Lifetime exposure to disinfection by-products and risk of cancer at six anatomic sites in Iowa, USA

Abstract

Background: Disinfection by-products (DBPs) are suspected risk factors for multiple cancers. In our population-based case-control study in Iowa, we previously found moderate risks of bladder, brain, and rectal (but not colon, kidney, or pancreatic) cancers with duration of use of chlorinated surface water, known to have elevated DBP levels. We reanalyzed these data using quantitative estimates of lifetime DBP exposure. Methods: We identified 1,225 bladder, 586 colon, 566 rectal, 311 brain, 334 kidney, and 302 pancreatic cancer cases and 2,106 controls (1986-1989). We developed historical annual estimates of total trihalomethanes (TTHM) and the sum of five haloacetic acids (HAA5) levels for Iowa public water supplies. Using residential water source histories for each participant, we computed lifetime average exposure (µg/L) and exposure years above the U.S. maximum contaminant levels (MCL) for TTHM (>80 µg/L) and HAA5 (>60 µg/L); private well users were assumed to have no DBP exposure. We estimated odds ratios (ORs) and 95% confidence intervals (CI) for exposure quantiles from logistic regression models adjusted for potential confounders.Results: Rectal cancer risk was associated with lifetime average TTHM exposure above the 93.5th percentile (≥51.7 µg/L; OR=1.56, 95%CI=1.0-2.6; p-trend=0.02) and with 25+ years of exposure to levels above the MCL (OR=1.65, 95%CI=1.0-2.7; p-trend=0.05) but not with shorter duration; results were similar for men and women. Lifetime average TTHM exposure was not significantly associated with other cancers. 25+ years of exposure ≥80 µg/L TTHM was associated with increased odds of colon (OR=1.96, 95%CI=1.1-3.5), bladder (OR=1.38, 95% CI=1.0-2.0), and brain (OR=2.59, 95%CI=1.0-6.4) cancer among men, but not women or for shorter duration of exposure at this level. We observed sporadic positive associations with HAA5 exposures across cancer sites.Conclusions: Our findings contribute new quantitative evidence of associations between high levels of TTHM and rectal, colon, bladder, and brain cancers.