Long-term, high-level exposure to bacterial endotoxin-- a contaminant found in raw cotton fiber and cotton dust -- is associated with a 40 percent decrease in lung cancer risk among female Chinese textile workers, according to a new study in the March 7 Journal of the National Cancer Institute.
Since the 1970s, studies in the U.S. and abroad have reported a lower than average risk of lung cancer for textile workers. Additionally, studies have shown that workers in other occupations with high endotoxin exposure, such as dairy farmers, have reduced lung cancer risks as well. Although many researchers thought endotoxin might be associated with reduced risk of lung cancer, no previous studies had quantified the relationship between endotoxin exposure and lung cancer risk.
George Astrakianakis, Ph.D., of the University of Washington in Seattle, and his colleagues compared the endotoxin exposure of 628 female cotton textile workers in Shanghai who were diagnosed with lung cancer with a group of 3,184 female workers without lung cancer who were matched by age to the cancer patients. They estimated the workers' total endotoxin exposure in textile factories based on their measurements of cotton dust exposure, which varied depending on the workers' jobs and length of employment.
The risk of developing lung cancer decreased as workers were exposed to greater amounts of endotoxin over many years. Twenty years of exposure to endotoxin reduced the incidence of lung cancer to approximately 7.6 per 100,000, compared with 19.1 per 100,000 for the average Shanghai woman. The risk was lowest for women whose endotoxin exposure occurred early in their career.
How endotoxins could reduced lung cancer risk is unclear. "Potential anticarcinogenic effects of endotoxin are probably mediated by the innate and acquired immune systems, although the specific mechanisms have yet to be elucidated," the authors write.
The researchers considered several other factors that could have influenced their results. The protective effect of endotoxin could not be explained by differences in smoking habits, but the authors could not exclude a potential bias in the study's design, which they call the healthy worker survivor effect. The authors also acknowledge that uncertainties exist in estimating endotoxin exposures in past years, but the findings remained virtually unchanged when different exposure scenarios were applied.
In an accompanying editorial, Paolo Boffetta, M.D., of the International Agency for Research on Cancer in France, discusses the importance of this finding for lung cancer research, but warns that the study's limitations make it too early to consider using endotoxins for lung cancer prevention. "Results of the study by Astrakianakis [and colleagues] are strongly suggestive that endotoxin exposure is associated with a reduced risk of lung cancer, but potential confounding [variables] and lack of strong supportive mechanistic evidence prevent stronger conclusions," Boffetta writes. "Great caution should be exercised by all when moving from the results of observational studies of the effects of complex mixtures to interventions aimed at cancer prevention."
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