© 2002 by Oxford University Press
Journal of the National Cancer Institute, Vol. 94, No. 7, 533-534,
April 3, 2002
© 2002 Oxford University Press
CORRESPONDENCE |
RESPONSE: Re: Night Shift Work, Light at Night, and Risk of Breast Cancer
Correspondence to: Johnni Hansen, Ph.D., Danish Cancer Society, Institute of Cancer Epidemiology, Strandboulevarden 49, DK-2100 Copenhagen Ø, Denmark (e-mail: johnni{at}cancer.dk).
I thank Dr. Kerenyi for his comments regarding the recent editorial on light at night and breast cancer risk (1), and appreciate the opportunity to further comment on this issue of light as a potential carcinogen.
Recent studies on light and breast cancer risk have primarily focused on people who work at night (2–4). Dr. Kerenyi refers to an interesting unpublished study regarding the opposite situation, i.e., of people who worked in darkness during the day in a film-producing factory in Canada. Compared with other workers with normal light exposure within the same factory, the workers with low light exposure had a reduced risk of breast cancer and melanomas, thus supporting the "melatonin-hypothesis", which, in this opposite situation with low exposure to light, may argue that excess darkness, including blindness, may decrease the risk of certain cancers, including breast cancer and melanomas (5–8).
To explore the results of Dr. Kerenyi, we estimated the relative risk of breast cancer and malignant melanomas among workers employed in photographic laboratories in Denmark, on the basis of our comprehensive nationwide case–control data linkage on occupational history (1964–1989) and cancer (1970–1989) (9). The odds ratio (OR), adjusted for sex and birth year, for female breast cancer (including at least 1 year of employment and a lag period of at least 10 years) was statistically significantly decreased (OR = 0.4; 95% confidence interval [CI] = 0.2 to 0.9; N = 11) among the photo laboratory workers compared with other employees. The corresponding OR values for melanomas among women and men, respectively, were 1.4 (95% CI = 0.5 to 3.7; N = 6) and 0.4 (95% CI = 0.1 to 1.8; N = 2). Considering 1) the limitations of a small number of included cases and 2) that no further information is available on exposure to light for this occupational group, we found that the results for breast cancer are in line with the findings by Dr. Kerenyi. This may put further impetus on the need for studies with detailed information on light exposure and cancer risk among workers with extreme exposure to light either as darkness during the day or as artificial light during the night.
REFERENCES
1
Hansen J. Light at night, shift work, and breast cancer risk. J Natl Cancer Inst 2001;93:1513–5.
2
Schernhammer ES, Laden F, Speizer FE, Willett WC, Hunter DJ, Kawachi I, et al. Rotating night shifts and risk of breast cancer in women participating in the nurses' health study. J Natl Cancer Inst 2001;93:1563–8.
3
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.
4 Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiology 2001;12:74–7.[CrossRef][Web of Science][Medline]
5
Stevens RG. Electric power use and breast cancer: a hypothesis. Am J Epidemiol 1987;125:556–61.
6 Hahn RA. Profound bilateral blindness and the incidence of breast cancer. Epidemiology 1991;2:208–10.[Medline]
7 Kerenyi NA, Pandula E, Feuer G. Why the incidence of cancer is increasing: the role of `light pollution'. Med Hypotheses 1990;33:75–8.[CrossRef][Web of Science][Medline]
8 Stevens RG, Rea MS. Light in the built environment: potential role of circadian disruption in endocrine disruption and breast cancer. Cancer Causes Control 2001;12:279–87.[CrossRef][Web of Science][Medline]
9 Olsen JH, Jensen OM. Occupation and risk of cancer in Denmark. An analysis of 93,810 cancer cases, 1970–1979. Scand J Work Environ Health 1987;13 Suppl 1:1–91.[Medline]
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