“Scientists Found A Scary Link Between Tap Water & Cancer
Read this before your next swig.” - Amanda Mactas on Delish
The premise of this lifestyle article is that tap water in the U.S. is unsafe to drink because some chemicals in the water can cause cancer. The article asserts that “your everyday drinking water may be causing some damage,” the best solution is to boil your water or use a water filtration system. However, quasi-scientific articles can be misleading, and the public should be exceedingly skeptical when so-called scientific articles end with a marketing campaign, in this case, selling water filtration systems.
Regrettably, the article raises questions about an issue largely settled with years of scientific data: trihalomethanes (THMs) in drinking water.
Trihalomethanes
THMs are a group of chemicals formed when chlorine, added to the water as a disinfectant, combines with natural organic chemicals present in the water. Chlorine, added for disinfection, is responsible for the rapid improvements in public health in the first 40 years of the 20th Century, reducing infant mortality by 75% and childhood mortality from typhoid fever, cholera, and dysentery by roughly 66%.
In 1979, the EPA first set regulations on total THMs (TTHMs), the four most commonly found THMs in water, trichloromethane (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and tribromomethane (TBM). The regulations were based on a study that showed that TCM was carcinogenic, causing bladder cancers in rats and mice tested by instilling high doses dissolved in corn oil through a feeding tube. In 1998, the regulation level for TTHMs was reduced to 80 ppb.
Further animal tests showed that TCM and BDCM were not carcinogenic when tested using water instead of corn oil. [1] The EPA concluded that these chemicals were “not likely to be carcinogenic below a dose threshold,” meaning that TCM and BDCM must be present at a certain level to cause cancer, and levels below that are not carcinogenic.
Examining Cancer Rates 45 Years After Regulation
It has been more than 45 years since the U.S. and other countries regulated TTHMs in drinking water. If there was a genuine causal relationship between TTHMs and bladder cancer, this should now be apparent in decreasing bladder cancer rates. A 2019 study investigated whether overall reduced exposures to TTHMs through regulations have resulted in lower incidences of bladder cancer worldwide. The study examined national trends in eight countries: the U.S., Canada, the U.K., Germany, The Netherlands, Sweden, Australia, and Japan.
The study also examined lung cancer rates because of its dominant association with smoking, a significant risk factor for bladder cancer. Tobacco smoke is a tumor-initiator carcinogen, causing inflammation that promotes many types of cancer. Lung cancer risk decreases by 21% within 5 years of stopping smoking and by half 10 years after smoking stops, and the risk of bladder cancer also decreases.
The study did not find linkages between TTHMs and bladder cancer trends in the eight countries. As shown in the table, national levels of TTHMs have declined or remained stable over the years in all countries studied. The U.S. average levels of TTHMs halved between 1975 and 2015, with the incidence of bladder cancer remaining stable. The same general trend was observed in Canada.
The Netherlands eliminated chlorine use in surface water supplies by 1993. Yet, bladder cancer rates have trended upward in women, and TTHM levels have historically been low in Sweden, yet they too have observed an upward trend in bladder cancer in women.
Meta-analysis
Amanda Mactas’s article in Delish cited the results of a meta-analysis as proof that tap water caused cancer. [2] While meta-analysis has its strengths, it also has shortcomings, as I have written in the past. Shortcomings include:
- There are subjective judgments every step of the way, giving teams of like-minded people plenty of room to steer in a desired direction if they want to
- It is not at all unusual for a meta-analysis to be heavily dominated by a single study
- A meta-analysis is a snapshot in time that can be out-of-date the day it is published, with new research coming almost daily.
This review considered 29 epidemiology papers representing 12 unique studies on THMs and cancer, a very small number out of the total (2,020) articles identified in the authors’ literature search.
In comparing the highest THM exposure in residential drinking water to the lowest results, there was an increased relative risk for bladder cancer of 33% with no increased risk for colorectal cancer.
The authors concluded that this represents limited-suggestive evidence that THMs in drinking water increase the risk of bladder cancer and colorectal cancer, which sounds like scientific gobbly gook for no evidence. These conclusions should be treated with a great deal of skepticism, especially in light of the 2019 study that did not see a decrease in bladder cancer in the U.S. or Canada after 45 years of TTHM regulation
Ultimately, the real danger isn’t in your tap water—it’s in how easily selective data and scientific jargon can sell fear (and filtration systems). Nearly five decades of regulatory oversight, declining THM levels, and stable cancer rates should give us confidence in the safety of public drinking water. When lifestyle articles trade rigorous science for clickbait and conclude with a call to buy, it’s not your health they’re protecting—it’s your wallet they’re targeting. The scariest contaminants may not be in your water but in how we consume information.
[1] Corn oil is used because it is better for dissolving many chemicals than water.
[2] ] Meta-analysis is defined as '’the statistical analysis of a large collection of analysis results from individual studies for the purpose of integrating the findings.”
Source: National Trends of Bladder Cancer and Trihalomethanes in Drinking Water: A Review and Multicountry Ecological Study Dose Response DOI: 10.1177/1559325818807781
