Estimating National Exposures and Potential Bladder Cancer Cases Associated with Chlorination DBPs in U.S. Drinking Water

Abstract

Background: Disinfection byproducts (DBPs) in public water systems (PWS) are an unintended consequence resulting from reactions between mostly chlorine-based disinfectants and organic and inorganic compounds in source waters. Epidemiology studies have shown that exposure to DBP (specifically trihalomethanes) was associated with an increased risk of bladder cancer.

Objective: Our goal was to characterize the relative differences in exposures and estimated potential bladder cancer risks for people served by different strata of PWS in the United States and to evaluate uncertainties associated with these estimates.

Methods: We stratified PWS by source water type (surface vs. groundwater) and population served (large, medium, and small) and calculated population-weighted mean trihalomethane-4 (THM4) concentrations for each stratum. For each stratum, we calculated a population attributable risk (PAR) for bladder cancer using odds ratios derived from published pooled epidemiology estimates as a function of the mean THM4 concentration and the fraction of the total U.S. population served by each stratum of systems. We then applied the stratum-specific PARs to the total annual number of new bladder cancer cases in the U.S. population to estimate bladder cancer incidence in each stratum.

Results: Our results show that approximately 8,000 of the 79,000 annual bladder cancer cases in the United States were potentially attributable to DBPs in drinking water systems. The estimated attributable cases vary based on source water type and system size. Approximately 74% of the estimated attributable cases were from surface water systems serving populations of $>\mathrm{10,000}$ people. We also identified several uncertainties that may affect the results from this study, primarily related to the use of THM4 as a surrogate measure for DBPs relevant to bladder cancer.

Discussion: Despite significant reductions in exposure over the past several decades, our study suggests that $\sim 10\%$ of the bladder cancer cases in the United States may still be attributed to exposure to DBPs found in drinking water systems. https://doi.org/10.1289/EHP9985.

Figure 1.

Overview of methodology used to estimate U.S. national exposures and potential bladder cancer cases for each stratum. This illustrates information sources and actions applied for each stratum.

Figure 2.

States and tribal lands that submitted THM4 compliance monitoring data for calendar year 2011 and comprising Six-Year Review 3 data set (shown with shading). Data for 20,760 disinfecting systems serving a total of nearly 198 million people. See Supplemental Material Table S1 for information about number of THM4 records by system size and source water type. Map was created in R (version 4.0.0; R Development Core Team). Note: THM4, trihalomethane-4.

Figure 3.

Comparison of THM4 and bladder cancer epidemiological dose–response information. The slope shows the odds ratio plot from Regli et al.; also shown is the central, upper, and lower curves from the Villanueva et al. pooled analysis; the shaded region is based on the lower and upper confidence intervals for the four exposure category results (males) from Costet et al. See Supplemental Material, “R Script of Data and Equations,” for Figure 3.

Figure 4.

Bromine Incorporation Factor (BIF) vs. THM4 for 903 free chlorine systems serving $\ge \mathrm{10,000}$ people stratified by water source. BIF was calculated for each annual system-level average THM4 sample based on the molar concentration and number of bromine atoms for each THM specie. Systems were grouped in $10\mathrm{\mu }\mathrm{g}/\mathrm{L}$ increments based on system average THM4 concentrations. For each group, standard error was estimated as the standard deviation divided by the square root. See Excel Table S8 for underlying data. Note: THM4, trihalomethane-4.

Figure 5.

Cumulative distribution for the mass-based ratio of THM3 (i.e., brominated THMs) to THM4 for common systems between 1998 and 2011 data sets ($n=127$). Researchers calculated mass-based ratio for each system from the 1997–1998 Information Collection Rule data set that provided both THM3 and THM4 concentrations and compared to matching ratio from 2011 Six-Year Review 3 data set. Statistical testing performed using R (version 4.1.2; R Development Core Team) for the hypothesis that the means and variances were not different. See Excel Table S9 for underlying data. Note: THM3, three brominated species of THM4; THM4, trihalomethane-4.

Figure 6.

Cumulative distribution for the ratio of HAA5 to THM4 for common systems between 1998 and 2011 data sets ($n=153$). Researchers calculated mass-based ratio for each system from the 1997–1998 Information Collection Rule data set that provided both HAA5 and THM4 concentrations and compared to matching ratio from 2011 Six-Year Review 3 data set. Statistical testing performed using R (version 4.1.2; R Development Core Team) for the hypothesis that the means and variances were not different. See Excel Table S10 for underlying data. Note: HAA5, haloacetic acid-5; THM4, trihalomethane-4.