High-voltage overhead power lines in epidemiology: patterns of time variations in current load and magnetic fields

In epidemiological studies of electromagnetic fields and health effects, exposure classification is crucial. There is no generally accepted biophysical interaction mechanism, but many studies are based on the hypothesis of a causal relationship with the strength of magnetic field. Some definition of the magnitude of exposure must be used, e.g., mean magnetic flux density, the integral of magnetic flux and time, or a peak value. Magnetic fields around a particular power line depend on the current load. The aim of the present study was to follow variations in line current load in the power supply system of the largest Norwegian city on a yearly, monthly, daily and diurnal basis. Fairly large variations in load were found, but increases in consumption were not necessarily reflected in current load on high voltage lines. The correlation between outdoor temperature and current load varied widely, depending on the type of power station feeding the line in question. The registered time variations are large enough to interfere with epidemiological classification of residences and testing of epidemiological hypotheses.