Hot tub chlorine vs bromine

Both work. The CDC lists both for hot tubs, with its own target for each. The choice comes down to cost, how much the tub gets used and how much you want to fuss with it.

One thing to know before you pick: going from bromine back to chlorine means draining the tub. So choose once.

The levels to keep

The CDC's hot tub numbers: free chlorine at least 3 ppm, or bromine 4 to 8 ppm. Above 10 ppm chlorine or 8 ppm bromine, the CDC says to check with the owner or operator before getting in. A bromine tablet label we checked (BCDMH, EPA Reg. 5185-490) also says not to enter above 8 ppm.

Spa makers often publish their own, lower ranges for home tubs, and they don't agree with each other. Pool & Spa News uses 3 to 5 ppm chlorine and 4 to 6 ppm bromine. Use your tub's manual, and treat the CDC numbers as the safety floor and ceiling.

	Chlorine	Bromine
CDC minimum	3 ppm	4 ppm
CDC upper limit	10 ppm	8 ppm
Pool & Spa News	3–5 ppm	4–6 ppm

How they're different

Chlorine is cheaper and works faster, Bullfrog says. Taylor agrees it's typically the lower-cost choice. Dichlor granules are a common choice; they dissolve fast and are easy to dose with the hot tub calculator. Raising a 400-gallon tub from 1 to 4 ppm takes about 0.29 oz of 56% dichlor.

Bromine holds up better in hot water. Taylor says it "offers superior performance at increased bather loads and higher pH," and Bullfrog puts bromine's working pH range at about 7.2 to 8.4, against about 7.2 to 7.8 for chlorine. It costs more and dissolves slower, usually from tablets in a floater or feeder, so it's steadier between doses.

Smell and skin are where opinions split. Taylor says bromine's byproducts are milder and usually less irritating than chlorine's. Bullfrog says reactions vary by person and some people think bromine smells worse. If someone in your house reacts to one, try the other.

The stabilizer catch with chlorine

The CDC recommends not using cyanuric acid (stabilizer) in hot tubs, or chlorine products that contain it. Dichlor contains it. Every 10 ppm of chlorine from dichlor adds about 9 ppm of stabilizer, and an AQUA Magazine article (sponsored by a spa-care brand) gives the example of a 300-gallon tub reaching about 50 ppm stabilizer in around four weeks.

Stabilizer only leaves when you drain. So if you use dichlor, change the water on schedule. Liquid chlorine and cal-hypo don't add stabilizer. Bromine doesn't use it at all; Taylor says cyanuric acid "does not work for bromine sanitizer."

Setting up bromine

Bromine needs a starter. Leslie's describes the usual routine: after each refill, add sodium bromide to build a "bank" of about 10 to 15 ppm, shock to activate it, then keep bromine tablets in a floater or feeder. Every drain resets the bank to zero, so repeat it each time.

Bromine also needs regular shocking. Taylor explains that used-up bromine has to be reactivated by shock, either chlorine or non-chlorine (MPS).

Check your tub's manual before you start. Salt-system spas make their own chlorine, and some makers restrict which sanitizers you can use. Never put bromine tablets in a feeder or floater that has held chlorine tablets, or the other way round.

Switching between them

Going from bromine to chlorine is the hard direction. Pool Solutions explains why: chlorine converts the leftover bromide back into bromine, so with even a few ppm of bromide in the water you still have a bromine tub.

The fix is a full drain. Trevi says to completely drain a spa, and Taylor adds flushing the lines. Clean the filter, refill and start fresh.

Testing

Use a test made for your sanitizer. Taylor recommends DPD or FAS-DPD for chlorine, and says total bromine reads fine on DPD too. Strips are handy before a soak, but the CDC says drop kits are more accurate. Hot tub test strips that read both are an easy start. Write your readings on the test log; it has hot tub targets printed on it.