Epidemiological studies and laboratory research suggest that exposure to extremely low frequency (< 300 Hz) magnetic fields is associated with an increase in risk of developing a number of rare diseases including leukaemia. Overall the risks to health appear to be small but a more accurate exposure assessment technique would help evaluate the true extent of any health effects. In this pilot study we aimed to identify and evaluate personal, work and environmental factors and their influence on measured exposure levels with a view to developing a method of reconstructing exposure. Office workers and power utility workers were studied using personal dosimeters to measure magnetic field exposure, along with frequent observation or measurement of factors related to exposure. Factors such as average and closest distance to source, time at position and current flow were combined in a series of metrics to investigate simple models of personal exposure. The results indicate that mean and peak magnetic field exposure levels are linked to current flow and the average distance of a worker from the source of the magnetic field. These factors are more accurate predictors of high exposures than they are of lower and average levels. It may be possible, with further work, to produce a model of exposure to magnetic fields for use in epidemiological studies.