals (VDTs). Female rats were exposed 2 weeks prior to and throughout pregnancy at a rate of 7 h/day. Three intensities of magnetic field (5.7, 23 or 66 microT) were used. All of these field intensities were much greater than those to which VDT users are exposed. A slight but statistically significant decrease in maternal lymphocyte count for the highest intensity field was found as compared with the control group. However, the lymphocyte count was within the normal range, and the observed changes in hematological parameters were considered mild. No other maternal or fetal parameters that were examined showed a significant difference for any of the three field intensities. Where minor variations in skeleton development were observed they were known to be the common "noise" that appears in every teratological evaluation.

Induction heating and operator exposure to electromagnetic fields.

Alternating magnetic fields are used in industry for induction heating of metals and semiconductors. Relatively high power, typically of a few to a few hundred of kW is used, and a frequency of operation ranges from 60 Hz to a few tens of MHz. A survey of the magnetic field strengths to which the operators are exposed has shown that these exposures are, in many instances, high compared with recommended exposure limits.

Extremely low frequency electromagnetic emissions from video display terminals and other devices.

Video display terminals (VDTs) are in widespread use and can be found in practically every modern office. Assessment of possible health effects of electromagnetic emissions from these devices is necessary. The intensity of the magnetic field and the frequency spectrum of the emissions at extremely low frequencies (ELF) (5-500 Hz) were measured for a few VDTs and compared with the background field intensities and emissions from other devices. The emissions in the frequency range investigated are of such low intensities that they are very unlikely to have any biological effects, let alone represent a health hazard.

Operator exposure to radiofrequency fields near a hyperthermia device.

Radiofrequency-induced hyperthermia has recently gained acceptance in cancer therapy. One of a few commercially available systems is the Magnetrode, a device operating at 13.56 MHz with an output power up to 1000 W. Heating of the prescribed part of the patient's body is accomplished by one of the coil electrodes. To assess exposure to the operator of the device, the intensities of the electric and magnetic fields in the vicinity of three Magnetrode coils were measured, while a human phantom or a human being was in the coil. Exposure to the device operator is compared with the recommendations of the Canadian safety code and the recent ANSI recommendations.

Exposure to the operator and patient during short wave diathermy treatments.

Short-wave diathermy is one of the most popular treatment modalities used to relieve pain through localized heating. In Canada its use in hospitals is as frequent as that of ultrasonic therapy devices and about 10 times more popular than microwave diathermy. Intensities of the electric and magnetic fields around applicators of 7 different diathermy units were measured at the operator position and at various untreated areas of the patient during 11 treatment regimes using 5 types of applicators. Exposure to the operator in the normal operating position, behind the device console, usually does not exceed the limits recommended by the Canadian Federal Government. The intense fields around the applicators and the cables extend up to 0.5 m and are a source of potential overexposure to the operator.