The science of shower water

What's really in your shower water?

Your daily shower is mostly a water story: chlorine that keeps tap water safe to drink, minerals picked up from the ground, and what happens when both meet hot water, skin and hair. Below, we've summarized the clinical and laboratory research in plain language, including the studies that found no effect. Each source is cited and given an evidence label.

Strong randomized trials, systematic reviews or primary standards   Moderate well-run controlled or large observational studies   Preliminary small, lab-only or pilot work

01 · Skin & breath

What your body absorbs in a hot shower

Tap water isn't only something you drink. Clinical exposure studies show chloroform, a disinfection byproduct in chlorinated tap water, enters the body through both the lungs and the skin while you shower.

When utilities disinfect water with chlorine, small amounts of byproducts called trihalomethanes (THMs) form; chloroform is the most common. Hot water and steam move these compounds into the air and onto your skin. In controlled studies with human volunteers, blood and breath THM levels rose more after a 10-minute shower or bath than after drinking a litre of the same water, and skin absorption was roughly equal to inhalation.

What the research does not show: that these short-term increases harm health. Researchers say that question still needs work. Not every byproduct behaves the same; some cross skin about 100 times less readily than chloroform.

Gauge: skin versus lungs, about 1 to 1A half-circle split into two equal halves, labelled skin and lungs. In one study, the chloroform dose absorbed through skin in a shower was approximately equal to the dose inhaled.SkinLungs≈ 1 : 1skin : lungs
Chloroform dose absorbed through the skin during a shower was approximately equal to the dose inhaled. [1]
ModerateHuman exposure study
Exhaled-breath chloroform after a normal shower was 6.0–21 µg/m³ vs 2.4–10 µg/m³ after inhalation-only exposure; dermal dose ≈ inhalation dose.
Jo WK, Weisel CP, Lioy PJ. (1990). Routes of chloroform exposure and body burden from showering with chlorinated tap water. Risk Analysis 10(4):575–580. Source [1]
Small controlled human study; consistent with later work.
ModerateHuman exposure study
Among 31 adults, blood THMs were highest after a 10-minute shower and lowest after drinking 1 L of tap water.
Backer LC, Ashley DL, Bonin MA, et al. (2000). Household exposures to drinking water disinfection by-products: whole blood trihalomethane levels. Journal of Exposure Analysis and Environmental Epidemiology 10(4):321–326. Source [2]
Controlled human study, 31 participants.
ModerateHuman exposure study
Average blood THM increases of 57–358 pg/mL after showering, bathing and hand dishwashing; authors say public-health significance needs more research.
Nuckols JR, Ashley DL, Lyu C, Gordon SM, Hinckley AF, Singer P. (2005). Influence of tap water quality and household water use activities on indoor air and internal dose levels of trihalomethanes. Environmental Health Perspectives 113(7):863–870. Source [3]
Controlled exposure study in occupied homes.
PreliminaryHuman exposure studyMixed finding
Chloroform skin permeability was about 0.015 cm/h; haloketone byproducts crossed the skin roughly 100 times less readily.
Xu X, Weisel CP. (2005). Dermal uptake of chloroform and haloketones during bathing. Journal of Exposure Analysis and Environmental Epidemiology 15(4):289–296. Source [4]
6 subjects; shows not all byproducts absorb alike (mixed finding).

02 · Hair

Chlorine, minerals and the hair fiber

Laboratory research links hard-water minerals to buildup on hair and a modest loss of strength. Chlorine damage to the cuticle is well documented, but mostly at swimming-pool strength, which is higher than typical tap water.

Hair is a protein (keratin) fiber protected by overlapping cuticle scales. In lab studies, chlorinated water roughened the cuticle and weakened wet fibers, but the test solutions held about 10 ppm chlorine. U.S. tap water is legally capped at 4.0 mg/L, and most systems run below that. In swimmers, frequent pool exposure has been linked to cuticle loss and lighter, discolored hair.

For hard water, a 2025 meta-analysis found calcium and magnesium deposit on hair and tensile strength drops slightly (moderate certainty). Not every study agrees: one laboratory study found no difference in strength or elasticity.

