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THM in the water

My city notified some residents of elevated levels of trihalomethane in the water, as a bi-product of disinfecting. One monitoring site recorded 81.7 ug/L while a second site recorded 89.05 ug/L, where 80.00 ug/L is the maximum allowable contaminant level. Apparently this has gone on for a about a year, and some neighbors have told me that its happened in many other years too. The director of public works says its no big deal and that I'd have to drink 2 L/day for 70 years to risk any damage as a result of trihalomethane. Trying to find answers, I ran into an EPA study project regarding THM that looks kind of alarming, but I know enough not to get freaked out just because I saw something on the internet. Should I be concerned or not?

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Contributor III

Re: THM in the water

Dear Jason,

Risk assessment is usually defined by specific metrics for a Risk Management professional such as myself. Individual (and corporate) risk assessments get much more unique and murkier based on individual risk-benefit understanding. ALL of life carries risks. Some are worse than others. Health factors can be difficult to evaluate because of the plethora of conditions in the subjects as well as the materials. So, the EPA and other health organizations always tend toward the ultra-conservative. That doesn’t mean that the prescribed “safe” levels aren’t good goals, just that exceeding them may not have an actual impact on you personally.

Compounds implicated in “cancer risks” are particularly difficult to pin down for individuals. First, cancer is a wide range of disease types and initiated by a host of individual factors – most particularly anything that causes more rapid changes to cellular structure or genetics. There are so many “causative” factors that it is realistically impossible to avoid them all.

Now, as a regulatory mandate, there should be some push-back on the agencies charged with maintaining “safe” levels. Corrections do need to be made, or the official risk assessment and allowable levels changed by the elected government representing the society. In the short term it is your own risk assessment as to whether the potential negative effects and probabilities of their occurrence are worth any preventive actions. You can switch to using only bottled water for drinking and cooking (although boiling may remove the contaminant anyway) – and how are you assured of the quality of the bottled water? Or you can use a specific filter or other methods proposed directly by the EPA on their website to remove residual contaminants:

Another site (slightly alarmist IMHO) also provides some good home alternatives:

I have my own well, but use both 10um filters for particulates and activated carbon for things like the halomethanes. You can obtain inexpensive home water filtration systems easily. I bought my from Amazon (others are available locally in hardware stores too) years ago, and only need to periodically change the disposable internal filters.

Best regards,


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New Contributor II

Re: THM in the water


   Steven Cooke provided some useful information. But the question is still the same: What are the "safe" levels of trihalomethanes? Others are likely to note that chloroform was used as an anesthetic as early as 1847. It was also used to treat asthma and as a component in cough syrups. But, as a loyal member of the faculty in the Department of Chemistry at Purdue, I have to invoke the work of one of our ex-chemistry professors, Harvey Washington Wiley, who is best known for his leadership in getting the Pure Food and Drug Act passed in 1906 and the FDA prohibited its use in humans in 1976 when it was found to cause cancer in lab animals. So, let's return to the question: Is there a safe limit to exposure to trihalomethanes?

   As you noted, the Maximum Contaminant Level (MCL) for total THMs in public drinking water that was established by the EPA is 80 ppb, which corresponds to 80 micrograms per liter. This raises the question: What does Maximum Contaminant Level mean?  MCL's are enforceable drinking water standards when one balances the adverse health effects of a substance against both the feasibility and costs of treating contaminated water as well as the beneficial effects of chlorinating water to minimize the risk of GI-tract diseases.

   It is easy to understand how the "2 L/day for 70 years" estimate was generated from existing standards for testing water contaminants. The "2 L/day" is the traditional starting point for risk assessment calculations. The calculation might be a bit misleading, however, because it assumes a 70 Kg body weight. I estimate that my water intake is, in fact, roughly 2 L per day, but I haven't weighed 70 Kg (154 lb) in perhaps the last 60 years. 

   Now that we have an MCL, and we recognize that the levels of THM's you cite is slightly above the value established by the EPA, we need to adjust it -- slightly -- to look at the effect this may have on you and your family. The typical state drinking water standard for THM's would be based on the generally recognized idea that THM's are not likely to cause cancer below the exposure limit that would lead to cell toxicity. This is estimated to be 70 ppb or 70 micrograms per liter for chloroform. 

   At this point you might start being worried because the measurements you cite might be considered significantly above typical values for the state drinking water standard. But, wait a second! By convention, the state drinking water standard is usually set at a value that is 20% of the total safe exposure. Why only 20%? To provide a margin of safety and to recognize that chloroform, at least, enters the body by three routes: inhalation, ingestion, and transdermal contamination because it can be absorbed through the skin. 

   Last evening, I was watching, once again, the movie "Bridge of Spies." On three separate occasions during the course of the movie, when Tom Hank's character notes that the Soviet spy he was working with did not seem to be worried,  Rudolf Abel responded each time: "Would it help?" Assuming an average of about 85 ppm in your water supply, this is 25% of the best estimate we have of a safe exposure. 

   So, let's think about your case? Are you going to avoid exposure to the local water system and drink only bottled water -- possibly exposing yourself to contaminants it might contain? But hoping it is better. Or are you going to recognize that the problem of exposure to THM's in your water supply is something you might pursue with the local authorities to bring levels down to perhaps 70 ppb? Or are you going to go about your life, assuming this is not a problem that is serious enough to worry about?

   In a superb book -- "Wolves Eat Dogs" -- Martin Cruz Smith takes his Russian police officer into the region around Chernobyl to solve a murder case. In the course of the book, he does a remarkably good job describing what happened, why it happened, and how the State responded to the exposure. If memory serves, which, I will admit, it often doesn't, it notes that estimates of fatalities due to the Chernobyl debacle range from, 51 to 300,000. The truth is somewhere between these estimates. After the tsunami hit the Fukishima reactors, calculations like the "2 L/day for 70 years" were issued for locations beyond the immediate answer. They were probably right, but not for the  "Fukishima 50," who remained on-site after 750 others were evacuated. 

Contributor III

Re: THM in the water

Thanks for a very clear explanation with examples of practical risk assessment and government regulation, George!  There is a very wide and fluid continuum from paranoia to carelessness.  Properly responsible government will represent the public's communal interest in safety.  Yet it cannot remove the responsibility of each individual to make their own choices.  Nor can it morally mandate required regimens or lifestyles.  A GOOD understanding of the risks in life from all quarters is essential to each individual for making better decisions regarding their own welfare.

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New Contributor III

Re: THM in the water

You should learn what the trend in THM monitoring data shows for your water supply. If it has been progressively increasing over years, that may be an indication of increasing organic matter in the source water leading to an increasing formation of disinfection by-products.

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