Why are we interested in fields greater than 0.4 microteslas?
Several epidemiology studies of magnetic fields and childhood leukaemia have used a field of 0.4 microtesla (µT) as a cutpoint in the analysis, to define the highest exposure group. Both the United Kingdom Childhood Cancer Study (UKCCS) and the Ahlbom pooled analysis used >0.4 µT as the top exposure category, and in the Ahlbom pooled analysis, it was only in this top category that a statistically significant association was found. Because of this, there is quite a bit of interest in homes with fields greater than 0.4 µT.
What exactly is the field we are interested in?
The epidemiological studies measured the background field in the home (often in the child’s bedroom). That is, they deliberately measured the field away from any domestic appliances. It is easy to find fields of greater than 0.4 µT close to appliances, we are interested here in homes where the field is greater than 0.4 µT even away from appliances.
They also measured the field for 24 hours or longer (often 48 hours). Fields vary over time. The field can go above 0.4 µT for short periods in quite a lot of homes. We are interested here only in homes where the 24 hour average is above 0.4 µT.
Finally, they measured the field in the home. Obviously we know that people spend time outside the home and are exposed to fields there as well, but it’s hard to measure that, and we often use the field in the home as a surrogate for total exposure. When people talk about “0.4 µT” they usually mean the field in the home.
How many UK homes are like this?
The largest set of measurements that we have of field in homes in the UK was done as part of the UKCCS.
They did measurements on 2226 homes of control children; just 9 (0.4%) were assessed as having fields greater than 0.4 µT.
44 (2.0%) had fields similarly assessed as greater than 0.2 µT.
Both these percentages are lower than in many other countries, particularly the USA.
Where do these fields come from?
We often assume that high magnetic fields must come from visible high-voltage power lines. Such lines certainly can produce fields greater than 0.4 µT. An average National Grid transmission line produces 0.4 µT within about 60 m (this is an average and the distance may be higher or lower for specific lines). More on fields from transmission lines and more specifically on how far from different overhead lines the field falls to 0.4 µT.
However, only some of the homes with these fields are close enough to a high-voltage power line to experience the high field from it. . The best figures we currently have (see above) are:
- Fields greater than 0.4 µT: 43% of these homes
- Fields greater than 0.2 µT: 23% of these homes
If the field does not come from a high-voltage power line, it probably comes from either distribution wiring outside the home or some feature of the wiring in the home. It is unusual for these sources to produce fields this high, but it is not impossible.
To find out exactly where these fields do come from, the electricity industry and the DTI jointly funded the then Leukaemia Research Fund to visit the homes. The work was done by HPA-RPD. The results were published in July 2005 as a Report and in March 2007 as a peer-reviewed paper.
They investigated 21 homes with fields (24-hour average in the general volume of the home) greater than 0.4 µT. 43% of these were due to high-voltage overhead lines. The breakdown is shown in the following diagram.