• Indirect estimates of human exposure to magnetic fields have been commonly used in epidemiology (e.g., calculations, ''wire codes" (see Glossary and Appendix B for definitions of wire codes), distance, and contemporary measurements).

  • Wire codes, the most commonly used estimates of possible exposure to electric and magnetic fields, are not strong predictors of magnetic-field strengths in homes. Within a given geographic region, however, wire codes to tend to distinguish relatively well between the higher and lower field strengths in homes.

  • Exposure of humans and animals to 60-Hz electric and magnetic fields induces currents internally. The density of these currents is nonuniform throughout the body. Also, the spatial patterns of the currents induced by the magnetic fields are different from those induced by the electric fields.

  • The endogenous current densities on the surface of the body (higher densities occur internally) associated with electric activity of nerve cells (as measured by electroencephalograph) are of the order of 1 milliampere per square meter (mA/m2) and are chiefly of even lower frequency than 60 Hz, peaking at 5-15 Hz. Human exposure to a 60-Hz field of 100 µT (1 G) is needed to produce an equivalent current density in the body. The induced current densities caused by typical residential fields (about 1 mG) are therefore about 1 µA/m2, or 1,000 times less than endogenous current densities.

  • Microscopic heterogeneity has not been accounted for in evaluations (experimental or theoretic) of local current densities within tissues and in and around cells and cell assemblies.

  • Several features are required for laboratory field-exposure systems to help eliminate potential experimental artifacts. Relatively few experimental studies have reported using systems that satisfy all these requirements. One requirement, obtaining and analyzing the data blind to the status of the exposure conditions, is extremely important. A large fraction of the published reports either do not provide sufficient information or do not satisfy many of the requirements for appropriate exposure-system design and operation.

DEFINITION OF TERMS

Electric and magnetic fields are produced by electric charges and their motion. A static electric field is produced by electric charges whose magnitude and position do not change in time. A static magnetic field can be produced either by a permanent magnet or by a steady flow of electric current (moving electric charges). The magnetic field produced by the latter means is frequently called a direct-current (dc) magnetic field. Alternating-current (ac) magnetic fields are produced by electric currents alternating in time. Electric and magnetic fields are vector quantities and thus are characterized by their magnitude and direction at every point in space and time. The behavior of electric and magnetic fields and their interrelationship are comprehensively described by Maxwell's equations



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