What dermatologists advise (American Academy of Dermatology)
The AAD's advice for swimmers includes wearing a swim cap, rinsing hair right after swimming, and using a swimmers' shampoo plus a deep conditioner. [22]
Gauge: hair strength about 8.1 percent lower in hard waterA gauge from 0 to 20 percent with the needle at 8.1 percent: hair soaked in hard water had about 8.1 percent lower tensile strength than hair soaked in deionized water in one laboratory study.0%10%20%8.1%lower tensile strength
Hair strands soaked in hard water vs deionized water: mean tensile strength 234.16 vs 254.84 (P = 0.001). A separate study, Srinivasan 2013, found no difference. [7]
PreliminaryLaboratory study
Hair exposed to 10 ppm chlorine for up to 30 hours was damaged most at acidic pH and gradually at neutral pH; alkaline conditions changed it little.
Fair NB, Gupta BS. (1987). The chlorine–hair interaction. II. Effect of chlorination at varied pH levels on hair properties. Journal of the Society of Cosmetic Chemists 38:371–384. Source [5]
Lab study on isolated fibers at pool-level chlorine.
PreliminaryObservational study (swimmers)
61% of 67 elite swimmers had discolored hair vs 0% of controls; microscopy showed cuticle loss.
Nanko H, Mutoh Y, Atsumi R, et al. (2000). Hair-discoloration of Japanese elite swimmers. The Journal of Dermatology 27(10):625–634. Source [6]
Observational; pool water, not shower water.
PreliminaryLaboratory studyNull result
Hair treated with hard water (212.5 ppm) showed no significant difference in tensile strength (P = 0.858) or elasticity (P = 0.874).
Srinivasan G, Srinivas CR, Mathew AC, Duraiswami D. (2013). Effects of hard water on hair. International Journal of Trichology 5(3):137–139. Source [8]
Null result; 15 participants.
ModerateSystematic review & meta-analysis
Across 9 studies, hard water raised calcium/magnesium deposits and lowered tensile strength by 0.28 SD.
Bikash C, Sarkar R, Sundaram SP. (2025). The effects of hard water on human hair: a systematic review and meta-analysis. International Journal of Trichology (Sep–Oct 2025). Source [9]
Moderate certainty for strength; very low for other outcomes.

03 · Skin & hard water

Hard water, skin barrier and eczema research

Living in a hard-water area is linked with more childhood eczema in large observational studies. But clinical trials of whole-home water softeners have not shown they improve eczema that already exists.

Hard water carries more calcium and magnesium. In a controlled study of 80 volunteers, washing with hard water left more soap (surfactant) residue on the skin, which raised water loss and irritation. Population studies from the UK found higher eczema rates in infants and young children living in harder-water areas.

The randomized clinical trial evidence is mixed. In the 336-child SWET trial, a water softener added no benefit over usual care. A small pilot (SOFTER) testing softeners from birth saw a trend that was not statistically tested; a full trial is needed.

