Endotoxin exposure and lung cancer mortality by type of farming: is there a hidden dose-response relationship?

Previous studies have suggested that those in occupations exposed to endotoxin have a reduced rate of lung/respiratory cancer. An initial investigation found a significantly reduced risk of all sites malignant neoplasms in white male crop and livestock farmers, and black male and female crop farmers. This study provides data on lung/respiratory cancers in the same workers. Data were obtained from occupation and industry-coded US death certificates collected from 26 states for the period 1984-1993. Cause, sex, and race specific proportionate mortality ratios (PMRs) were calculated using a National Institute of Occupational Safety and Health computer program. A pooled relative risk (PRR) was obtained by summing up separately and then dividing the sex-race specific observed and expected cases, separately in crop and livestock farmers. Deaths from respiratory cancer were 12,482 and 2,290, and deaths from lung cancer were 12,091 and 2,201. In each sex and race group respiratory and lung cancer PMRs are generally lower than unity. Lung cancer PRR was 0.80 (0.78-0.81) in crop farmers and 0.70 (0.67-0.73) in livestock farmers, a significant difference (p < 0.0001). Comparison of our findings with those by Nieuwenhuijsen et al. [1999] reporting personal exposure measurements in groups of Californian farmers (endotoxin averaging 132.5 EU/m(3) during livestock farming against 19.9 EU/m(3) during field crop and fruit farming), suggests a decreasing lung cancer risk with increasing endotoxin exposure, and supports a possible dose-response relationship between the two.