A deep dive into THMs, the chemicals behind NWT water advisories

Some NWT communities have faced advisories over elevated levels of trihalomethanes – or THMs – in the drinking water. Scientists think that could become more common due to climate change.

So how worried should you be about THMs?

Researchers with whom Cabin Radio spoke said more work is needed to better understand the possible health effects of long-term and short-term exposure to the chemicals. They said following the recommendations of public health officials will help reduce the associated risks.

THMs are a byproduct of disinfection, created when chlorine reacts with organic matter, such as dead leaves, in the water.

Health Canada guidelines suggest the annual average of THM concentrations in a community’s drinking water shouldn’t exceed 100 µg/L.

According to those guidelines, THMs refer to the total amount of four chemicals: chloroform, bromodichloromethane, dibromochloromethane and bromoform.

Last November, the territorial government issued a THM-related public health advisory for communities in the NWT’s South Slave region including Hay River, Enterprise, Kátł’odeeche First Nation and Kakisa.

“No illnesses have been reported, however, we are advising residents out of an abundance of caution as lifetime exposure (70 years or more) to high levels of THMs may potentially increase the risk of cancer in individuals,” read the advisory issued at the time.

For nearly three months, residents were advised by the territorial government to filter their drinking water or leave it uncovered in a container for 24 hours before drinking. They were also asked to use bottled or filtered water for food preparation and take shorter baths and showers to limit exposure. 

In December, a separate health advisory was issued for Fort Liard after elevated levels of THMs were detected there. That advisory remains in effect.

High concentrations of THMs aren’t a phenomenon unique to the NWT. Elevated levels have previously been found in parts of British Columbia, Newfoundland and Labrador and Ontario.

While links between long-term exposure to THMs and certain cancers appear to be well established, some experts say the chemicals could have an effect on reproductive outcomes. Public health bodies say there isn’t yet enough evidence to support that conclusion. 

Experts agree that more research is needed to better understand the possible effects of consuming elevated levels of THMs, and water treatment upgrades may be needed if climate change makes the chemicals more common.

How THMs are typically managed

Manuel Rodriguez-Pinzon, an environmental studies professor and Drinking Water Research Chair at Université Laval in Quebec, said chlorine is an important part of water treatment. Using it nearly eliminates the risk of microbial pollution that could make people sick in the short term.

This is a common notion in public health guidance – the short-term gain of killing bacteria in the water outweighs the possible long-term risks. 

Adding sophisticated filtration systems to remove organic matter from the system is considered a best practice to reduce THMs, Rodriguez-Pinzon said, though that isn’t always the simplest nor cheapest solution, as it sometimes requires infrastructure upgrades and skilled staff.

He said in many cases, communities look to switch their water source from surface water – such as lakes and rivers – to ground water.

Ground water, Rodriguez-Pinzon said, generally tends to be higher quality because it is naturally filtered by the soil.

“Very small municipalities prefer to use ground water because the requirements of treatment are much less and the cost will be much less,” said Rodriguez-Pinzon.

(Many communities in the NWT, such as Yellowknife, rely on surface water, though Fort Liard is an exception, hamlet senior administrator John McKee told Cabin Radio.)

Low water levels and climate change could each play a role in THM concentrations, Rodriguez-Pinzon said, because the concentrations of organic matter in water can increase during droughts. Higher water temperatures facilitate the chemical reactions that make THMs.

“When chlorine reacts with organic matter to form key chains [of molecules], this reaction depends very much on kinetics – so the rapidity of the reaction is very much influenced by the temperature of water,” he said.

While research on the health effects of consuming THMs is evolving, Rodriguez-Pinzon stressed the risks of short-term consumption are minimal. 

“People have to understand that they are not going to have an impact – health impact – in the short term, but being exposed to chemicals in drinking water [over] years can produce some issues,” said Rodriguez-Pinzon.

Weighing the risk: Cancer and reproductive outcomes

In Barcelona and other areas of Spain, THMs have historically been a big problem as the country deals with water scarcity.

That’s according to Cristina Villanueva, associate research professor and head of the Environment and Health program at the Barcelona Institute for Global Health.

As a result, Villanueva said, a lot of research into THMs comes from this part of the world, where investments have been made in water treatment and desalinization to improve water access and quality while reducing THM levels.

Based on the available research, Villanueva said, there appears to be an established link between bladder cancers, colorectal cancers and THMs when consumed over prolonged periods, but more research is needed to understand the mechanisms that could cause those cancers.

Studies on animals where they are exposed to higher concentrations of THMs show a prevalence of various types of cancers, Villanueva added.

This analysis is echoed in draft updated guidelines on THM consumption prepared by Health Canada, which reference some of Villanueva’s work. 

She said a lot of the research conducted so far has looked into chronic or long-term exposures to the chemical byproducts, and more research would be needed to explore shorter-term effects of exposure.

Zorimar Rivera-Núñez, an environmental health scientist and assistant professor at Rutgers University in New Jersey, said studying the effect of THMs is complicated because there can be as many as 700 different chemicals in water, not all of which are regulated.

Similarly, Health Canada’s draft guidelines note there are so many different disinfection byproducts in drinking water that it can be difficult to identify the chemicals responsible for health effects.

The agency states that in humans, some studies have shown associations between prolonged exposure to THMs in drinking water and bladder cancer. Studies in animals have also seen an effect on the livers and kidneys of test subjects.

Health Canada also acknowledges a potential association between exposure to THMs in drinking water and reproductive and developmental effects. However, it notes “the epidemiological evidence is insufficient to determine whether any observed associations are causal,” meaning there isn’t enough evidence to associate the effects witnessed with THMs. 

