Bromine vs Chlorine in a Hot Tub, and Why an Aluminum Tub Uses Bromine
Bromine and chlorine both sanitize hot tub water, and for a conventional acrylic spa either works. Chlorine is cheaper and acts faster. Bromine is more stable in hot water, keeps working across a wider pH range, produces less of the sharp smell people associate with chlorine, and is gentler on skin and eyes. For a hot tub with an aluminum interior the choice is not a preference: chlorine is incompatible with the metal and bromine is the sanitizer.
Here is how the two differ, and why the answer changes with what the tub is made of.
Not sure where your water is? Check your readings.
How each one works
Both are halogens, and both kill bacteria by oxidizing them. Chlorine in a hot tub forms hypochlorous acid, which is fast and effective and also volatile: it burns off quickly at hot tub temperatures and its effectiveness collapses as pH rises above about 7.6. When it reacts with sweat and body oils it forms chloramines, which are the source of the chemical smell and the eye sting; the smell means the chlorine is used up, not that there is too much.
Bromine forms hypobromous acid, which is slower and steadier. It stays effective at higher temperatures and up to a pH of about 8, and when it reacts with organic matter it forms bromamines, which still sanitize and smell far less. Spent bromine can also be reactivated by an oxidizer, so a bromine reserve builds in the water rather than burning off.
Bromine against chlorine, in a hot tub
| Chlorine | Bromine | |
|---|---|---|
| Speed | Fast acting | Slower, steadier |
| In hot water | Burns off quickly | Stable |
| pH tolerance | Effectiveness drops above 7.6 | Effective to about 8 |
| Smell and irritation | Chloramines: sharp smell, eye sting | Bromamines: mild, still sanitizing |
| Working range in a hot tub | Typically 1 to 3 ppm | Typically 3 to 5 ppm on acrylic, 2 to 3 ppm on aluminum |
| Cost | Lower | Higher per dose, but less is lost to heat |
| Aluminum interior | Not compatible | Compatible |
Why chlorine and aluminum do not mix
Two reasons. Chlorine tablets sold for hot tubs are usually trichlor, which is strongly acidic and drives pH down where it sits, and aluminum's protective oxide layer is least stable at low pH. And chloride ions, which every chlorine product leaves behind as it works, are the specific thing that attacks that oxide layer, which is why salt water and salt chlorine generators are ruled out for the same reason. Bromine works without either problem, and it is the sanitizer every aluminum hot tub manufacturer specifies. Do aluminum hot tubs corrode covers the mechanism.
How bromine is used
Standard 1 inch bromine tablets in a dispenser, held at the working range and tested a couple of times a week. On a tub with filtration the dispenser is in-line with the filter, so the tablets dissolve at a steady rate as water passes. On a tub without a pump a floating dispenser does the same job more slowly. Granular bromine is best avoided: powder that does not fully dissolve settles and concentrates on the base. Never put two tablet types in one dispenser, and never a chlorine tablet in a bromine dispenser. Why bromine stops holding covers what uses it up.
Bromine is more effective in balanced water, so alkalinity first, then pH, then calcium hardness, then sanitizer. Sanitizer added to unbalanced water is far less effective, and the usual response is to add more, which is how tubs end up over-dosed.
If you have an AlumiTub
Bromine, held at 2 to 3 ppm and never above 3. That is lower than the 3 to 5 ppm typical of an acrylic spa because there are no jets, no internal plumbing and no filter chamber for biofilm to establish in, so demand stays low once the water is in range. Never chlorine in any form, never a salt chlorine generator, and nothing containing copper or silver.
Bromine is not part of Good Clean Living Water Care, because regulatory approval cannot be held for a single product across both Canada and the United States, so it is sourced separately: standard 1 inch bromine tablets from any pool and spa supplier. New Electric, Hybrid and Wood Fired with filtration tubs ship with an in-line dispenser fitted; a plain Wood Fired uses a floating dispenser. Everything else in the collection, the fill filter, the balancers, the stabilizer and the cleansers, is built around the ranges above and around aluminum.
Common questions
Is bromine or chlorine better for a hot tub?
For an acrylic spa either works; bromine is more stable in hot water, effective across a wider pH range and produces less smell and irritation, while chlorine is cheaper and faster. For a hot tub with an aluminum interior, bromine is the only option, because chlorine products and the chlorides they leave behind attack the metal's protective layer.
Why does a hot tub smell of chlorine?
The smell is chloramines, formed when chlorine reacts with sweat and body oils. It means the chlorine has been used up rather than that there is too much. Bromine forms bromamines instead, which still sanitize and smell far less.
Can you use chlorine in an aluminum hot tub?
No. Chlorine tablets are strongly acidic and the chloride ions every chlorine product leaves behind attack aluminum's protective oxide layer. Salt chlorine generators are ruled out for the same reason. Bromine at 2 to 3 ppm is the specified sanitizer.
What bromine level should a hot tub have?
Typically 3 to 5 ppm on a conventional acrylic spa. On an aluminum tub with no jets or internal plumbing, 2 to 3 ppm and never above 3, because there is nowhere for biofilm to establish and demand stays low once the water is balanced.
Can you switch a hot tub from chlorine to bromine?
Yes. Drain and clean the tub, clean or replace the filter, refill and balance, then start bromine tablets in a fresh dispenser. Never mix the two products, and never put chlorine tablets in a dispenser that has held bromine.
Not sure where your water is? Check your readings.