For decades, utilities in the U.S. have used a family of disinfectants known as chloramines to disinfect drinking water. Municipalities turned to these substances as an alternative to chlorine, because chlorine’s byproducts in water have been associated with possible adverse health effects such as elevated risks of colon and bladder cancer, low birth weight, and miscarriage. However, scientists have long suspected that at least one decomposition product of chloramine might also be detrimental to human health.
That byproduct has at last been identified, 40 years after it was first observed, but whether it’s dangerous remains an open question. The decomposition products of chloramines are difficult to isolate and identify, and this substance has proven particularly elusive. For decades, it has been referred to as simply “unidentified product.”
A paper published November 21 in Science appears to have solved the mystery. The paper , a small molecule with the formula ClN2O2 and the structure Cl-N-NO2. The anion is formed when monochloramine–the most commonly used chloramine disinfectant–decomposes into dichloramine (NHCl2), which then goes through a series of reactions with water, atmospheric oxygen, and other monochloramine molecules to form chloronitramide (HClN2O2). This substance dissociates in water to produce the chloronitramide anion.
Now, however, scientists need to determine whether their long-standing suspicions about potential toxicity of the formerly “unidentified product” are true. As of now, scientists do not know for sure. Daniel McCurry, author of the Perspective accompanying the study, says, “Unfortunately, we have no idea yet how toxic [the] chloronitramide ion is, or if it’s toxic at all–but estimates by the authors suggest that it probably is.”
It’s important to maintain a perspective on how the dangers posed by disinfectants and their byproducts compare to the benefits of their use. For those of us who’ve been lucky enough to grow up with clean drinking water, it’s easy to underestimate just how important it is to public health. As McCurry points out, “Water treatment is responsible for approximately half of the life expectancy gains seen in the U.S. over the first half of the 20th century, and a large portion of that is due to disinfection. It would be unthinkable to give that up to avoid the much smaller hazard posted by disinfection byproducts.”
Even chlorine, with its well-understood and well-documented risks, continues to be used to disinfect water–both in the U.S. and elsewhere–because the benefits of clean drinking water far outweigh chlorine’s potential dangers. McCurry says that at individual level, “The risk presented by a typical water contaminant is not worth losing sleep over on an individual basis.”
With that said, McCurry also says such a risk “is worth doing something about on a public health scale [by] doing what we can to minimize exposure to disinfection byproducts within realistic constraints (cost, technology, etc).” Making such judgements is essentially the key purpose of public health policy—almost all public health decisions involve weighing the costs of a given action against its benefits.
There are certainly alternatives to chlorine and chloramines, which come with their own benefits and detriments: many European countries disinfect their water with ozone, and McCurry says that another option is the use of UV light. A switch to either of these options in the U.S. would require investment in new infrastructure—ozone’s instability means that it needs to be produced on site, while UV light requires the construction of UV reactors. The benefit of these forms of treatment is that they leave no residue in the water, and therefore form no decomposition products.
However, the fact that chlorine and chloramines do remain in water means that they can continue to break down contaminants on an ongoing basis. This is not the case for ozone or UV light–they disinfect the water at the point of treatment, but offer no protection against any further contamination. Such ongoing protection is also currently required by U.S. law. As McCurry explains, “U.S. regulations require water leaving the waterworks to contain a ‘disinfectant residual’ (i.e., [a] measurable amount of disinfectant lingering in the water from the plant to the tap). That rule [exists] to prevent re-infection, should a pathogen get in there after the water leaves the plant.”
Ultimately, the decision on what measures—if any—need to be taken will depend on further research into chloronitramide’s effects. The EPA, which is ultimately responsible for the standards that govern drinking water in the USA, will then need to decide whether any further action needs to be taken.
“If the EPA decides to regulate [chloramines], then utilities switching back to chlorine will have to do something to control chlorine’s disinfection byproducts,” McCurry says. He notes that “there are plenty of other options to control [these byproducts]”: these include “removal of the organic precursors of [disinfection byproducts] by activated carbon or membrane treatment … but those are more costly than chloramination.”
Additional reporting by Lauren Leffer.
