The Childhood Cancer Research Group (CCRG) at the University of Oxford have published a series of epidemiology papers looking at childhood cancer and high-voltage overhead power lines. In 2015, they published a further paper, this time looking at underground cables.
Why look at underground cables?
The previous CCRG papers found associations between childhood leukaemia rates and overhead lines. There are good reasons to think these can't be caused (or at least not solely) by magnetic fields - they extended too far from the lines, and they changed over just a few decades. But overhead lines, as well as producing magnetic fields, do multiple other things as well - they produce electric fields and corona ions, and their visual presence may have an effect on a neighbourhood. So it is rather difficult disentangling magnetic fields from all the other factors for overhead lines.
Underground cables, by contrast, produce magnetic fields (albeit with a smaller range) - but nothing else. No visible presence, no electric fields, no corona. So if you found an association between childhood leukaemia and underground cables, it would almost certainly have to be caused by magnetic fields.
What did they find?
In summary, the paper found no associations with underground cables.
This needs qualifying slightly. Few people live near underground cables so the statistical power was limited, and that means the conclusion is not very robust. Also, there were suggestions of an association for brain tumours, but the paper suggests these were probably chance.
Nevertheless, the paper concludes:
We stated, before knowing the results of our study, that an elevation of leukaemia rates close to UGCs would almost certainly have to be caused by MFs, as UGCs produce no other discernable effects in their vicinity. We have, in fact, overall found no evidence of increased risks for leukaemia. Despite the tiny fraction of people living close to UGCs, which severely limits the statistical power, the large overall size of this study means that this finding still carries some weight ..... We conclude that these results are modest, though far from conclusive, evidence against MFs being a cause of leukaemia, and therefore, indirectly, provide some further suggestive evidence that where risk elevations are found near OHLs, they may be caused by factors other than MFs.
The methods in more detail
- 55,525 cases
- children born and diagnosed under the age of 15 in England and Wales from 1962-2008
- taken from the National Registry of Childhood Tumours
- each case has one or two individually matched controls selected from birth registers, matched for age, sex and birth registration sub-district.
- The resulting 116,815 subjects are largely the same as for the "follow on" study of OHLs, except:
- confined to residency in England and Wales as they did not have the UGC records for Scotland.
- some 3000 subjects fewer, largely cases diagnosed in 2008 and their controls together with a small number from earlier years, as the dataset for this analysis was frozen sooner than the one for the OHL analysis
The underground cables
Based on National Grid’s digital mapping of the locations of its UGCs, extracted for the purposes of this study as a single snapshot in 2011.
- High-voltage AC (400 kV, 275 kV and a very few 132 kV) and High-Voltage DC
- Records nearly 2000 km of underground cables, but the paper excluded UGCs entirely within substations, short lengths of UGC connecting a substation to the terminal tower of an OHL immediately outside it, and cables in deep bored tunnels (typically 20 m or more underground).
- The records are compiled for operational purposes and show all currently “live” cables, plus disconnected cables where the cable is still physically present in the ground.
- They do not necessarily show all redundant cables or instances where a cable route has been changed at some point, which will have led to some errors
- recorded by 65,260 straight-line segments, of variable length as necessary to capture curves
- generally accurate to 0.1 m or better.
- digital mapping information supplemented with relevant engineering information obtained from mainly internal National Grid records:
- year of construction
- spacing of the individual conductors
- depth of burial
- when two or more cables are in close proximity, relative phasing
- Some approximations were necessary:
- cables that are buried directly in the ground (as opposed to being laid in concrete troughs) often have variable depths along their route, which were standardised as a single depth
- as phasing often changes at joints, where joints between two different cables following the same route are staggered, the relative phasing will often be different for that length between joints, which was ignored.
- loads obtained from the same historical records as for studies of OHLs
Identifying location of addresses
- Grid reference of birth address obtained from Address-Point
- For the 308 subjects where the Address-Point grid reference was within 40 m of the nearest UGC (the chosen criterion for when the spatial extent of the home is potentially significant compared to the distance), attempted to identify the home on large-scale mapping and to record the grid reference of the closest and farthest points of the building from the UGC.
