Why is personal exposure different to the background field?
Most measurements in epidemiological studies and most measurements made in homes have measured the field at one or more specific places, away from appliances; that is, the measurements are of the background field. Ignoring for the moment variations of field with time, a person moving around a home experiences a varying field, partly because the background field varies from place to place, and partly because of fields from appliances and other local sources.
What do we know about personal exposure?
Various measurements have been made by asking people to wear personal-exposure monitors and also measuring the background field in their homes. Average personal exposures are usually bigger than measurements of background field in the same home. The ratio in eight studies ranged from 1.09 to 2.3 with an average of 1.4. Recent work in the USA, although not directly comparing personal exposures to fields measured in homes, found a geometric-mean personal exposure of 0.09 microtesla (µT) in one thousand people, consistent with exposures being a few tens of percent higher than fields measured in homes. Simplistically, this can be thought of as the contribution from appliances (and any other local sources within the home).
How much do appliances add to personal exposure?
The contribution of appliances to exposure will vary with background field, because the higher the background field, the smaller the physical area around an appliance in which its field is significant. It is possible to create models of these effects and fit them to available data. In the UK the conclusion is that, on average, appliances contribute one-third of exposure (in accord with the simplistic approach already mentioned). The actual fraction is roughly 50% for a home at the tenth percentile of the distribution of background fields, 10% for a home at the ninetieth percentile, and just 3% for a high-field home such as might be found near a transmission line.
Example of an exposure measurement
This graph shows the reading on a personal exposure monitor worn by an adult for one day, with some of the sources of exposure marked. The peak exposure (from the electric drill) was 149 µT; the average exposure was 0.11 µT.