What dermatologists advise (American Academy of Dermatology)
The AAD's self-care tips for atopic dermatitis include bathing in warm, not hot, water for 5–10 minutes with a mild, fragrance-free cleanser, then moisturizing. [23]
Gauge: odds ratio 1.28 for childhood eczema in harder-water areasA gauge from 1.0 (no link) to 2.0 with the needle at 1.28 and an amber arc for the 95 percent confidence interval from 1.09 to 1.50. Certainty of the evidence is very low.1.0 no link1.52.0OR 1.28odds of eczema in children
Pooled odds of eczema in children living in harder vs softer water areas (observational studies); amber arc = 95% confidence interval 1.09–1.50. GRADE certainty: very low. [14]
StrongRandomized clinical trialNull result
Eczema improved 20% with a softener vs 22% with usual care alone (difference not significant, p = 0.53).
Thomas KS, Dean T, O'Leary C, Sach TH, Koller K, Frost A, Williams HC; SWET Trial Team. (2011). A randomised controlled trial of ion-exchange water softeners for the treatment of eczema in children. PLoS Medicine 8(2):e1000395. Source [10]
Randomized, observer-blind, 336 children.
PreliminaryPilot randomized clinical trialNot statistically tested
Visible eczema at 6 months: 33% (softener) vs 48% (control); feasibility pilot, no statistical testing.
Jabbar-Lopez ZK, Ezzamouri B, Briley A, et al. (2022). Randomized controlled pilot trial with ion-exchange water softeners to prevent eczema (SOFTER trial). Clinical & Experimental Allergy 52(3):405–415. Source [11]
Small pilot RCT; not powered to test effect.
ModerateObservational study (infants)Association only
Among 1,303 infants, high water calcium carbonate was linked with more eczema (adjusted OR 1.87); the role of chlorine remained uncertain.
Perkin MR, Craven J, Logan K, et al.; EAT Study Team. (2016). Association between domestic water hardness, chlorine, and atopic dermatitis risk in early life: a population-based cross-sectional study. Journal of Allergy and Clinical Immunology 138(2):509–516. Source [12]
Cross-sectional; shows association, not cause.
ModerateClinical study (80 participants)
Hard water increased soap residue on skin, raising irritation and water loss; ion-exchange softening reduced this. No clear effect of chlorine.
Danby SG, Brown K, Wigley AM, Chittock J, Pyae PK, Flohr C, Cork MJ. (2018). The effect of water hardness on surfactant deposition after washing and subsequent skin irritation in atopic dermatitis patients and healthy control subjects. Journal of Investigative Dermatology 138(1):68–77. Source [13]
Controlled mechanistic study, 80 participants.

04 · Standards

NSF/ANSI 177: what it is and why it exists

NSF/ANSI 177 is the American National Standard for shower filters. It sets requirements for materials, construction and labeling, and its performance test addresses free available chlorine.

Before the standard, shower-filter chlorine claims were made using inconsistent test methods. NSF developed NSF/ANSI 177, "Shower Filtration Systems – Aesthetic Effects," through its Joint Committee on Drinking Water Treatment Units using the ANSI consensus process, so that claims could be compared on a common basis. [18][24] In the standard's test, a system is challenged with 2.0 mg/L of free chlorine in warm (about 104°F), pH 7.5 water and must reduce it by at least 50%. [18]

NSF describes shower filters under the standard as reducing free available chlorine only. It is an aesthetic-effects standard, not a health-effects standard, and it addresses free available chlorine rather than chloramine, chloroform/THMs, hardness, metals or PFAS. NSF maintains a public listing of shower filters under the standard. [16]

VitalFlow Filtered Showerhead: tested to NSF/ANSI 177 (July 2026).

Gauge: at least 50 percent chlorine reduction requiredA gauge from 0 to 100 percent. The shaded zone from 50 to 100 percent marks the minimum reduction of a 2.0 milligram per liter free chlorine challenge in the standard's test.0%50%100%≥ 50%of a 2.0 mg/L challenge
Minimum reduction of a 2.0 mg/L free chlorine challenge in the standard's test method. [18]
StrongThe standard
Purpose: minimum requirements for materials, design, construction and performance of filters that reduce aesthetic free available chlorine.
NSF International / ANSI. (2019). Shower Filtration Systems – Aesthetic Effects. NSF/ANSI 177-2019 (preview pages, ANSI Webstore). Source [15]
Primary source: the standard itself.
StrongStandards body
NSF: shower filters under NSF/ANSI 177 "are certified to only reduce free available chlorine."
NSF. (2026). NSF Standards for Water Treatment Systems. nsf.org consumer resources (accessed Oct 23, 2026). Source [17]
Primary source: standards body.
ModerateTrade-press summary
Developed because manufacturers used inconsistent test methods; test water 104 ± 4°F, pH 7.5, 2.0 mg/L chlorine, ≥50% reduction; not a health-effects standard.
Plumbing & Mechanical. (2005). NSF/ANSI Standard 177: A Level Playing Field for Shower Filter Claims. pmmag.com, June 1, 2005. Source [18]
Trade-press summary of the standard's launch.
StrongPublic listing
NSF's public listing of shower filters under NSF/ANSI 177.
NSF. (2026). Listings: NSF/ANSI 177 Shower Filtration Systems – Aesthetic Effects. NSF Product and Service Listings (DWTU). Source [19]
Primary source: certifier's database.

