摘要：More than 250 million U.S. residents, and millions more throughout the world, drink water treated by chemical disinfectants. Although it is important to kill harmful pathogens in water, chemical disinfection results in the formation of disinfection by-products (DBPs). DBPs represent a continuous, ubiquitous exposure, with levels much higher than other emerging contaminants that currently capture the public’s attention, such as pharmaceuticals and per- and polyfluorinated alkyl substances (PFAS).
1–
2
Of all the chemicals that people are exposed to through their drinking water, DBPs are one of only a handful with a convincing body of evidence for adverse human health risks. Bladder cancer is the health effect noted most consistently in epidemiologic studies of DBP exposure.
In their groundbreaking paper, Weisman et al. conducted the largest risk assessment of DBPs in the United States to date, focusing on bladder cancer cases associated with chlorinated drinking water.
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The authors, most of them from the U.S. Environmental Protection Agency (EPA) Office of Water, obtained DBP data from public water systems serving 198 million people from 42 states. These data were collected as part of the third Six-Year Review of the DBP regulations.
From this risk characterization, it is clear that the Safe Drinking Water Act (1974) and subsequent Stage 1 and Stage 2 DBP Rules (finalized in 1998 and 2006, respectively) reduced trihalomethane (THM) levels and potential bladder cancer cases attributed to DBPs significantly. This is hugely encouraging and shows that regulations can work. However, Weisman et al. note that
∼
8,000
of the 79,000 annual bladder cancer cases can still be attributed to DBPs.
Despite the difficulty in proving causality for suspected carcinogens, the weight of evidence supporting the link between DBPs and bladder cancer has increased since the Stage 2 DBP Rule.
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Something interesting (and a bit unexpected) was the finding by Weisman et al. of greater risk of bladder cancer from large drinking water systems compared with small systems.
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It is well known that DBP exceedances occur more often in small systems,
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which typically do not have the resources of larger systems to install innovative treatment methods to reduce DBP levels.
Weisman et al. also considered sources of uncertainty and whether the four regulated THMs (also known as THM4) are good surrogates for unregulated DBPs that have higher toxicity.
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Emerging evidence suggests that haloacetonitriles and iodoacetic acids should be considered for future drinking water regulations. For example, a recent forcing factors study, which measured 72 DBPs and whole water cytotoxicity in a variety of impacted drinking waters across the United States, found that haloacetonitriles are important drivers of cytotoxicity.
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In vitro studies have also shown that haloacetonitriles are much more toxic than the regulated THMs and haloacetic acids,
2,
6
and one haloacetonitrile—dibromoacetonitrile—is a rodent carcinogen in two species.
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Iodoacetic acids have been also found to be important toxicity drivers for chloraminated drinking waters when iodide is present.
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These chemicals are among the most genotoxic and cytotoxic DBPs studied, with one of these— iodoacetic acid (IAA)—the most genotoxic of all DBPs studied to date.
2,
6
IAA also causes tumors in mice.
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Unfortunately, the lack of national occurrence data seems to have prevented the U.S. EPA from moving forward in considering haloacetonitriles or iodoacetic acids for regulation. The fifth Unregulated Contaminant Monitoring Rule (UCMR-5) would have been an ideal way to collect much-needed occurrence data for these highly toxic DBPs, and haloacetonitriles can be easily measured using existing U.S. EPA methods. However, 29 of the 30 contaminants on the new UCMR-5 are PFAS,
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which I believe may be the result of intense public pressure due to their presence in the news. However, unlike PFAS, DBPs have a substantial body of evidence for increased cancer in human populations, with, as this new paper states,
∼
8,000
U.S. cases annually that can be attributed to DBPs. The public might think that because 11 DBPs are regulated, the problem is solved. However, there is clearly more to do to eliminate this health risk.
