Cohort study of cancer risk among male and female shift workers.
OBJECTIVE
Melatonin, a hormone that inhibits experimentally induced cancers, is suppressed by nighttime exposure to light so that nighttime shift workers may be at an increased risk of cancer. Previous studies of shift workers found an increased risk of breast cancer among women and suggested a possible increased risk of colon cancer among women and prostate cancer. The present study was conducted to see whether these previous findings could be confirmed and whether shift workers are at elevated risk for cancer at additional sites.


METHODS
Altogether 2 102 126 male and 1 148 661 female workers were identified who worked in both 1960 and 1970. Their jobs were classified according to the percentage of shift workers, and they were followed from 1971 through 1989 or until they were diagnosed with cancer or died. Standardized incidence ratios (SIR) were used to compare the adjusted cancer incidence rates for shift workers with those for nonshift workers.


RESULTS
Cancer rates were not elevated for the male shift workers [all sites combined: N=6524 cases among shift workers, SIR 1.02, 95% confidence interval (95% CI) 1.00-1.05; prostate: N=1319, SIR 1.04, 95% CI 0.99-1.10] or for the female shift workers (all sites combined: N=268, SIR 1.00, 95% CI 0.89-1.13; breast: N=70 cases, SIR 0.94, 95% CI 0.74-1.18).


CONCLUSIONS
No evidence was found for an association between shift work and breast or prostate cancer, or all cancer sites combined among shift workers.
Schwartzbaum
Judith
J
Ohio State University, School of Public Health, Division of Epidemiology and Biometrics, Starling-Loving Hall, 320 W. Tenth Ave, Columbus, Ohio 43210, USA. schwartzbaum.1@osu.edu
Ahlbom
Anders
A
Feychting
Maria
M
D000293:Adolescent; D000328:Adult; D001943:Breast Neoplasms / Q000453:epidemiology; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D008550:Melatonin / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008875:Middle Aged; D009369:Neoplasms / Q000145:classification / Q000453:epidemiology / Q000209:etiology*; D011471:Prostatic Neoplasms / Q000453:epidemiology; D018570:Risk Assessment; D013548:Sweden / Q000453:epidemiology; D014940:Work Schedule Tolerance / Q000502:physiology*
D008550:Melatonin