How fields vary with time

Magnetic fields are produced by currents.  So as the current varies (up and down over a day or a year, and generally upwards over time), magnetic fields tend to vary as well. 

Variations over a day and a year

These graphs show the variation of magnetic field in one particular house over a day and a year. Magnetic fields vary roughly in the same way as demand for electricity does.

 graph of variation of magnetic field over one day

Variation of magnetic field in a home over 24 hours
(This may not be completely typical - fields would normally be lower during the night - this house may have been on a circuit with significant electricity use during the night, perhaps Economy 7.)
 

 graph of variation of magnetic field over a year

Variation over one year

Longer-term variations

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.  This is shown in this graph.  The solid red line shows the predictions of the model.  For the earlier years, we don't have the knowledge to model exposures in this detail, but we do know they must have risen from zero at about the turn of the century or a bit before when electricity first started being used.  This is shown by the dotted red line.

graph of variations in magnetic fields over decades 

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.  The abstract is at the bottom of this page.  The following graph compares the increase in exposures with the increase in childhood leukaemia rates in the UK.  (Actual incidence data are available for the later years, shown by the solid blue line.  Before that, we don't have incidence data, but we do have mortality data. As childhood leukaemia was almost invariably fatal at that period, that should give a good measure of incidence, and is shown by the dotted blue line.)

trends-both-curves

Both exposure and incidence have gone up, but their biggest increases were at different times.  Incidence went up quickest in the 1920s, 30s and 40s, when exposure went up more slowly.  In the 1950s and 60s, exposure increased more rapidly, but incidence increased more slowly.  This makes it hard to claim that this evidence supports a link between incidence and exposure.  But, as the paper says, there are so many approximations, neither is this good evidence against a link. 

 

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.