Is your hot tub water actually clean — or just compliant?

Every time you soak in a hot tub, the water you're sitting in is carrying dissolved load that has been building since the day it was filled. Minerals, chemical residues, sanitiser by-products, body oils, sweat — all of it dissolves in and stays in. The industry calls this Total Dissolved Solids (TDS), and the official upper limit before a drain and refill is mandated sits at 1,500 parts per million.

That sounds like a technical compliance number. It is. What it isn't is a wellness benchmark — and that distinction matters more than most hot tub owners realise.

The BISHTA and HSG282 guidelines were developed primarily to control Legionella risk in commercial spa settings. The TDS ceiling exists as an operational trigger — the point at which water is considered too chemically degraded to be safely managed. It was never designed to answer the question: what level of dissolved solids is actually good for the person soaking in it?

Here's a number that tends to stop people. A standard portable hot tub holds around 800–1,200 litres of water. A swimming pool holds around 250,000. Put two people in your hot tub and the organic concentration — body oils, sweat, cosmetics, dead skin cells — is equivalent to approximately 200 people in that pool. That's not a reason to panic. It is a reason to think seriously about what's happening to your water chemistry between fill and drain.

That organic load is what drives TDS rise faster than anything else. When chlorine reacts with it, it forms chloramines — the real culprit behind stinging eyes, skin irritation, and that harsh chemical smell. Chloramines are inert dissolved load: they don't sanitise, they just accumulate in the water adding to TDS and making every soak less comfortable. The more chloramines form, the more you dose, the faster TDS climbs.

Breaking that cycle requires tackling the organic load at source. Two tools make a meaningful difference: non-chlorine shock (potassium monopersulfate) oxidises chloramines immediately after each use, restoring sanitiser efficiency without adding chlorine. Enzyme treatment goes further — plant-derived enzymes physically break down oils, sunscreen, and bather residues into particles small enough for your filter to capture and remove. Unlike chlorine, which sanitises organics but leaves them dissolved in the water, enzymes physically remove them. That's the difference between managing TDS and actually slowing it.

But enzymes are only as effective as the filter catching what they break down. And the filter that came with your hot tub is almost certainly failing you faster than you think.

The four-lever system that keeps TDS genuinely low: fast heating to reduce evaporative loss per session, permanent filtration to consistently capture what enzymes break down, enzyme treatment to remove organic load before it feeds the chloramine cycle, and non-chlorine shock after every use to clear chloramines immediately.