| Exposure at night-time
The Ahlbom pooled analysis
found a raised relative risk for childhood
leukaemia for 24-hour exposure in the home greater than 0.4 µT:
|
Field
|
<0.1
µT
|
0.1-0.2
µT
|
0.2-0.4
µT
|
>0.4
µT
|
|
Relative risk
|
1
|
1.08
|
1.11
|
2.00
|
A year later, a new study
from Germany
found a similar result for 24-hour exposure, but also found the same thing as a
smaller, earlier German study, that there was a higher relative risk for exposure
just during the night-time. The results
from these German studies were:
|
Field
|
<0.1
µT
|
0.1-0.2
µT
|
0.2-0.4
µT
|
>0.4
µT
|
|
New German study
|
|
|
|
|
|
Relative risk: 24 hour
|
1
|
1.15
|
1.16
|
5.81
|
|
Relative risk: night-time
|
1
|
1.42
|
2.53
|
5.53
|
|
Pooled German studies:
|
|
|
|
|
|
Relative risk: 24 hour
|
1
|
1.10
|
1.58
|
2.48
|
|
Relative risk: night-time
|
1
|
1.37
|
3.26
|
3.44
|
This prompted a new study to see whether this same "night-time exposure"
effect was present in other comparable studies.
The results from the German studies were pooled with the USA, UK and Canadian studies from the
Ahlbom pooled analysis. This new pooled
analysis found almost exactly the same relative risks for night-time and
24-hour exposure:
|
Field
|
<0.1
µT
|
0.1-0.2
µT
|
0.2-0.4
µT
|
>0.4
µT
|
|
Relative risk: 24 hour
|
1
|
1.09
|
1.2
|
1.98
|
|
Relative risk: night-time
|
1
|
1.11
|
1.37
|
1.93
|
If there had been higher relative risks for
night-time exposure, this could be either because exposure at night-time is
more biologically relevant, or because the studies assess night-time exposure
(when the child is in the home) more accurately. But these results "do not support the
hypothesis that nighttime measures are more appropriate" (the authors' own
words).
Of course, it is still possible to
postulate a mechanism whereby night-time exposure could be biologically relevant,
and that would not necessarily be contradicted by the epidemiological
evidence. But there is no longer the
epidemiological evidence in favour of that idea.
|
Am J Epidemiol.2007 May 7;
Nighttime Exposure to Electromagnetic Fields and Childhood Leukemia: An Extended Pooled
Analysis
Schuz J, Svendsen AL, Linet MS, McBride ML, Roman E, Feychting M, Kheifets L, Kheifets L, Lightfoot T, Mezei G, Simpson J, Ahlbom A.
Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark.
It has been
hypothesized that nighttime bedroom measurements of extremely low frequency
electromagnetic fields (ELF EMF) may represent a more accurate reflection of
exposure and have greater biologic relevance than previously used 24-/48-hour
measurements. Accordingly, the authors extended a pooled analysis of
case-control studies on ELF EMF exposure and risk of childhood leukemia to
examine nighttime residential exposures. Data from four countries (Canada, Germany, the United Kingdom, and the United States)
were included in the analysis, comprising 1,842 children diagnosed with
leukemia and 3,099 controls (diagnosis dates ranged from 1988 to 1996). The
odds ratios for nighttime ELF EMF exposure for categories of 0.1-<0.2 muT,
0.2-<0.4 muT, and >/=0.4 muT as compared with <0.1 muT were 1.11 (95%
confidence interval (CI): 0.91, 1.36), 1.37 (95% CI: 0.99, 1.90), and 1.93 (95%
CI: 1.11, 3.35), respectively. The fact that these estimates were similar to
those derived using 24-/48-hour geometric mean values (odds ratios of 1.09,
1.20, and 1.98, respectively) indicates that the nighttime component cannot, on
its own, account for the pattern observed. These results do not support the
hypotheses that nighttime measures are more appropriate; hence, the observed
association between ELF EMF and childhood leukemia still lacks a plausible
explanation.
|
For simplicity, in the tables earlier on
this page we gave just the relative risks without the confidence
intervals.For completeness, the
following table gives the same relative risks together with 95% confidence
intervals.
|
Field
|
<0.1 µT
|
0.1-0.2 µT
|
0.2-0.4 µT
|
>0.4 µT
|
|
Ahlbom pooled
analysis
|
|
|
|
|
|
Relative risk: 24
hour
|
1
|
1.08 (0.89,1.31)
|
1.11 (0.84,1.47)
|
2.00 (1.27,3.13)
|
|
New German study
|
|
|
|
|
|
Relative risk: 24
hour
|
1
|
1.15 (0.73,1.81)
|
1.16 (0.43,3.11)
|
5.81 (0.78,43.2)
|
|
Relative risk:
night-time
|
1
|
1.42 (0.90,2.23)
|
2.53 (0.86,7.46)
|
5.53 (1.15,26.6)
|
|
Pooled German studies:
|
|
|
|
|
|
Relative risk: 24
hour
|
1
|
1.10 (0.75,1.60)
|
1.58 (0.74,3.36)
|
2.48 (0.70,8.86)
|
|
Relative risk:
night-time
|
1
|
1.37 (0.94,2.00)
|
3.26 (1.49,7.11)
|
3.44 (1.03,11.4)
|
|
New pooled
analysis:
|
|
|
|
|
|
Relative risk: 24
hour
|
1
|
1.09 (0.89,1.32)
|
1.2 (0.89,1.63)
|
1.98 (1.18,3.35)
|
|
Relative risk:
night-time
|
1
|
1.11 (0.91,1.36)
|
1.37 (0.99,1.90)
|
1.93 (1.11,3.35)
|
|
