Topic
Water temperature and hair washing
Nobody has studied shower temperature on hair or scalp, so lukewarm water is the sensible choice based on skin data, and cold-rinse shine claims are untested.
Key takeaways
- Direct evidence is essentially absent. Our PubMed searches for water temperature with hair, scalp or shampoo returned no study of hair or scalp outcomes. Claims that cold water "closes the cuticle" or hot water "strips hair" are untested.
- The closest data are on skin. In 50 healthy volunteers, immersing hands in hot water raised water loss through the skin from 25.75 to 58.58 g/h/m2, raised pH and caused redness. Cold water raised water loss to 34.96 and pH too. Hot water was more damaging than cold, and the authors advised cold or lukewarm water (Herrero-Fernandez 2022). It was hands, not scalp, and the exposure was prolonged.
- Heat plus surfactant plausibly increases irritation on scalp skin, but this is extrapolation. Dry or eczema-prone scalps are the most likely to benefit from lukewarm water.
- Cold rinses: there is no evidence that they add shine, reduce frizz or grow hair. A final cool rinse is harmless and may feel good after conditioner.
- Water mineral content (hardness) has more evidence than temperature; see hard water and hair.
- Wet hair is weaker, because keratin swells in water and stretches more easily, so be gentle (see brushing and detangling).
The science
Skin barrier function responds to water exposure and temperature (Herrero-Fernandez 2022). Applying this to the scalp is plausible because scalp skin has the same outer layer, but we found no scalp-specific measurements. For hair fibres, there are lab studies of heat damage from styling tools (see heat tools), but none of shower temperature.
What it means by hair profile
- Dry, flaky or eczema-prone scalp: lukewarm water is the choice closest to the evidence.
- Oily scalp: no evidence that hot water helps cleaning or that cold water reduces oil.
- Curly, coily and high-porosity hair: no study; lukewarm to cool water is common practice for dryness-prone hair.
Myths and facts
- Myth: cold water seals the cuticle and adds shine. Fact: untested.
- Myth: hot water causes hair loss. Fact: no evidence, though hot showers can irritate the scalp.
- Myth: very hot water is needed to remove oils. Fact: surfactants do the removing; heat is not required.
Bottom line
Wash with lukewarm water and avoid scalding. A cooler final rinse is optional. If your scalp is itchy or inflamed after showers, try lower temperature and shorter time. Put your effort into shampoo choice, water hardness and technique instead.
Studies 1
Herrero-Fernandez M et al. Impact of Water Exposure and Temperature Changes on Skin Barrier Function. 2022
Tested: hot versus cold water
Result: Hot water raised transepidermal water loss from 25.75 to 58.58 g/h/m2 and cold water to 34.96; both raised skin pH (6.33 to 6.65 and 6.62); hot was worse and the authors advised cold or lukewarm water.
Read the study ↗Related
- Hard water and hairHard water leaves calcium on hair and can irritate skin, but there is no good human evidence that it causes hair loss, and chelating shampoos are untested in trials.
- Shampoo pHHair and scalp prefer mildly acidic conditions, but most shampoos are less acidic than that and the case for lower pH rests on lab mechanism rather than wear trials.
- How often to wash your hairYour scalp sets how often you need to wash and your hair lengths set how rarely you can, so the right number depends on your hair type and scalp.
- Brushing and detanglingMost combing damage comes from tangles, so conditioner and the right technique cut breakage more than the choice of brush.
- Heat styling toolsFlat irons, curling tools and hot combs damage hair more with higher temperature, repetition and wet hair; dermatologists advise using them about once a week or less.