Hamster cells bring byproducts into the library
Are nitrogen-containing disinfection byproducts more toxic than other organic halogens?
Some haloacetonitriles may be the most toxic byproducts unintentionally created when water is disinfected with chlorine or chloramines, according to new research. The study, published today on ES&T’s Research ASAP website (DOI: 10.1021/es0617441), represents a first step toward toxicological and epidemiological assessments of these compounds, which are a nitrogenous twist on already regulated drinking-water disinfection byproducts (DBPs).
U.S. EPA regulations specifically limit water treatment plants’ release of nine DBPs—four trihalomethanes (THMs) and five haloacetic acids. But as analytical methods improve, more and more DBPs have been documented. Experts in the research community and elsewhere have voiced growing concern over the unknown and potentially more toxic nitrogen-based DBPs, such as nitrosamines.
The new report by Michael Plewa, a toxicologist and geneticist at the University of Illinois at Urbana–Champaign, and colleagues revisits the haloacetonitriles. Two decades ago, assays of members of this class of compounds were shown to kill cells and damage genetic material. Haloacetonitriles are found at relatively low concentrations in drinking water.
Plewa and his co-workers used Chinese hamster ovary (CHO) cells in an in vitro COMET assay that the team previously modified and calibrated specifically for DBPs. The new research marks the first time haloacetonitriles have been assessed with the same protocols and materials that Plewa’s team has used to build a whole library of quantitative and qualitative in vitro toxicity information for other DBPs.
This new research allows for the direct comparison of individual DBP compounds as well as comparisons within and among classes of compounds, Plewa says. And within the haloacetonitriles class, the team has built a ladder of relative cytotoxicity and genotoxicity for seven compounds.
Other researchers note that the in vitro tests are only a step in identifying potential toxic properties and not an indication of actual comparative human risk with regard to better known and regulated DBPs. “If you are really trying to compare these things toxicologically, the thing to do is look at them in vivo,” says Richard Bull, an independent toxicologist who has worked for EPA, Washington State University, and Battelle. Bull notes that the data gathered by the new research are more qualitative than quantitative. “These are screening tests,” he says, noting the lack of a dependable quantitative relationship between mutagenic and carcinogenic potency. “But the tests themselves are really useful for qualitative information, [such as,] are these carcinogens?”
The researchers combined genotoxicity and cell toxicity into an index to judge the different classes of DBPs, and they concluded that the toxicities of haloacetonitriles and halonitromethanes are several magnitudes greater than that of the regulated haloacetic acids. However, the monohalo-forms of many of these compounds are clearly much more toxic than the di- and trihalo-forms, points out Dave Reckhow of the University of Massachusetts, Amherst.
These higher toxicity substances may be produced at elevated concentrations by water utilities that switch to alternatives to chlorine treatments such as chloramination, Plewa says. Such treatments are more efficient at minimizing THMs and trihaloacetic acids, the main DBPs regulated by EPA in its controversial Stage 2 ruling that went into effect on January 4.
At the same time, these smaller concentrations of more highly toxic forms could negate the advances in getting rid of the lower toxicity ones, Reckhow says. This conundrum “brings into question the idea of monitoring them all as a group instead of singling out” the more toxic monohalo-forms. It also highlights “the fact that we don’t have good, sensitive methods for these monohalogenated DBPs,” he adds.
“The most toxic of the haloacetonitriles in the study are not normally detected in water,” says Stuart Krasner of the Metropolitan Water District of Southern California. Utilities that have switched from chlorination to chloramination, to limit THMs or other DBPs, have not seen an increase in haloacetonitrile concentrations, and chloramination controls the formation of the haloacetonitriles, says Krasner. However, he and others point out that some DBPs, such as N-nitrososodimethylamine (better known as NDMA) and iodinated compounds, do increase under certain conditions.
Plewa’s team suggests, however, that switching to chloramination or other methods could lead to higher levels of nitrogenated species. Plewa points out that Krasner’s data show that about 70% of the total organic halogens in drinking water remain unknown and that these could include emerging nitrogenous and iodinated DBPs that are structurally related and potentially toxic. The Plewa team’s library of data will be useful for examining the toxicity of any newly identified compounds, some researchers say.
