shower-water-disinfection-byproducts

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Shower water disinfection byproducts are chemicals that form when disinfectants such as chlorine react with organic material in water. The two sources used for this guide measured several of them. One study followed seven people through household water-use activities in two homes and measured chloroform and bromodichloromethane in indoor air, exhaled breath and blood. The other measured haloacetonitriles and chloral hydrate in a laboratory model of human skin. Neither source measured illness, so this guide does not say what the measured levels mean for anyone’s health.

Quick answer

• In the 2006 Environmental Health Perspectives study, showering and bathing were the only activities that significantly raised chloroform in exhaled breath, even though machine washing and opening a dishwasher raised indoor air levels.
• The 2011 Journal of Applied Toxicology paper calls haloacetonitriles and chloral hydrate “important cytotoxic DBPs.” It measured how these compounds pass through skin. It did not measure disease.
• Neither source gives a safe level for shower water, a health threshold, or results from people with health problems. Both used healthy or laboratory subjects.
• Neither source compared shower heads or filters, so neither can say whether a product changes these levels.

Which disinfection byproducts the sources name

The Environmental Health Perspectives paper focuses on trihalomethanes, which it calls “the major class of disinfection by-products of water treated with chlorine.” It measured four of them: chloroform, bromodichloromethane, dibromochloromethane and bromoform. Chloroform and bromodichloromethane were the two that produced measurable breath results. The paper states that “Neither dibromochloromethane nor bromoform gave measurable breath concentrations for any of the activities investigated because of their much lower tap-water concentrations.” That is a result about the two study homes, not a statement that these two compounds are harmless.

The Journal of Applied Toxicology paper focuses on a different family. Its introduction describes disinfection byproducts broadly, listing “trihalomethanes, haloacetic acids, haloacetonitriles (HANs) and haloketones.” The paper then studies five haloacetonitriles, abbreviated in its tables as CAN, DCAN, TCAN, BCAN and DBAN, plus chloral hydrate (CH). Its abstract names dibromoacetonitrile, which had the highest permeability of the group, and chloroacetonitrile, which had the lowest.

The reason these compounds form is the same across both sources. The Journal of Applied Toxicology introduction explains that “These compounds are formed by reactions of disinfectants such as chlorine with humic substances and other organic material commonly found in water.” In other words, the byproducts are created in the water supply and in pools, and a shower simply delivers them. The sources do not say how much of a given byproduct a particular city’s water contains today.

How the studies measured them

The Environmental Health Perspectives study was run in two single private homes, “one in North Carolina (NC) and the other in Texas (TX).” Both were ranch-style houses with three bedrooms and two bathrooms. The authors chose the homes because they expected different trihalomethane patterns in the two water supplies. The paper says that “Seven subjects performed the various water-use activities in two residences served by water utilities with relatively high and moderate total THM levels.” The seven subjects were described as “Seven healthy, young subjects.” All were nonsmokers. Each subject slept in the study home the night before to limit earlier exposure.

Each subject took part in several water-use activities over two days about a week apart. The activities included a hot shower, a bath, drinking tap water, hand washing, dishwashing and a clothes washer. The paper reports that “Water, indoor air, blood, and exhaled-breath samples were collected during each exposure session for each activity, in accordance with a strict, well-defined protocol.” Indoor air was collected in evacuated stainless steel canisters during each activity. Breath samples were taken 5 minutes after each activity ended, and blood was drawn through a catheter before and after each activity.

The authors controlled conditions so that comparisons between activities would mean something. They describe a design in which “To maintain a consistent exposure environment, the activities, exposure times, air exchange rates, water flows, water temperatures, and extraneous THM emissions to the indoor air were carefully controlled.” The paper gives the shower length as 10 minutes and the bath as 14 minutes, so its results describe those durations.

