shower-water-chlorine-exposure
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Shower water chlorine exposure is the question of how much of the disinfectant in tap water reaches your body while you shower. The two studies used for this guide did not measure chlorine itself. They measured trihalomethanes (THMs), a group of by-products that form when disinfectants react with organic matter in water, with chloroform as the largest of them. The 2005 study followed seven subjects in two homes and measured chloroform in shower air, exhaled breath and blood. This guide explains what those numbers show, what they do not show, and why neither study says anything about which shower head to buy.
Quick answer
- Showering raised chloroform in the air of the bathroom. The 2005 study reports that “only showering and bathing caused significant increases in the breath chloroform levels” among the activities it tested.
- In that study, the mean normalized shower-air chloroform value for a 10-minute shower was 2.28 µg/m3 per µg/L, compared with 0.68 for a 14-minute bath (Table 4 of the paper).
- Breath levels after a shower were much lower than air levels. Breath was sampled 5 minutes after the exposure ended.
- Neither source gives a safe level, measures a health outcome from showering, or tests a shower head or filter.
Where the chlorine by-products come from
The 2005 study describes the THMs chloroform, bromodichloromethane, dibromochloromethane and bromoform as “major by-products of water-disinfection processes involving chlorine”. The 2011 study adds how these by-products form: they are “formed by reactions of disinfectants such as chlorine with humic substances and other organic material commonly found in water”.
This means the amount of by-product in your shower depends on the water supply, not only on the chlorine dose at the treatment plant. The 2005 study used two residences served by water utilities with different levels of THM. The authors describe them as having “relatively high and moderate total THM levels”. The sources do not name the utilities or give a formula for predicting a home’s level.
How the 2005 study was run
The study by Gordon and colleagues appeared in Environmental Health Perspectives online in 2005 and in print in 2006. The authors selected 12 common water-use activities. The result this guide focuses on is the shower and bath protocol. “Seven subjects performed the various water-use activities in two residences served by water utilities with relatively high and moderate total THM levels”.
The authors say they tried to keep conditions the same across the sessions: “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”. Each session collected several kinds of samples: “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”.
The study’s calculations used a specific set of assumptions: “10-min shower duration with 8.0 L/min flow rate”, a shower water temperature of 41°C, and a shower stall volume of 1.74 m3. The paper does not name a shower head model, so the 8.0 L/min figure describes the study’s setup, not a product that readers can buy.
For readers who think in gallons, the arithmetic is simple. Eight litres per minute for 10 minutes is 80 litres. Using the standard conversion of 3.785 litres per US gallon, 80 litres is about 21 US gallons, and 8.0 litres per minute is about 2.1 gallons per minute. These two conversions are calculated for this guide. They are not figures stated in the sources.
What the measurements showed
The paper reports water concentrations for each site. For chloroform, the mean water level at the North Carolina site was 126.2 µg/L on the first exposure day and 90.4 µg/L on the second. At the Texas site it was 28.5 µg/L and 22.2 µg/L. The authors describe the air results as “appreciable increases in air concentrations for all subjects at both sampling sites, despite the large difference in tap-water CHCl 3 concentration between the two sites”.
The normalized values in Table 4 divide the air or breath concentration by the water concentration, so that homes with different water levels can be compared. For chloroform, the table gives these means:
| Chloroform, normalized mean (± SE) | 10-minute shower | 14-minute bath |
|---|---|---|
| Air during the activity (µg/m3 per µg/L) | 2.28 ± 0.20 | 0.68 ± 0.12 |
| Exhaled breath, 5 minutes after (µg/m3 per µg/L) | 0.19 ± 0.02 | 0.25 ± 0.04 |
The table shows that shower air had a higher normalized chloroform value than bath air, while breath values were close for the two activities. The paper also reports that breath levels after a shower were higher than before it: the post-exposure breath values were “consistently higher than the initial pre-exposure levels for the 10-min showering activity”. The breath difference is small compared with the air difference, and the paper’s own discussion notes that breath concentrations drop quickly after exposure.
The authors also compared their normalized shower-air values with three earlier home studies. The table below uses the paper’s Table 5 for chloroform only. Tap-water values differ widely between the studies, while the normalized air values fall in a narrower band.
| Study | Mean tap water chloroform (µg/L) | Normalized shower air chloroform (µg/m3 per µg/L) |
|---|---|---|
| May et al. (1995), as reported in the 2005 paper | 55 | 1.72 |
| Kerger et al. (2000), as reported in the 2005 paper | 47 | 3.52 |
| Egorov et al. (2003), as reported in the 2005 paper | 198 | 1.67 |
| Gordon et al. (2005), this study | 82.8 | 2.23 ± 0.18 |
The paper concludes that the normalized indoor air values are “quite similar” across the studies. That is the authors’ reading of their own table. The three earlier studies are cited as the paper reports them. The guide did not open them.
Shower length and when breath was sampled
The 2005 study used a 10-minute shower. The authors estimate that a shorter shower would lower the air level: “The shorter showering period in our study would have resulted in a decrease in the air levels by as much as 50%”. This is the authors’ estimate, made in comparison with the 15 to 20 minute showers reported in the 2003 study. The sources do not test shorter showers directly.
