Magnesium vs chlorine pools — what's actually different?

Most pools in Australia use chlorine. Bathhouses like Sonder use magnesium. The two are doing fundamentally different jobs — and that's the thing that surprises most first-time guests when they step into mineral water for the first time.

Chlorine: a sanitiser

Chlorine has one job: kill things. Bacteria, algae, the stuff that makes pool water unsafe. It's a powerful disinfectant, and it works — which is why backyard pools, public swimming centres and most gym pools rely on it.

The trade-off is that chlorine is harsh. It strips natural oils from skin, dries hair out, can irritate eyes and noses, and has that distinctive "pool smell" — which, contrary to popular belief, isn't the chlorine itself but the chemical compounds it forms when it reacts with skin, sweat and other organic matter. The smell is the by-product of chlorine doing its job.

Magnesium: a mineraliser

Magnesium chloride does sanitise water — it has natural antimicrobial properties — but its main job in a bathhouse is to mineralise the water. The water itself becomes the experience, rather than being a delivery system for chemicals.

The water is softer. There's no chemical smell. Your skin doesn't feel stripped after a soak — it feels conditioned, like you've been in seawater on a warm day.

Most magnesium bathhouses (Sonder included) use a combination of magnesium chloride and small amounts of other sanitisation methods (ozone, UV, or low-residual chlorine) to keep the water safe without compromising the mineral content. It's the best of both worlds.

What you actually notice as a guest

Stepping into a magnesium pool for the first time, there are a few things that stand out immediately:

• The water feels silky. Magnesium changes the surface tension of water — it glides over your skin differently.
• There's no smell. No chlorine note in the air, no chemical burn in your nose.
• Your eyes don't sting. You can dip your head under without the usual chlorine sting.
• Your skin feels different afterwards. Not stripped or dry — softer and more pliable, almost moisturised.

The transdermal magnesium question

A lot of magnesium bathhouse marketing makes big claims about absorbing magnesium through the skin and how that fixes sleep, muscle tension, anxiety, etc. The honest answer: the science is genuinely mixed.

Studies have shown that magnesium can be absorbed through the skin, particularly when the skin is warm and the water is concentrated (both true in a bathhouse). Many guests anecdotally report better sleep, reduced muscle soreness and easier wind-down after regular soaks. Whether that's the magnesium itself, the heat, the contrast bathing, the act of putting your phone away for 90 minutes, or some combination of all of them — researchers are still figuring out.

What we'd say: don't visit Sonder because you've read a wellness blog promising specific benefits. Visit because the water feels good, the experience is calming, and your body tends to thank you afterwards. The rest is a bonus.

"The water itself becomes the experience, rather than being a delivery system for chemicals."

So is magnesium objectively better?

For a bathhouse experience: yes, almost certainly. The lack of harsh chemicals, the silkiness of the water, the way your skin and hair feel afterwards — these are real, noticeable differences that you'll feel within minutes of your first soak.

For a backyard family pool? Chlorine is still much easier and cheaper to maintain. Mineral pools require more careful balancing and slightly more attention. But for a wellness facility designed around mineral water as the experience, magnesium is the right choice — which is why every bathhouse worth visiting has made it.

Curious to feel the difference yourself? Sonder opens soon — see our three magnesium pools and session options, or read our first-visit guide if you're planning a session.