05 · How we source data

Methodology & sourcing

Our state water summaries are compiled from publicly available Consumer Confidence Reports (CCRs), the annual water quality reports that community water systems are required to publish under federal rules. For each state and Washington, D.C., we start with the largest community water systems by population served, read the disinfectant (chlorine or chloramine) and disinfection-byproduct figures each utility reports, and compare them with EPA drinking water standards, such as the maximum residual disinfectant level (MRDL) for chlorine of 4.0 mg/L (as Cl2).

This is compiled public data, not proprietary lab testing, and it is not a test of your tap. Coverage varies by state: in many states our summary reflects the largest systems serving roughly a quarter of residents, and some states have limited published data. Each state page lists the systems included, the report year and a link to every source report. Your own utility's CCR is the authoritative source for your water.

We built a state-by-state summary from public utility reports, with every number linked to its source report.

Readings are presented in coordination with EPA drinking water standards. VitalFlow is not affiliated with, endorsed by, or partnered with the EPA or any water utility. [20] [26] [21] [25]

Explore your state: state water reports · interactive water report

References

  1. Jo WK, Weisel CP, Lioy PJ. (1990). Routes of chloroform exposure and body burden from showering with chlorinated tap water. Risk Analysis 10(4):575–580. https://doi.org/10.1111/j.1539-6924.1990.tb00541.x
  2. Backer LC, Ashley DL, Bonin MA, et al. (2000). Household exposures to drinking water disinfection by-products: whole blood trihalomethane levels. Journal of Exposure Analysis and Environmental Epidemiology 10(4):321–326. https://doi.org/10.1038/sj.jea.7500098
  3. Nuckols JR, Ashley DL, Lyu C, Gordon SM, Hinckley AF, Singer P. (2005). Influence of tap water quality and household water use activities on indoor air and internal dose levels of trihalomethanes. Environmental Health Perspectives 113(7):863–870. https://doi.org/10.1289/ehp.7141
  4. Xu X, Weisel CP. (2005). Dermal uptake of chloroform and haloketones during bathing. Journal of Exposure Analysis and Environmental Epidemiology 15(4):289–296. https://doi.org/10.1038/sj.jea.7500404
  5. Fair NB, Gupta BS. (1987). The chlorine–hair interaction. II. Effect of chlorination at varied pH levels on hair properties. Journal of the Society of Cosmetic Chemists 38:371–384. https://library.scconline.org/cdn-1708705422691/Chlorine-Hair-Interaction-II.pdf
  6. Nanko H, Mutoh Y, Atsumi R, et al. (2000). Hair-discoloration of Japanese elite swimmers. The Journal of Dermatology 27(10):625–634. https://doi.org/10.1111/j.1346-8138.2000.tb02243.x
  7. Luqman MW, Ramzan MH, Javaid U, Ali R, Shoaib M, Luqman MA. (2018). To evaluate and compare changes in baseline strength of hairs after treating them with deionized water and hard water and its role in hair breakage. International Journal of Trichology 10(3):113–117. https://doi.org/10.4103/ijt.ijt_115_16
  8. Srinivasan G, Srinivas CR, Mathew AC, Duraiswami D. (2013). Effects of hard water on hair. International Journal of Trichology 5(3):137–139. https://doi.org/10.4103/0974-7753.125609
  9. Bikash C, Sarkar R, Sundaram SP. (2025). The effects of hard water on human hair: a systematic review and meta-analysis. International Journal of Trichology (Sep–Oct 2025). https://doi.org/10.4103/ijt.ijt_86_25
  10. Thomas KS, Dean T, O'Leary C, Sach TH, Koller K, Frost A, Williams HC; SWET Trial Team. (2011). A randomised controlled trial of ion-exchange water softeners for the treatment of eczema in children. PLoS Medicine 8(2):e1000395. https://doi.org/10.1371/journal.pmed.1000395
  11. Jabbar-Lopez ZK, Ezzamouri B, Briley A, et al. (2022). Randomized controlled pilot trial with ion-exchange water softeners to prevent eczema (SOFTER trial). Clinical & Experimental Allergy 52(3):405–415. https://doi.org/10.1111/cea.14071