Rivera-Núñez has focused her research on the possible association between THMs and reproductive outcomes. Several of the papers she has authored were referenced in Health Canada’s draft guidelines.

She cautions against anyone using a single paper to draw any conclusions, especially in the field of environmental epidemiology.

“You’re going to find some papers that say there’s some impact on birth weight or preterm birth, and there are other papers that said they find nothing” related to that, said Rivera-Núñez.

She acknowledged studies in this area of research are based on observational data, not randomized control trials, as would be the gold standard.

“We cannot do that with chemical exposure because we cannot dose people with THMs or lead, or any other chemical,” said Rivera-Núñez.

“So the best that we can find is observational data.”

She said some of the strongest studies she’s seen lately suggests there may be a relationship between THM consumption in pregnant women and birth defects.

One of the leading theories on how THMs may affect reproductive outcomes, Rivera-Núñez said, is that when metabolized, disinfection byproducts create a type of unstable molecule that easily reacts with other molecules in a human cell.

These can cause stunted growth in infants, premature births and sometimes miscarriages, because they react with hormones like progesterone that are important in maintaining a pregnancy, Rivera-Núñez said.

Rivera-Núñez pointed to a literature review from July 2025 that suggests high THM exposure may increase the risk of growth and urinary tract defects in infants. (A literature review analyzes a series of recent studies to summarize findings in one particular area of research.)

That review called for stricter standards to protect vulnerable populations, such as pregnant people.

How the guidelines are made

To minimize consumption of THMs when advisories are issued, Rivera-Núñez reiterated a lot of the advice issued by the territorial government. 

She said people can install tap filters designed to reduce or eliminate THMs. Similar filters are also available for shower heads to reduce exposure through the skin or inhalation of steam.

“You can also minimize the time that you spend in the shower or bathing,” said Rivera-Núñez. “If you’re under one of these warnings, maybe [it’s] not a good idea to take a bath if you’re a small child, older adult or any other at-risk populations, like pregnant women – I would not do that.”

In an email to Cabin Radio, a spokesperson for Health Canada said its guidelines are typically updated every five years “to incorporate emerging science or address urgent priorities.” 

“Health Canada’s review of the scientific literature, including studies published up to 2023, informs updates to the THM guidelines and considers vulnerable populations, such as pregnant individuals,” the spokesperson said. 

“Recent meta-analyses highlighting potential health impacts in these groups are reflected in the draft guidelines, which propose values based on the most up-to-date science and risk assessment. This ensures that regulatory thresholds for THMs are aligned with current evidence and protective of all populations.” 

A GNWT spokesperson said there wasn’t sufficient evidence to warrant mentioning a possible risk to reproductive outcomes, adding the advice issued by the department was already protective for everyone, including pregnant people and other vulnerable populations. 

“After reviewing the available research, the Department found that current scientific evidence does not show a clear or consistent link between THM levels in drinking water and reproductive or developmental problems,” the spokesperson said. 

“Because the studies are mixed and do not demonstrate a proven cause‑and‑effect relationship, we determined there is no scientific basis for recommending extra precautions specifically for pregnant people beyond what already protects the general population.

“We also want to avoid causing unnecessary worry by highlighting risks that the science does not clearly support.” 

Michelle Driedger, a professor at the University of Manitoba’s College of Community and Global Health and the former Canada Research Chair in Environment and Health Risk Communications, said the job of communicating public health risks is simpler when the evidence is more clear-cut. 

“In some of these studies, it becomes a challenge when there are all kinds of things that could be contributing to some of the reproductive outcomes that are being measured, because humans are not good lab rats,” said Driedger. “We can’t control the environment in exactly the same way as we can with animal-based studies.”

She said there are other considerations when it comes to setting guidelines and issuing public health advice, such as whether it may even be technologically feasible to reduce THMs concentrations below guideline levels. 

Water plant upgrades to reduce THMs

This month, the federal government committed $20.1 million toward a new water treatment plant in Hay River, an upgrade the town had long sought. 

The funding will cover about 75 percent of the costs of the project, and the town said it is now exploring partnerships with neighbouring communities to cover the remainder.

At a press conference about the funding announcement, Hay River senior administrative officer Glenn Smith said the new plant could help reduce THMs in tap water.

“Right now, a lot of reduced water in the Hay River and into even Great Slave Lake, warming temperatures – all of that can increase organics in the drinking water,” said Smith. “So we’re quite happy with the new plant that will be able to address those types of concerns.” 

He hopes to see the design phase of the project complete within the year and have construction begin soon after, though it may not be complete until at least 2029. 

In Fort Liard, senior administrative officer McKee said the community is working with the territorial government to change the levels of chlorine with which the water is treated, in an effort to bring down THM levels while still properly disinfecting the water. 

In the long term, he said, the hamlet is implementing upgrades and working with its engineers to put filters in place that would also work to reduce levels of the disinfection byproducts. 

McKee said the hamlet is looking to get those upgrades installed by the summer, though supply chain issues could cause delays. 

Rodriguez-Pinzon said that as new research emerges, the guidelines for consuming these chemicals could change. 

For now, Villanueva said, Canadian guidelines are fairly consistent with what is recommended in Europe. 

By contrast, regulations in the United States list the maximum acceptable concentration of THMs as 80 µg/L, lower than that suggested by Health Canada. 

Villanueva said there is some evidence to suggest lower levels of disinfection byproducts than those recommended in Canada and Europe could pose some risk, but more research would be needed to prove that conclusively.