American Journal of Epidemiology Advance Access originally published online on July 11, 2006
American Journal of Epidemiology 2006 164(4):402; doi:10.1093/aje/kwj265
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Letter to the Editor |
RE: "OCCUPATIONAL RISK FACTORS FOR ESOPHAGEAL AND STOMACH CANCERS AMONG FEMALE TEXTILE WORKERS IN SHANGHAI, CHINA"
Department of Community and Family Medicine, School of Public Health, Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China
(e-mail: iyu{at}cuhk.edu.hk)
We read with great interest in a recent issue of the Journal an article on a Shanghai, China, case-cohort study by Wernli et al. (1
). They reported that female textile workers exposed to silica dust for more than 10 years had a 15.8 times' increased risk of esophageal cancer (95 percent confidence interval: 3.5, 70.6), on the basis of three incident cases (1). On the other hand, cumulative exposure to endotoxin was inversely related to risk of esophageal cancer (p-trend < 0.01). We wonder if there was an actual excess risk of esophageal cancer among the silica-dust-exposed workers in comparison with urban Shanghai women, because the overall observed incidence was 46 percent lower than the expected incidence (standardized incidence ratio = 0.54, 95 percent confidence interval: 0.4, 0.7) for the entire cohort (2).
The authors failed to explore the potentially confounding effects of dietary risk factors. This is an intrinsic methodological defect in this type of case-cohort analysis—inadequate exploration of major potential confounding factors for individual outcomes. Controlling for major potential confounding factors is more likely to be possible in a properly conducted nested case-control study. Intake of pickled vegetables (rich in carcinogenic N-nitroso compounds) daily or more frequently could significantly increase the risk of esophageal cancer (18.1 times (3)). Moreover, confounding effects from occupational exposure to polycyclic aromatic hydrocarbons cannot be ruled out in the interpretation of the apparent association between silica and esophageal cancer, because workers classified as having silica dust exposure in Wernli et al.'s study were all involved in foundry work. Foundry workers might also be exposed to polycyclic aromatic hydrocarbons, but such exposures were not assessed, as mentioned by the authors (1).
The findings of the study by Wernli et al. would have been more informative if potentially confounding effects from both dietary and concomitant occupational carcinogenic exposures had been taken into consideration.
ACKNOWLEDGMENTS
Conflict of interest: none declared.
References
- Wernli KJ, Fitzgibbons ED, Ray RM, et al. Occupational risk factors for esophageal and stomach cancers among female textile workers in Shanghai, China. Am J Epidemiol 2006;163:717–25.
[Abstract/Free Full Text] - Wernli KJ, Ray RM, Gao DL, et al. Cancer among women textile workers in Shanghai, China: overall incidence patterns, 1989–1998. Am J Ind Med 2003;44:595–9.[CrossRef][Web of Science][Medline]
- Cheng KK, Day NE, Duffy SW, et al. Pickled vegetables in the aetiology of oesophageal cancer in Hong Kong Chinese. Lancet 1992;339:1314–18.[CrossRef][Web of Science][Medline]
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  K. J. Wernli, E. D. Fitzgibbons, R. M. Ray, D. Li Gao, W. Li, N. S. Seixas, J. E. Camp, G. Astrakianakis, Z. Feng, D. B. Thomas, et al. THE AUTHORS REPLY Am. J. Epidemiol., August 15, 2006; 164(4): 402 - 403. [Full Text] [PDF]
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