Scientists are still uncertain about the best way to do this, because experiments have shown that several aspects of the fields may be relevant to biological effects. Even though the average field level has been used widely to represent EMF exposure, it is possible that other definitions may relate more closely to any possible effects. Second, EMF in the environment is very complicated. We are usually exposed to EMF from a large number of sources every day.
Fields change both in time and space.
A person's EMF exposure depends to a large degree on what he or she is doing in the field at the time. Several kinds of small meters are now available that can be carried or worn by a person to record magnetic field exposures automatically. The figure on the next page is an example of data collected with one of these meters. The magnetic field was measured every 24 seconds over a 24-hour period. For this person, field exposure at home was very low.