Bromine vs. chlorine for hot tubs
Both bromine and chlorine keep a hot tub sanitary when kept at the right level. Chlorine is cheaper and quicker to adjust. Bromine is more forgiving in hot water and is usually fed from a floating dispenser. Pick the one that fits how you use the tub, then stick with it.
Target levels
The CDC recommends, for hot tubs:
- Chlorine: at least 3 ppm free chlorine.
- Bromine: 4–8 ppm.
- pH: 7.0–7.8 for either.
Source: CDC, what you can do to stay healthy in hot tubs.
Side by side
| Chlorine | Bromine | |
|---|---|---|
| How it's usually added | Granules (dichlor) or liquid after each use | Tablets in a floating dispenser, plus a bromide "bank" when the water is new |
| Adjusting the level | Fast: a dose raises it within minutes | Slower: dispenser output is changed by its setting |
| In hot water | Used up quickly by heat and jets | Generally holds up better |
| Stabilizer | Dichlor adds it; the CDC recommends against stabilizer in hot tubs | Can't be stabilized, and doesn't need to be in a covered tub |
| Smell | Little when the level is right; strong smell means chloramines | A different, often milder smell |
| Cost | Usually lower per week | Usually higher per week |
| Testing | Common chlorine kits and strips | Bromine strips, or a DPD kit with a conversion |
How bromine works in a hot tub
Bromine tablets dissolve slowly in a floater or inline feeder. When bromine sanitizes, it turns into bromide, which stays in the water. A shock or oxidizer turns that bromide back into active bromine. That's why bromine systems pair tablets with regular shocking, and why new water often gets a dose of sodium bromide to build up a bromide reserve. Follow the product labels for both.
Because the bromide stays in the water, you can't simply switch to chlorine: adding chlorine to water with bromide turns it into bromine. See "Switching" below.
Testing bromine
Bromine test strips read it directly. If you have a DPD chlorine kit, many kits include a bromine scale. Otherwise, a DPD chlorine reading can be converted: multiply it by 2.25 (the ratio of bromine's atomic weight, 79.9, to chlorine's, 35.45). A reading of 2 ppm "chlorine" on a DPD test of bromine water means about 4.5 ppm bromine. Check your kit's instructions, since some are already calibrated for bromine.
Worked example: chlorine dose
A 400-gallon hot tub drops to 1 ppm free chlorine after a soak. Raising it to 4 ppm takes about 2.6 fl oz of 6% unscented bleach, according to the chlorine calculator (3.4 fl oz raises it from zero to 4 ppm, so 3 ppm takes three-quarters of that). Bromine doesn't work this way: its level is set by the floater and topped up with shock, so follow the product label for bromine doses.
Which to choose
- Choose chlorine if you use the tub regularly, test before each use, and want the lowest cost and quick control.
- Choose bromine if you prefer a dispenser that feeds sanitizer between uses, or find bromine's smell milder.
- Either way, test before use, keep pH in range, and replace the water as your manual recommends.
Switching between them
- Chlorine to bromine: you can start bromine in the existing water once chlorine has dropped, but many owners drain and refill to start fresh.
- Bromine to chlorine: drain, clean and refill. The bromide left in the water would turn any chlorine you add back into bromine.
Frequently asked questions
Can I use bromine in a pool?
It's possible, but rare for outdoor pools, because bromine can't be protected from sunlight by stabilizer and is used up fast outdoors.
Is bromine gentler on skin?
Some people find it so, but irritation is more often caused by pH out of range or by combined chlorine from too little sanitizer. Keep levels and pH in range either way.
Is bromine safe if I'm sensitive to chlorine?
It's a different chemical, and some people react to one and not the other. If you have a skin condition, ask a healthcare provider.