What is thermal water and how does it work in skin care?

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If you walk into a pharmacie in France, you’ll immediately bump into a giant display of thermal water spray cans. A whole host of French skin care brands like Avène, La Roche-Posay, Uriage and Vichy sell thermal water sprays. What is it, what’s in it, what does it do and how is it different from regular water?

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What is thermal water?

Thermal water comes from hot springs. The water in these hot springs come from deep in the ground, where it’s heated by geothermal activity (the Earth’s natural heat which also causes lava to be molten).

What’s in thermal water?

It’s mostly water, of course, but it isn’t “just water in a can”! As the thermal water rises to reach the spring, it passes through rocks and soil which dissolve to add minerals to the water. The mineral content of a particular thermal water depends on where it comes from. The minerals include the ones found in your skin’s natural moisturising factor (NMF), like chlorides, sodium, potassium, calcium, and magnesium.

Here are the compositions of the 4 most popular thermal waters (source: Bacle et al., Int J Dermatol 1999, brand marketing materials). There’s some variation between batches of course, since it’s a natural mixture.

Composition (mg/L or ppm)AvèneLa Roche-PosayVichyUriage
Total dry residue207444512011000
Sodium4.81018602360
Potassium0.72.299.645.5
Calcium42.7140150.6600
Magnesium21.24.912.3125
Chlorides5.4253573500
Bicarbonates226.73964776.3402
Sulfates1430Trace2862
Nitrates1.4TraceTrace< 100
Silica (SiO2)10.630-42
Selenium-0.060--
Strontium-0.260--
Zinc-0.022-0.16
Copper-0.005-0.075
Manganese---0.154
Iron---0.015
pH7.46.97.006.77

As you can see, the composition varies a fair bit, with Uriage, the thermal water with the highest mineral content, being 55 times more concentrated than Avène, which has the lowest mineral content.

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Residue remaining after room temperature evaporation of 10 mL of Uriage Thermal Water in a 250 mL beaker.

“-” in the table means no data, since different thermal water brands like to highlight different aspects of their water. La Roche-Posay talks a lot about the selenium content of their water, while Uriage emphasises the high calcium concentration. Avène talks a lot about the 2:1 ratio of calcium and magnesium in their water.

There’s also nitrogen gas inside the can that acts as a propellant, to push the water out as a spray. Paula’s Choice writes that the nitrogen “can generate free-radical damage and cause cell death”, which luckily isn’t true, since nitrogen gas (N2) make up 78% of the air we breathe! It’s actually very unreactive, so unreactive that it’s commonly used in laboratories to flush out more reactive things like oxygen and water. (The papers cited in the Beautypedia article actually involve chemicals that just contain nitrogen atoms, not nitrogen gas itself.)

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