Sources of EMFs >
Long-term variations in fields
Direct measurements of fields in the same property
are available only up to about 5 years apart. One piece of work
measured fields in 56 homes in the USA in 1990 which had already
been measured in 1985. Spot measurements had increased, on average,
by 35%. For longer periods, changes in fields have to be estimated
from models, taking account of changes in loads, numbers of consumers,
lengths of circuits, etc. Simple models look just at some measure
of per capita consumption. National Grid scientists developed a
more sophisticated model which looks at changes in electricity systems
and wiring practices as well. Even so, there are some changes (principally
perhaps in the USA) which such models cannot easily take into account,
so the results should be interpreted with caution. The models all
suggest that average fields have increased over time, for example
by a factor of 4.2 in the UK from 1949 to 1989.
A recent paper analysed what is known about long-term variations and
fields and what can be deduced from comparing this with changes in cancer rates:
| Bioelectromagnetics. 2006 May 24;27(7):545-552
Childhood leukemia, electric and magnetic fields, and temporal trends.
Kheifets L, Swanson J, Greenland S.
During the past 25 years concern has been raised about the possible
health effects of extremely low frequency (ELF) electric and magnetic fields
(EMFs), particularly regarding childhood leukemia. Comparison of changes in
electricity consumption (a surrogate for exposure) to changes in childhood-leukemia
rates, known as ecologic correlation, have been used to argue both for and against
the association between magnetic fields and childhood leukemia. In this paper we
explore what can be learned from such an ecologic approach. We first examine
separately the evidence on trends in exposure to EMFs and on trends in leukemia
rates, and then compare the two. Both incidence rates and exposures have increased,
but there are so many approximations and assumptions involved in connecting the
two trends that we cannot regard the ecologic evidence as providing any
meaningful evidence for or against a causal link. |
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