American Journal of Epidemiology Advance Access originally published online on August 16, 2007
American Journal of Epidemiology 2007 166(7):859; doi:10.1093/aje/kwm225
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LETTERS TO THE EDITOR |
RE: "NIGHTTIME EXPOSURE TO ELECTROMAGNETIC FIELDS AND CHILDHOOD LEUKEMIA: AN EXTENDED POOLED ANALYSIS"
1 Office of the Vice Chancellor, University of Sunderland, Sunderland SR1 3SD, United Kingdom
2 Department of Physics, Faculty of Science, University of Bristol, Bristol BS8 1TL, United Kingdom
3 Powerwatch, Ely CB6 2QA, United Kingdom
(e-mail: jonathan.ward{at}bris.ac.uk)
In a recent pooled analysis, Schüz et al. (1) examined the link between nighttime extremely low frequency (ELF) electric and magnetic field (EMF) exposure and childhood leukemia. There appears to be a non sequitur in the abstract of their paper: "The fact that these estimates were similar to those derived using 24-/48-hour geometric mean values ... indicates that the nighttime component cannot, on its own, account for the pattern observed" (1, p. 263). This reads as if nighttime exposure could not on its own be the main or sole component underlying, or accounting for, the observed association with the elevated incidence of childhood leukemia. That would be wrong. Consider the hypothetical example of an underlying association solely with the nighttime component and none with daytime exposure, examined under constant intensity of exposure over 24 hours. The apparent association with daytime exposure would then be simply due to confounding by nighttime exposure.
Schüz et al.'s concluding discussion, however, is clear and correct, including the claim that "our data provide no support for the suggestion that exposure incurred during the night is biologically more relevant" (1, p. 269). Equally, they do not suggest that nighttime exposure is not biologically more relevant. They simply do not distinguish between daytime and nighttime exposure. It would be interesting to know whether personal, rather than bedroom, measurements were also well correlated between night and day.
There is a slight but consistent subdiagonal bias in Schüz et al.'s table 2 (1, p. 266), suggesting that exposures in the pooled data tend to be slightly lower at night than by day, though the two metrics remain well correlated. Such differences in well-correlated metrics can lead to the 24-hour metric misrepresenting a nighttime effect. It would be interesting to see the disaggregation of the table 2 data by country.
The abstract concludes, "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" (1, p. 263). We question the generality of that conclusion.
Apart from the melatonin hypothesis, evidence continues to develop at several levels. For example, Juutilainen et al. (2) have analyzed data from in vitro studies and short-term animal studies that combined ELF EMF exposure, albeit at higher exposure levels than those associated with childhood leukemia, with known carcinogenic, mutagenic, or otherwise harmful physical or chemical agents. Of 65 studies, the overall percentage of positive studies differed very significantly (p < 0.0001) from that expected by chance. The evidence lends support for the "low field effect" of the radical-pair mechanism, which could theoretically operate with fields as low as 0.033 µT (3).
However, these pooled results are important in showing that, at least over these countries and with the respective measurements, nighttime and daytime exposures are too well correlated to provide a basis for distinguishing their possible effects. There may be distinct effects, but either a differently exposed population or personal rather than bedroom measurements would be needed for epidemiology to reveal them.
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ACKNOWLEDGMENTS |
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Professors Michael O'Carroll and Denis Henshaw acted as expert witnesses for objectors at the Beauly-Denny power line public inquiry in Scotland in March 2007. Andrew Cohen is employed by A & J Philips (Ely, United Kingdom), which supplies measurement and screening equipment for ELF EMFs.
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References |
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 TOP References |
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- Schüz J, Svendsen AL, Linet MS, et al. Nighttime exposure to electromagnetic fields and childhood leukemia: an extended pooled analysis. Am J Epidemiol (2007) 166:263–9. [Epub 2007 May 7].
[Abstract/Free Full Text] - Juutilainen J, Kumlin T, Naarala J. Do extremely low frequency magnetic fields enhance the effects of environmental carcinogens? A meta-analysis of experimental studies. Int J Radiat Biol (2006) 82:1–12.[CrossRef][Web of Science][Medline]
- Ritz T, Thalau P, Phillips JB, et al. Resonance effects indicate a radical-pair mechanism for avian magnetic compass. (Letter). Nature (2004) 429:177–80.[CrossRef][Medline]
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