I recently had a clogged drain requiring the services of a plumber. While discussing the details of the job, he took out brochures and a “fact sheet” prepared by his company explaining that my city tap water was going to kill me. Fortunately, they could provide a solution – a home-wide water filtration system.

The plumber seemed naively sincere, and genuinely fearful of the cancer-causing contaminants found in drinking water. He invited me to read through the material he provided while he unclogged by drain. I did better than that. I took the time to do a quick search for some more objective information on the topic.


The focus of this particular scaremongering is the additive monochloramine, which is added to city water. According to the Environmental Protection Agency (EPA):

Chloramines are disinfectants used to treat drinking water. Chloramines are most commonly formed when ammonia is added to chlorine to treat drinking water. The typical purpose of chloramines is to provide longer-lasting water treatment as the water moves through pipes to consumers. This type of disinfection is known as secondary disinfection. Chloramines have been used by water utilities for almost 90 years, and their use is closely regulated. More than one in five Americans uses drinking water treated with chloramines. Water that contains chloramines and meets EPA regulatory standards is safe to use for drinking, cooking, bathing and other household uses.

Chlorine is still used as a primary disinfectant for drinking water, but most systems switched to or added chloramines as secondary disinfectants because they do not dissipate quickly, and therefore are more effective. Also they produce fewer potentially-harmful byproducts than chlorine, and are therefore useful in meeting the new regulations about the levels of these byproducts.

Ozone, UV light, and chlorine dioxide are also used as primary disinfectants, usually at the water processing plant, not in the distribution system. Disinfectants are necessary to prevent waterborne diseases, like cholera and Legionnaires.

Disinfection byproducts (DBPs)

Of course, everything we do has potential risks and benefit. There is evidence to suggest that some DBPs have potential health risks, such as increasing the risk of cancer, anemia, low birth weight (but not other reproductive outcomes) and neurological symptoms.

As always, however, the dose makes the toxin. With questions of toxicity there are multiple lines of evidence, and we tend to see the same picture emerge over and over.

Toxicology studies look at the effects of the substance of concern on cell cultures. These often demonstrate toxic effects, and DBPs are no exception. Such studies, however, only show the toxic potential of substances, not what actually happens with human exposure. Direct exposure is likely to cause greater toxicity than what occurs in a living animal.

When considering actual toxicity, exposure, bioavailability, and detoxification need to be taken into consideration. While a substance may be toxic in a petri dish, it is not clear if will get to the target cells in a living animal in high enough doses to cause the same toxicity.

The next step is therefore animal studies. The purpose of such studies is not to see if a substance is toxic – everything it toxic in high enough dose – but rather to see the dose-response of toxicity. This information will then be extrapolated to humans (which is admittedly tricky) to develop safety limits on dose.

Human toxicity studies are rare because of ethical considerations. Therefore most human data is epidemiological – looking at exposure and outcomes out in the world (not in a controlled setting). This type of data can be tricky to interpret because of uncontrolled variables, but can provide reassurance that exposures are not associated with adverse outcomes.

The EPA uses all of this data to set safety limits on known DBPs, with a wide margin of safety in order to account for uncertainties in the data. Generally, levels of DBPs in drinking water are very low, and have been lowered over the years because of the use of chloramines and other techniques.

There are many DBPs, however, and it is possible that we have yet to discover all the possible DBPs that are forming in drinking water. Also, DBPs generally form as a reaction with organic contaminants in the water, and therefore the amount and type of such contaminants will alter the amount and type of DBPs that form.

Further, some DBP levels are regulated, while others are not. However, even unregulated DBPs are often still measured, and you can find this information about your local drinking water.

Another concern is that disinfectants can leach lead and copper out of pipes, increasing their levels in drinking water. These, however, are also measured and carefully regulated.


The brochure handed to me by my plumber contained all accurate information, much of it obviously copy-and-pasted from the EPA website. However, it was all cherry-picked with the obvious intent of scaring the average homeowner into thinking they will get cancer unless they buy an expensive filtration system.

The brochure mentioned that DBPs exist, they are potentially carcinogenic, there are some that are unregulated, and we may not have even discovered all the ones that exist. It further mentioned the leaching of lead and copper into the drinking water.

The brochure did not mention that monochloramines have been used for 90 years, that even unregulated DBPs are measured, that epidemiological evidence has not revealed any harm from DBPs, that monochloramines actually decrease their presence, and that the EPA carefully regulates and monitors their levels to keep them well below safety limits, with a wide margin to account for our current uncertainty in the scientific data.

I was also not given information on my local drinking water, information that is readily available online, and which shows that lead, copper, and DBP levels (regulated and unregulated) are well-below established safety limits.

There is also no information indicating that spending thousands of dollars on a home-wide water filtration system has any health or other benefits.

In short, I was given half the story – the scary half, with the clear intent on scaremongering into an expensive purchase. This was further justified using the precautionary principle – “shouldn’t we be safe, just in case?” Admittedly, where one draws the line with the precautionary principle is a bit subjective. Everyone can decide for themselves how much risk they are willing to take, and how much they are willing to spend to decrease potential risk.

Such decisions, however, should be made with full information, and in a proper context. It’s also useful to consider cost-effectiveness. How much money are you willing to spend to achieve what measure of risk reduction? Perhaps that money could be better spent somewhere else, and perhaps there is greater risk in the lost opportunity (for those of us without unlimited funds) from making a major purchase for dubious benefits.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.