American Journal of Epidemiology Advance Access originally published online on July 13, 2005
American Journal of Epidemiology 2005 162(4):393-394; doi:10.1093/aje/kwi227
American Journal of Epidemiology Copyright © 2005 by the Johns Hopkins Bloomberg School of Public Health All rights reserved
THE AUTHORS REPLY
Ulla M. Forssén1,2,
Lars Erik Rutqvist3,
Anders Ahlbom1 and
Maria Feychting1
1 Institute of Environmental Medicine, Karolinska Institutet, Stockholm, SE-171 77 Sweden
2 Department of Epidemiology, School of Public Health, University of North Carolina, Chapel Hill, NC 27599
3 Department of Medicine, Huddinge University Hospital, Karolinska Institutet, Stockholm, SE-141 86 Sweden
We thank Dr. Erren (1
) for his comments about our study (2). Dr. Erren discusses the possibility that exposure to light at night might confound the results found for extremely-low-frequency electromagnetic fields in relation to breast cancer. The hypothesis that light at night might increase breast cancer risk through a mechanism involving suppression of the hormone melatonin is very interesting and has been addressed in studies of, for example, shift workers (3–5) and the blind (6). At present, the evidence in favor of this hypothesis is suggestive but not very strong regarding either the effect of light at night on breast cancer risk or a possible protective effect of melatonin on breast cancer development (7). However, the hypothesis is intriguing, and we continue to follow this research area with great interest.
In our study (2), we were unable to adjust for exposure to light at night because of a lack of data, but we believe it is unlikely that doing so would substantially change the results. To our knowledge, no study has investigated the association between light at night and exposure to occupational electromagnetic fields, but there is nothing that hitherto indicates an existence of such a relation that could mask a risk from electromagnetic fields. Furthermore, the epidemiologic studies that have investigated light as a risk factor for breast cancer have observed an increase in risk that is very moderate and not entirely consistent. At most, risks of twofold or less have been reported, substantially lower than those for the association between smoking and lung cancer, as Dr. Erren (1) brought up as an example. Exposure to light can therefore not be considered a "potentially strong" risk factor for breast cancer, should the effect be real.
In using the experimental studies as a comparison, Dr. Erren (1) also seems to suggest an interactive effect between light and electromagnetic fields. As far as we know, no studies on humans support this effect. Given the level of exposure to light common in Sweden, electromagnetic fields do not seem to affect breast cancer risk.
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ACKNOWLEDGMENTS
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Conflict of interest: none declared.
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References
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- Erren TC. Re: "Occupational magnetic fields and female breast cancer: a case-control study using Swedish population registers and new exposure data." (Letter). Am J Epidemiol 2005;162:392–3.[Free Full Text]
- Forssén UM, Rutqvist LE, Ahlbom A, et al. Occupational magnetic fields and female breast cancer: a case-control study using Swedish population registers and new exposure data. Am J Epidemiol 2005;161:250–9.[Abstract/Free Full Text]
- Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiology 2001;12:74–7.[CrossRef][ISI][Medline]
- Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. J Natl Cancer Inst 2001;93:1557–62.[Abstract/Free Full Text]
- Schernhammer ES, Laden F, Speizer FE, et al. Rotating night shifts and risk of breast cancer in the Nurses' Health Study. J Natl Cancer Inst 2001;93:1563–8.[Abstract/Free Full Text]
- Feychting M, Osterlund B, Ahlbom A. Reduced cancer incidence among the blind. Epidemiology 1998;9:490–4.[CrossRef][ISI][Medline]
- Travis RC, Allen DS, Fentiman IS, et al. Melatonin and breast cancer: a prospective study. J Natl Cancer Inst 2004;96:475–82.[Abstract/Free Full Text]
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