An honest comparison of the two main hot tub sanitisers, with the four questions that will decide which suits your household best.
Both chlorine and bromine kill the same bugs. The differences come down to smell, skin sensitivity, the temperature you run at, and how often you can be at the tub to test. There is no universal winner, but for most UK hot tub owners, bromine wins on three out of four counts.
Chlorine is faster to act and faster to drop off. It oxidises organic matter quickly and then dissipates, especially in warm water. You typically dose little and often.
Bromine works in a slower, steadier way. It tolerates higher temperatures (it is the same active species at 30C and 40C, whereas chlorine becomes less effective above about 35C) and tends to live in floating feeders or cartridges that release it gradually.
Bromine smells less. The compounds it forms after killing organics are less volatile than the chloramines chlorine produces, which is why bromine tubs feel less harsh on the eyes.
For sensitive skin, bromine is usually kinder. Eczema and dermatitis sufferers often switch to bromine and stop reacting.
Bromine is more stable at the temperatures hot tubs run at, which is the single biggest reason most UK hot tub specialists recommend it for a domestic tub.
If you only use the tub at weekends and want to dose-and-forget all week, chlorine in slow-dissolving tablets in an in-line feeder works well.
If you have a pool that you want to share chemicals with, chlorine across both makes life simpler.
If cost matters more than convenience, chlorine granules are noticeably cheaper per kilo than bromine tablets at retail.
Not cleanly. The two reseed each other's depleted cells in the water and you spend weeks chasing readings. Switch at a refill.
They use different test strips and different drops. Both are simple but you must use the correct test for the sanitiser you have, or the readings are meaningless.