  12. Perkin MR, Craven J, Logan K, et al.; EAT Study Team. (2016). Association between domestic water hardness, chlorine, and atopic dermatitis risk in early life: a population-based cross-sectional study. Journal of Allergy and Clinical Immunology 138(2):509–516. https://doi.org/10.1016/j.jaci.2016.03.031
  13. Danby SG, Brown K, Wigley AM, Chittock J, Pyae PK, Flohr C, Cork MJ. (2018). The effect of water hardness on surfactant deposition after washing and subsequent skin irritation in atopic dermatitis patients and healthy control subjects. Journal of Investigative Dermatology 138(1):68–77. https://doi.org/10.1016/j.jid.2017.08.037
  14. Jabbar-Lopez ZK, Ung CY, Alexander H, et al. (2021). The effect of water hardness on atopic eczema, skin barrier function: a systematic review, meta-analysis. Clinical & Experimental Allergy 51(3):430–451. https://doi.org/10.1111/cea.13797
  15. NSF International / ANSI. (2019). Shower Filtration Systems – Aesthetic Effects. NSF/ANSI 177-2019 (preview pages, ANSI Webstore). https://webstore.ansi.org/preview-pages/NSF/preview_NSF+ANSI+177-2019.pdf
  16. NSF. (2026). NSF/ANSI 177 – Shower Filter Certification. nsf.org (accessed Oct 23, 2026). https://www.nsf.org/water-systems/water-filtration/nsf-ansi-177-shower-filter-certification
  17. NSF. (2026). NSF Standards for Water Treatment Systems. nsf.org consumer resources (accessed Oct 23, 2026). https://www.nsf.org/consumer-resources/articles/standards-water-treatment-systems
  18. Plumbing & Mechanical. (2005). NSF/ANSI Standard 177: A Level Playing Field for Shower Filter Claims. pmmag.com, June 1, 2005. https://www.pmmag.com/articles/87589-web-exclusive-br-nsf-ansi-standard-177-a-level-playing-field-for-shower-filter-claims
  19. NSF. (2026). Listings: NSF/ANSI 177 Shower Filtration Systems – Aesthetic Effects. NSF Product and Service Listings (DWTU). https://info.nsf.org/Certified/DWTU/Listings.asp?Standard=177
  20. U.S. EPA / eCFR. (2026). Maximum residual disinfectant levels (chlorine: 4.0 mg/L as Cl2). 40 CFR 141.65. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-D/part-141/subpart-G/section-141.65
  21. U.S. EPA / eCFR. (2026). Consumer Confidence Reports: content of the reports. 40 CFR 141.153. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-D/part-141/subpart-O/section-141.153
  22. American Academy of Dermatology. (2026). How to stop damaging your hair (Dermatologists' recommendations for swimmers). aad.org (accessed Oct 23, 2026). https://www.aad.org/public/diseases/hair-loss/insider/stop-damage/
  23. American Academy of Dermatology. (2026). Atopic dermatitis: Self-care (Bathing tips). aad.org (accessed Oct 23, 2026). https://www.aad.org/public/diseases/eczema/types/atopic-dermatitis/self-care
  24. NSF International / ANSI. (2023). Shower Filtration Systems – Aesthetic Effects. NSF/ANSI 177-2023 (ANSI Webstore listing). https://webstore.ansi.org/standards/nsf/nsfansi1772023
  25. U.S. EPA. (2026). Safe Drinking Water Act: Consumer Confidence Reports (CCR). epa.gov. https://www.epa.gov/ccr
  26. U.S. EPA / eCFR. (2026). Compliance requirements (disinfectant residuals, running annual average). 40 CFR 141.133. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-D/part-141/subpart-L/section-141.133

Disclaimer. This page is for general education about water and is not medical advice. It summarizes published research about water, not about any specific product, and the VitalFlow Filtered Showerhead is not intended to diagnose, treat, cure or prevent any disease. If you have a skin, scalp or hair condition, talk to a board-certified dermatologist. State water summaries reflect utilities' own published reports for their largest systems and are not a test of your home's water. Study findings are summarized; read the linked sources for full details. Last reviewed: October 2026.