Does Chlorine in Tap Water Affect Your Gut Microbiome?

After writing about Auckland's tap water and what happens to rural households on tank and bore supplies, I kept getting a version of the same question. Not about what's in the water exactly, but about something more specific: what does the chlorine actually do? Not to pathogens — we know it kills those, which is the point — but to everything else. To us.

It's a reasonable question. Chlorine is an antimicrobial agent. That's why it's in the water. The question is whether that property switches off when it reaches your glass, or whether it keeps doing what it does once you drink it.

What chlorine is actually doing in your water

Auckland's water is disinfected with sodium hypochlorite — a form of chlorine — at the treatment plant, and a residual amount travels through the pipe network to your tap. That residual is intentional. The pipes are long, the network is old in places, and without that residual disinfectant, bacterial regrowth along the way becomes a real risk.

The amount at your tap is small — typically somewhere between 0.2 and 0.5 milligrams per litre, well within the New Zealand Drinking Water Standards limit of 5 mg/L. Watercare monitors this daily across hundreds of sites.

So far, unremarkable public health infrastructure.

The complication comes from two things: what chlorine reacts with in the water, and what it might do in the gut.

The byproducts problem

When chlorine meets organic matter — naturally occurring compounds from leaves, soil, and catchment runoff — it forms a class of chemicals called trihalomethanes, or THMs. Chloroform is the most common one. These aren't added intentionally. They're a byproduct of the disinfection process itself.

Auckland's Waikato River source water is particularly organic-rich — it travels through farmland and wetland before treatment — which means more organic matter to react with, and more THM formation potential. Watercare tests for these regularly and results have generally sat within standards, but it's a known management challenge with that source.

Long-term exposure to elevated THMs has been associated in epidemiological studies with small increases in bladder cancer risk. The evidence is not definitive, and the absolute risk at NZ levels is low. But it's the reason THM limits exist in drinking water regulations worldwide, and it's why some people who've looked into it closely end up filtering.

The gut microbiome question

This is where the science gets genuinely unsettled — and worth being honest about.

The gut microbiome is the community of bacteria, fungi, and other microorganisms living in your digestive system. Research over the past decade has established that this community plays a significant role in immune function, metabolism, mental health, and disease susceptibility. Disrupting it — through antibiotics, diet, or other exposures — appears to have real downstream effects.

Chlorine is, at its core, an antimicrobial. The question researchers have started asking is whether drinking chlorinated water at treatment levels affects the gut microbiome. The honest answer is: we don't fully know yet, and the studies that exist point in different directions.

A 2024 mouse study published in Science of The Total Environment found that exposure to chlorinated drinking water did reduce microbial diversity in the gut — with differences also seen between standard chlorine and chloramine treatment. But mouse studies don't translate directly to humans, and the concentrations and conditions aren't identical to household tap water exposure.

Human studies have been harder to design cleanly. The most rigorous one to date looked at children in Bangladesh where chlorination devices were installed in shared taps. It found no substantial impact on overall gut microbiome richness or diversity after a year. That's reassuring — but the Bangladesh setting involves different baseline microbiomes, different diets, and different levels of pathogen exposure than a household in Auckland. A 2024 study of infants in Haiti found some reduction in microbial diversity among those exposed to detectable chlorine residuals in their household water, though the sample was small.

A concept paper that has circulated in microbiome research circles — not a clinical trial, more a call for research — made the case that persistent low-level chlorine exposure in tap water was a plausible but under-studied risk factor for gut dysbiosis, and proposed a straightforward trial design to test it. As of now, that trial hasn't been done.

What this adds up to: there's a biologically plausible mechanism, some suggestive evidence from animal studies, inconclusive human evidence, and a significant gap in research on healthy adults in high-income countries drinking chlorinated municipal water over years. That's not a reason for alarm. But it's a genuinely open question.

Chloramine is a different consideration

Some water utilities have moved to chloramine — a compound formed by combining chlorine with ammonia — as a secondary disinfectant. It produces fewer THMs than free chlorine, which is why it's increasingly used. It also persists longer in the distribution system, which is useful for large pipe networks.

Auckland's primary treatment uses free chlorine. Some parts of the network may also use chloramine — Watercare's documentation describes disinfection as "chlorine or chloramine in some areas" — though free chlorine is the dominant method across most of the system.

The relevant difference for the gut microbiome question is this: free chlorine dissipates. If you let a glass of Auckland tap water sit on the bench for thirty minutes, most of the free chlorine will have offgassed. Chloramine doesn't do this. It's more stable and requires active removal — a catalytic carbon filter or a vitamin C-based filter specifically rated for chloramines. Letting it sit does nothing.

If you want to know which your area uses, Watercare's water quality reports for your specific zone will show it. The practical upshot: if you're relying on the jug-in-the-fridge method, it works for free chlorine. For chloramine, you need a filter.

This distinction matters more than most filtration marketing makes clear.

What this means practically

There's a version of this topic that becomes a rabbit hole of anxiety, and I'm not interested in going there. The reason chlorine is in the water is sound — waterborne illness is a serious public health problem, and chlorination has saved enormous numbers of lives globally. Filtering it out at the tap while leaving the distribution system protected is a reasonable middle position, not a rejection of public health infrastructure.

If you're on Auckland town supply and you have an RO filter or a quality carbon filter, you're removing the chlorine residual at the point of use. That's roughly what I do. The RO unit I wrote about previously removes chlorine, chloramines, THMs, and most other dissolved compounds as a matter of course.

If you don't have a filter and aren't planning to get one, leaving a jug of water in the fridge overnight will allow free chlorine to dissipate. It won't help with chloramines, but Auckland primarily uses free chlorine, so this is a practical low-cost step.

If you're particularly focused on gut health — you're managing dysbiosis, you're recovering from a course of antibiotics, you have a young infant whose microbiome is still developing — the question of chlorinated water is worth thinking about more carefully than it usually is. The research isn't settled enough to make a firm claim, but "this is an open question" is not the same as "this is fine."

For everyone else: the evidence for concern is real but preliminary. The evidence for chlorination's public health benefits is not preliminary at all. Hold both.

This piece is part of the OFT water series. The previous articles cover what's in Auckland's tap water, why I bought an RO filter, and what rural households on tank and bore water need to know.