Timing also matters for breath. The paper states that “postexposure exhaled-breath concentrations of CHCl 3 decrease rapidly with time, especially in the first few minutes after exposure”. The authors explain that the 2003 study collected breath samples within one minute of the shower, while this study waited 5 minutes. They write that differences in these factors “may explain the large differences in the observed breath values” between the two studies. Breath numbers from different studies should therefore be compared with care.
What the study says about health
The 2005 paper’s introduction notes that “a number of studies have found an association between elevated levels of THMs in drinking water and adverse health effects”. Those cited studies looked at drinking water and disease outcomes. The 2005 study did not measure disease, and it did not follow people over time. The paper’s stated purpose is to examine “which common household water-use activities result in an increase in exhaled-breath concentrations and, hence, the internal dose of THMs in people conducting these activities”.
The sources therefore support one narrow statement: during these tests, showering in water containing THMs raised chloroform in the air, and in the breath and blood of the subjects. They do not support a statement that showering causes any illness. The 2011 paper says that haloacetonitriles and chloral hydrate “have been reported to be mutagenic and teratogenic” in earlier studies. That is an attributed statement about other work, not a finding of the 2011 paper, and it concerns compounds the 2005 study did not measure.
If you have a health question about your water, your shower or your breathing, talk to your doctor. If you have a question about your skin, see a dermatologist. Neither source is a substitute for that advice.
What the sources do not say
- They do not measure free chlorine in the air or in the water after it leaves the tap. They measure THMs, mainly chloroform.
- They do not test a shower head, a filter, a hot-water setting, or a water-saving device as a way to lower exposure.
- They do not give a safe limit for THMs in shower water or in the air of a bathroom.
- They do not report any health outcome from showering. The 2005 study measured air, breath, blood and water, not illness.
- They do not describe how often the subjects showered, so they cannot estimate a yearly dose for a household.
- They tested seven subjects in two homes under controlled conditions. The sources do not say how the results would change in other homes or bathrooms.
How we chose
This guide uses only two sources. Both are peer-reviewed articles on PubMed Central that deal with chlorine by-products and showering or bathing. Both were opened and checked against saved copies of the text. The 2005 study was chosen because it measured showering directly in air, breath and blood. The 2011 study was chosen as the second source because it covers the same by-product family and the routes of exposure from water use, although it looks at a different compound group. Other sources were not used for the claims in this guide. The guide does not rank any products and does not claim that any shower head performs better than another.
What we could not verify
The sources do not cover the following, so this guide does not make claims about them:
- How much free chlorine is in the tap water of any particular city or home.
- How any particular shower head, hose or filter changes the amount of THM in the air or the body.
- Exposure for children, older adults, pregnant people, or people with lung or skin conditions.
- Exposure from showers of other lengths, apart from the authors’ own estimate for a shorter shower.
- Chloroform levels in homes with a bathroom fan, an open window, or a different room size.
Where the sources give a figure, this guide reports it as the source states it. The only figures calculated here are the gallon conversions in the section on the 2005 study.
What this means for a shower routine
The one shower factor the sources quantify is time. The 2005 study’s own estimate is that a shorter shower would reduce the air level, by as much as half in the authors’ comparison with a 15 to 20 minute shower. The sources do not say how short a shower should be, and they do not say whether shorter showers reduce breath or blood levels in the same proportion. Shower length is a factor the sources describe. It is not a recommendation this guide can make on health grounds.
The water supply itself is the other factor. The study’s two homes had very different tap-water levels, and the paper reports appreciable air increases at both. That suggests the water level is the starting point for exposure, and the sources do not show how any household can change it. A household that wants to know its own water level needs a water test from its utility or a laboratory. The sources do not describe that process, so this guide does not give a method.
FAQ
Does showering put chlorine by-products into the air?
In the 2005 study, showering produced “appreciable increases in air concentrations for all subjects at both sampling sites”. The compound measured was chloroform, one of the THMs, not chlorine itself.
Did the study find chloroform in the blood after a shower?
The study collected blood samples. Its abstract reports correlations for showering between tap water and blood, and between indoor air and blood, for chloroform. Correlations show association among these subjects. They do not show how much chloroform reaches the blood in a typical home.
Is there a safe chlorine or THM level in shower water?
The sources do not give one. The 2005 paper gives water concentrations for two homes, and the 2011 paper does not give a shower water level. Neither gives a safe level.
Will a filter or a different shower head reduce exposure?
Neither source tested a filter or a shower head. The sources cannot answer this question, so this guide does not make a claim about it.
Should I change my shower because of these studies?
The studies describe exposure in a small controlled test. They do not establish a health risk for any person. If you have a concern about your health, talk to your doctor before changing your routine.
Bottom line
The two studies show that showering in water containing THMs raised chloroform in the air, and in the breath and blood of the subjects, in a controlled test of seven people in two homes. Shower length and the timing of breath samples changed the numbers. The sources do not show a safe level, a health outcome, or any effect of a shower head or filter. Read them as measurements of exposure, not as a verdict on your shower.
Sources
- Gordon SM, Brinkman MC, Ashley DL, et al. “Changes in Breath Trihalomethane Levels Resulting from Household Water-Use Activities.” Environmental Health Perspectives, 2005; 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.” Journal of Applied Toxicology, 2011; 32(6): 387-394. https://pmc.ncbi.nlm.nih.gov/articles/PMC3199364/ (opened 2026-10-28)