- also attempted to view each of these on Google StreetView to estimate the lowest and highest inhabited floor and to allow for sloping ground
- site visits for 44 out of 51 addresses where either the spatial extent or the inhabited floors could not be estimated with acceptable confidence
- This resolved 35 of these ambiguities.
- The remaining 9 were mainly apartment blocks with no public access and were dropped from the analysis
The exposure metrics
- calculated the horizontal distance to the nearest UGC that had existed in the relevant birth year (including UGCs disconnected but still physically present).
- two distance estimates:
- “closest”, the distance to the nearest point of the building, using only those subjects where this point was positively identified
- “centre”, the distance to the midpoint of the building, this time including the distance to the Address-Point grid reference for subjects without robust nearest and farthest points.
- calculated magnetic fields for those subjects with successfully identified closest and farthest points and the occupied floors.
- aim was to attempted a calculation for every subject where the field could possibly be >0.1 µT
- attempted calculations for all subjects within 40 m of any UGC
- For these subjects and cables, all cable segments within 100 m included in the calculation
- calculations at:
- 1 m above floor level,
- for each occupied floor (taking the height between floors as a constant 3 m),
- at five points equally spaced from the nearest to the farthest point.
- two exposure estimates:
- “closest”, the field at the nearest point on the lowest occupied floor
- “average”, the mean of the field at all the points for that address.
The results in detail
As explained above, the paper calculates "closest" and "centre" distances, and "closest" and "average" magnetic fields.
The summary results given in the abstract are:
- Trend estimates for leukaemia as shown by the odds ratio (and 95% confidence interval) per unit increase in exposure were:
- reciprocal of distance 0.99 (0.95-1.03),
- magnetic field 1.01 (0.76-1.33).
The full results tables given in the paper are:
Odds ratios (OR) and confidence intervals (CI) of distance to underground cables, based on “closest” distance estimate. ORs are adjusted for socioeconomic status. Trend is OR for each increase of 100/d. Bold indicates P<0.05. Italics indicates a combined, post hoc category (see text).
“closest” distance estimate
“centre” distance estimate
Odds ratios (OR) and confidence intervals (CI) of distance to underground cables, leukaemia only: “closest” distance estimate for earlier and later periods; “centre” distance estimate for whole period. ORs are adjusted for socioeconomic status. Trend is OR for each increase of 100/d. 0-9.9 and 10-19.99 categories are combined for “centre” distance estimate.
The abstract in full
Magnetic fields and childhood cancer: an epidemiological investigation of the effects of high-voltage underground cables
K J Bunch1,2, J Swanson3*, T J Vincent1, M F G Murphy1,4
1 formerly Childhood Cancer Research Group, University of Oxford, New Richards Building, Old Road Campus, Headington, Oxford, OX3 7LG, UK
2 National Perinatal Epidemiology Unit, Nuffield Department of Population Health, University of Oxford, Old Road Campus, Oxford, OX3 7LF, UK
3 National Grid, 1-3 Strand, London, WC2N 5EH, UK
4 Nuffield Department of Obstetrics & Gynaecology, University of Oxford, Women's Centre, Level 3, John Radcliffe Hospital, Oxford, OX3 9DU, UK
Epidemiological evidence of increased risks for childhood leukaemia from magnetic fields has implicated, as one source of such fields, high-voltage overhead lines. Magnetic fields are not the only factor that varies in their vicinity, complicating interpretation of any associations. Underground cables (UGCs), however, produce magnetic fields but have no other discernible effects in their vicinity. We report here the largest ever epidemiological study of high voltage UGCs, based on 52,525 cases occurring from 1962-2008, with matched birth controls. We calculated the distance of the mother’s address at child’s birth to the closest 275 or 400 kV AC or high-voltage DC UGC in England and Wales and the resulting magnetic fields. Few people are exposed to magnetic fields from UGCs limiting the statistical power. We found no indications of an association of risk with distance or of trend in risk with increasing magnetic field for leukaemia, and no convincing pattern of risks for any other cancer. Trend estimates for leukaemia as shown by the odds ratio (and 95% confidence interval) per unit increase in exposure were: reciprocal of distance 0.99 (0.95-1.03), magnetic field 1.01 (0.76-1.33). The absence of risk detected in relation to UGCs tends to add to the argument that any risks from overhead lines may not be caused by magnetic fields.