Water hardness, water temperature and towel friction act on the film that covers the skin, just as much as the cleanser. Hard water leaves more cleansing residues on the skin, and hot water accentuates the effect of surfactants on the barrier.
These three factors are often forgotten. Yet they are present at every shower, and Switzerland is directly concerned by the first.
Hard water in Switzerland
Water hardness measures its content of dissolved calcium and magnesium. In Switzerland, it is expressed in French degrees (°fH). Drinking water there is classified into six levels of hardness, from “very soft” (below 7 °fH) to “very hard” (above 42 °fH) (SSIGE, 2008).
This hardness depends on the subsoil the water passes through. Water that flows through limestone rocks becomes loaded with minerals. Depending on the municipality, tap water can therefore be very soft or frankly hard. Your water supplier can tell you the hardness of the water that reaches your home.
Limescale does not impair the quality of drinking water. It does, however, change its behaviour with cleansing products. The SSIGE points out that calcium combines with soaps to form insoluble salts, known as “lime soaps” (SSIGE, 2008). Part of the dull deposit visible on shower walls comes from this.
What hard water changes on the skin
A British study examined this mechanism on the skin in detail (Danby et al., 2018). Eighty volunteers, some with a naturally more fragile skin barrier, had their skin washed with a common surfactant, sodium lauryl sulfate, in waters of different hardness.
The results are clear:
- more residues. The areas washed with hard water retained more surfactant after rinsing;
- more water loss. These residues increased water loss through the skin, an indicator of the state of the barrier;
- more irritation, especially in participants whose barrier was the most fragile;
- water softening by ion exchange attenuated these effects.
The authors attribute these results to the interaction between the calcium in hard water and the surfactant, which then rinses off less well. The chlorine in the water, tested in the same study, showed no clear effect.
Hot water
A very hot shower provides a pleasant sensation. Yet it places greater demands on the skin.
In a controlled study, the same cleansing solution was applied to the forearm of ten volunteers at three temperatures: 4 °C, 20 °C and 40 °C. After four days, the barrier was more impaired at the highest temperatures. The authors observed a strong correlation between temperature and irritation (Berardesca et al., 1995).
Water itself
In Europe, tap water often has a pH close to 8, and a simple rinse is enough to raise the skin's pH for a few hours (Lambers et al., 2006).
At every shower, these factors meet: slightly alkaline water, more or less hard, often hot, then drying. Taken in isolation, each weighs little. Their effects combine.
Friction
The stratum corneum is made of flattened cells, stacked and bound together. Firm friction with a glove, a brush or a towel adds a mechanical stress to the action of water and cleanser. It can detach superficial cells earlier than natural renewal would.
In beauty salons, therapists learn to dry the skin by patting it. This simple gesture limits friction after the shower.
Habits adapted to your water
- Know the hardness of your water. The information is available from your local supplier.
- Rinse for longer if the water is hard, to reduce cleanser residues.
- Prefer lukewarm water to very hot water.
- Limit the duration of the shower.
- Dry by patting, without rubbing.
- Keep exfoliation occasional.
If your skin remains uncomfortable despite these precautions, seek advice from a dermatologist.
This article is part of the PHYTO 5 Library.
Sources
- Berardesca E, Vignoli GP, Distante F, Brizzi P, Rabbiosi G. Effects of water temperature on surfactant-induced skin irritation. Contact Dermatitis. 1995;32(2):83-87. DOI : 10.1111/j.1600-0536.1995.tb00751.x
- Danby SG, Brown K, Wigley AM, Chittock J, Pyae PK, Flohr C, Cork MJ. 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. 2018;138(1):68-77. DOI : 10.1016/j.jid.2017.08.037
- Lambers H, Piessens S, Bloem A, Pronk H, Finkel P. Natural skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science. 2006;28(5):359-370. DOI : 10.1111/j.1467-2494.2006.00344.x
- SSIGE / SVGW (Société suisse de l'industrie du gaz et des eaux). Trinkwasserinformation TWI 13/2008 : Wasserhärte, was muss beachtet werden ? Zurich, 2008. Diffusé par le canton de Soleure : https://so.ch/fileadmin/internet/ddi/ddi-gesa-lk/Umgang_mit_hartem_Wasser__Wasserhaerte__Entkalkungsanlagen_.pdf