The Journal of Applied Toxicology study did not sample homes at all. It used diffusion cells, which are small chambers that hold a sample of skin between two fluids, and it tracked the compounds as they passed from a water-like solution through the skin into a receiving fluid. The skin was human cadaver skin. The chemicals were measured by gas chromatography using US EPA Method 551.1, a standard drinking-water method. The paper’s steady-state experiments used a donor concentration of 0.8 g/L for the haloacetonitriles and chloral hydrate. That figure is roughly 33,000 times the 24 µg/L the paper gives for how high haloacetonitrile levels typically reach in drinking water. The comparison is this guide’s arithmetic from the paper’s own figures, and it shows that the skin experiment used concentrations far above what most people encounter.

What the breath study found

The table below shows the normalized averages from the Environmental Health Perspectives study. Each value is the concentration in air or breath divided by the tap-water concentration, in micrograms per cubic metre per microgram per litre, averaged over all subjects. The values are means with standard errors, as reported in the paper’s Table 4.

Compound	Shower, air	Bath, air	Shower, breath	Bath, breath
Chloroform	2.28 ± 0.20	0.68 ± 0.12	0.19 ± 0.02	0.25 ± 0.04
Bromodichloromethane	2.09 ± 0.27	0.55 ± 0.10	0.23 ± 0.05	0.22 ± 0.06

The air values during showering were much higher than during bathing for both compounds. Breath values were about the same for the two activities. The paper notes that breath levels were essentially the same for the two activities even though shower air levels were higher, and it reads that pattern as a sign that the skin route matters more in the bath. The companion article covers the skin route in more detail. The paper also reports that “The exhaled-breath levels are approximately an order of magnitude lower than the levels in the indoor air” for the same activities. Breath levels rose clearly after a shower. The paper reports “showering (5-fold increase) and bathing (6-fold increase)” relative to baseline breath levels.

The before-and-after numbers make the same point in simpler terms. For the 10-minute shower, the paper gives mean normalized chloroform in breath of “0.19 ± 0.02 vs. 0.04 ± 0.01” after and before the shower. That is an increase in what subjects breathed out, measured five minutes after they stopped showering.

The study also looked at how the measurements related to one another. For chloroform from showers, the paper reports strong correlations between indoor air and breath, between breath and blood, and between air and blood. For bathing, only the water-to-breath and blood-to-air links were significant. The paper also states that “the breath concentration can be used as a proxy to estimate body burden or changes in body burden with time.” That statement is the authors’ view. It describes how breath can track blood levels in this study, not a clinical measure of harm.

The breath results show one clear finding. Showering and bathing “caused significant increases in the breath chloroform levels,” while machine washing and opening a dishwasher caused large jumps in indoor air but no comparable rise in breath. This is the paper’s main result, and it is about how much people take in, not about what that intake does.

What the skin study measured

The Journal of Applied Toxicology paper does not measure a shower or a person. Its main output is a permeability coefficient for each compound. The paper defines this as “The skin’s permeability was measured in vitro as the permeability coefficient (Kp) using Fick’s law assuming steady-state conditions.” Readers who want the numbers will find them in the companion article on chlorine dermal absorption. In this guide the point is the type of measurement: a lab model that shows how fast a compound crosses skin, not whether that crossing causes harm.

The paper’s own introduction says why it was written: “Potential dermal exposures to HANs and CH have not been reported.” It then estimates doses for a general population using Monte Carlo simulations, which are computer models that run many randomized combinations of inputs. The paper states that “It was assumed that all of the consumed drinking water was unfiltered tap water to provide an estimate of the maximum ingestion dose.” That assumption means the model does not describe what a filter would do to the dose.

The skin used was frozen before use. The paper says “Human cadaver skin was frozen prior to use, so no metabolic activity was expected.” The paper’s reason is that frozen skin should show no metabolic activity. The skin was still not living skin in a person, which limits what the result can say about the body.

