Abstract
Epidemiologists have used trihalomethanes (THMs) as a surrogate for overall disinfection byproduct (DBP) exposure based on the assumption that THM concentrations are proportional to concentrations of other DBP classes. Toxicological evidence indicates THMs are less potent toxins than unregulated classes like haloacetonitriles (HANs). If THMs are not proportional to the DBPs driving toxicity, the use of THMs to measure exposure may introduce non-trivial exposure misclassification bias in epidemiologic studies. This study developed statistical models to evaluate the covariance and proportionality of HAN and THM concentrations in a dataset featuring over 9500 measurements from 248 public water systems. THMs only explain ∼30% of the variance in HANs, whether the data is pooled in a classic linear regression or hierarchically grouped by water system in a multilevel linear regression. The 95% prediction interval on HANs for the median THM concentration exceeds the interquartile range of HANs. Mean HAN:THM ratios range from ∼2.4% to ∼80% across water systems, and varied with source water category, season, disinfectant sequence and distribution system location. The HAN:THM ratio was 265% higher in groundwater systems than in surface water systems and declined by ∼40% between finished effluent and maximum residence times in surface water systems with chlorine-chlorine disinfection. A maximum likelihood approach was used to estimate the misclassification bias that may result from using THMs to construct risk-ratios, assuming that HANs represent the “true” DBP exposure risk. The results indicate an odds ratio of ∼2 estimated with THM concentrations could correspond to a true odds ratio of 4–5. These findings demonstrate the need for epidemiologic studies to evaluate exposure by measuring DBPs that are likely to drive toxicity.
Highlights
Epidemiologic studies have employed the four chlorinated and brominated trihalomethanes (THMs) as a surrogate for exposure to total disinfection byproducts (DBPs) (Grellier et al, 2015)
For studies within single water systems, this assumption requires that THMs are highly correlated with HANs, and the intercept is small such that the HAN:THM ratio does not vary substantially with THM concentration
Using THMs to measure DBP exposure implicitly assumes that THM concentrations are proportional to concentrations of more toxic DBPs in the mixture, and that this proportionality is robust to variables within water systems, such as distribution system residence time, and between water systems, such as differences in source waters
Summary
Epidemiologic studies have employed the four chlorinated and brominated trihalomethanes (THMs) as a surrogate for exposure to total disinfection byproducts (DBPs) (Grellier et al, 2015). The significance and magnitude of the association between high THM concentrations and bladder cancer (Costet et al, 2011), colorectal cancer (Rahman et al, 2010), and adverse reproductive outcomes (Grellier et al, 2010; Wright et al, 2017) has been inconsistent, meta-analyses suggest that the most consistent association is with bladder cancer (Hrudey et al, 2015). DBPs occur in a complex mixture in disinfected drinking waters, with over 700 species identified, and it remains unknown which classes are toxicity drivers (Li and Mitch, 2018; Richardson and Kimura, 2020). When concentrations of individual DBPs in drinking waters were weighted by their CHO cytotoxicity LC50 values, haloacetonitriles (HANs) consistently comprised the majority of the toxic potency-weighted concentrations, while THMs contributed very little (Chuang et al, 2019; Furst et al, 2019). Recent research has demonstrated that the CHO cytotoxicity of defined mixtures of regulated and unregulated DBPs is additive (Lau et al, 2020), supporting the use of toxic potency-weighted DBP concentrations to compare the contributions of individual DBPs to cytotoxicity
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