The study investigated certain types of cancer that have been implicated with power lines or EMFs: leukaemia, brain tumours, malignant melanoma, and female breast cancer.  It compared incidence of these cancers to certain "control" cancers that are not implicated with EMFs.  It calculated the distance of each case to the nearest National Grid overhead line (275 kV and 400 kV in England and Wales, plus a very few 132 kV lines) that existed in the year of diagnosis (and also five years before diagnosis).  It also calculated the magnetic field exposure of each case from the distance, information about the line, and an estimate of the current carried by the line in the year of interest.

The methods have been described in detail in a previous published paper.

The study is broadly negative.  For some of the cancer types (most notably breast cancer), there is a suggestion of an association with proximity to the power lines in the unadjusted risks.  But any associations largely disappear after adjustment for a number of factors including deprivation.  This could mean that there is a difference in the socioeconomic status of the people who live close to power lines compared to the rest of the population (see what we already know about this), and that difference means that they contract some cancers at different rates, but there is no direct effect of the power lines or the magnetic fields they produce.

Whatever the explanation, the authors conclude:

"Our results do not support an epidemiologic association of adult cancers with proximity to residential magnetic fields from high-voltage overhead power lines."

They actually go on to say:

"Unless new biological hypotheses emerge, our findings should help to settle a long-standing debate on the safety of residential exposures to extremely low-frequency magnetic fields from high-voltage overhead power lines and adult cancers."

This last statement is something different people will have different opinions on.  But the study is a negative one, even if there may be debate about exactly how much weight to place on it.

Results for distance from power lines

The Odds Ratios given are the adjusted ones (the paper also gives unadjusted), for year of diagnosis (the paper also gives 5 years prior to diagnosis), and, for female breast cancer, are for 100% of cases (the paper also gives a 50% sample of cases).

Results for magnetic field calculated from power lines

The Odds Ratios given are the adjusted ones (the paper also gives unadjusted), for year of diagnosis (the paper also gives 5 years prior to diagnosis), and, for female breast cancer, are for a 50% sample of cases.

Results presented graphically

by distance	by magnetic field
Leukaemia
p=0.280	p=0.818
brain / central nervous system cancers
p=0.657	p=0.902
malignant melanoma
p=0.406	p=0.177
female breast cancer
p=0.275	p=0.832

Note: the graphs and tables are reconstructed by us from the data in the published paper, they are not the originals from the paper.

Adjusted and unadjusted results

For breast cancer, the findings before the adjustments for socioeconomic status etc were slightly higher (see also what we already know about socioeconomic status near power lines):

Strengths

• The size of the study - over 80,000 cancer cases, over a 35 year period, with tens of cases at the higher exposure levels
• The objectivity of the exposure assessments, all performed from records and blind to case-control status

Weaknesses

• The study is based on the address of the subjects at diagnosis, with no information about how long or short a time they had lived there, and no information on cumulative exposure
• The study had to use selected other cancers as controls
• The magnetic field calculations are only approximate (principally because of the limited historical load information)

Interestingly, each of these main weaknesses would act so as to dilute, in the study findings, the size of any true effect.

The study came about as an initiative of National Grid, who considered there ought to be a parallel study on adult cancers to the CCRG or "Draper" study on childhood cancers.

National Grid provided the data on the locations of the power lines and performed the calculations of proximity and exposure, using the same methods as for the Draper study.  Unlike that study, National Grid was not involved in the analysis or in writing the paper, which were the sole responsibility of the Imperial College group.  The study was formally under the auspices of the Department of Health (DH). Most of the funding came from the Energy Networks Association (ENA), but ENA provided that funding to DH and Imperial College's contract was with DH.