What the sources do not say

• Health outcomes. The Journal of Applied Toxicology abstract says that disinfection byproducts “have long been a human health concern and many are known carcinogens and teratogens.” That is background from the wider literature. The paper does not measure disease, cancer or birth outcomes in any group. For the haloacetonitriles it states that “These compounds have been reported to be mutagenic and teratogenic,” citing earlier studies that are not part of the two sources checked for this guide.
• Thresholds. Neither source gives a level of shower-water exposure that is considered safe or unsafe. The breath study reports concentrations, not risk.
• Who was studied. The breath study used seven healthy young adults in two homes. The paper does not include children, older adults, pregnant people, or anyone with lung or skin conditions. The skin study used cadaver skin, not living people.
• Water supplies. The breath study homes were served by utilities that used “combined chlorine as the residual disinfectant.” The paper does not claim its two homes represent other water systems, and it does not compare them with others.
• Shower equipment. Neither source compared shower heads, handhelds or filters. The skin model assumed unfiltered water. Nothing in the two sources shows whether a product reduces byproduct levels in the air, in breath or in the body.
• Future work. The breath study authors conclude with “Future studies are warranted to explore changes in these activities that can affect exposure.” The paper does not give that work.

How we chose

The brief named two sources, and this guide uses only those two. Both are peer-reviewed papers available through PubMed Central. Both were read from saved full-text copies in the site research folder, checked on 2026-10-28. A third saved file in the same folder was not used, because the brief limits this guide to these two sources. Both papers measure disinfection byproducts in water-use settings, which is why they were paired. They were then split between two articles: this one covers which compounds were measured, how they were measured, and what the sources leave out for health. The companion article covers the skin permeation numbers and the modeled dermal doses.

Every direct quotation in this guide was checked against the saved text of its source. Figures from the tables were copied from the papers’ tables rather than from summaries. Statements drawn from the papers’ introductions or from older studies they cite are labelled as such.

What we could not verify

• Current byproduct levels in any city or in the reader’s own water supply. Both studies measured specific homes or lab conditions, and this guide did not check newer utility data.
• The 24 µg/L figure for typical haloacetonitrile levels in drinking water. It comes from a 1991 review cited inside the Journal of Applied Toxicology paper. That review was not one of the two sources checked.
• The claim that many byproducts are “known carcinogens and teratogens.” The paper states it as background. This guide did not open the studies behind it.
• Whether the breath results from seven adults in two homes apply to anyone else. The sample is small and was chosen for a research design.

FAQ

Do these two sources show that shower water disinfection byproducts cause illness?

No. Neither source measured illness. The Journal of Applied Toxicology paper cites earlier work describing some of these compounds as mutagenic or teratogenic, but its own experiments measured how they pass through skin. The Environmental Health Perspectives study measured breath, air and blood levels in seven healthy adults, with no health outcome recorded.

Did chloroform levels in breath rise after a shower?

Yes. In the Environmental Health Perspectives study, breath chloroform rose about five times above baseline after a 10-minute shower, with the same increase reported across all subjects at both sites. The study did not measure symptoms, so the increase should not be read as a health effect.

Do the sources say whether a shower filter removes these compounds?

No. Neither source evaluated or compared a shower filter. The skin model assumed unfiltered tap water. Filter claims need a separate source that measures the filter itself.

Are these results typical of every home?

No. The breath study used two homes and seven subjects. The skin study used cadaver skin and concentrations far above typical drinking water. Water utilities, disinfectants and home plumbing all vary.

Who should I talk to about shower water concerns?

If you have breathing problems or skin symptoms, or you are worried about exposure, talk to a doctor. For skin concerns, a dermatologist is the right specialist. This guide cannot assess any individual’s risk.

Bottom line

The two sources show that showering and bathing in chlorinated tap water can raise trihalomethanes in breath, and that haloacetonitriles cross human skin in a lab model. They do not show what those levels mean for health, and they do not test any shower product. Treat them as exposure measurements from small, specific settings, and take any health question to a doctor.

Sources

• Gordon SM, Brinkman MC, Ashley DL, et al. Changes in Breath Trihalomethane Levels Resulting from Household Water-Use Activities. Environ Health Perspect. 2006;114(4):514-521. https://pmc.ncbi.nlm.nih.gov/articles/PMC1440773/ (opened 2026-10-28)
• Trabaris M, Laskin JD, Weisel CP. Percutaneous Absorption of Haloacetonitriles and Chloral Hydrate and Simulated Human Exposures. J Appl Toxicol. 2011;32(6):387-394. doi:10.1002/jat.1657. https://pmc.ncbi.nlm.nih.gov/articles/PMC3199364/ (opened 2026-10-28)