Etiologic clues to lip cancer from epidemiologic studies on farmers

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Abstract
Objectives This study examined the risk factors for lip cancer among farmers. Methods A series of meta-analyses of peer-reviewed studies of lip cancer and farming were performed using 21 studies published between 1981 and 1997. Prior to the meta-analyses, all the studies were reviewed and evaluated for heterogeneity and publication bias. Combined relative risks (RR) were calculated using the random effect model. Results The combined RR for lip cancer was 2.0 [95% confidence interval (95% CI) 1.74-2.30] for all the studies and 1.28 (95% CI 0.79-2.08) for studies involving female farmers. Additional meta-analyses were undertaken on a subset of studies reporting on skin cancer among farmers. The combined RR for nonmelanotic skin cancer was 1.0 (95% CI 0.89-1.14), and for malignant melanoma it was 0.88 (95% CI 0.74-1.05). Conclusion The findings suggest that male farmers have a significantly elevated risk for lip cancer. Lip cancer and skin cancer do not share a common etiologic factor. Besides sunlight exposure, other factors such as viral infection or reduced immunity may play a role in the etiology of lip cancer.