Generally speaking, the brominated organics are more troublesome than the chlorinated ones. I list some of the ones the EPA notes below for chlorine and bromine (or combo) compounds:
Chlorine alone
Chloroform (CHCl
3) -- EPA lists this as a carcinogen "likely under high exposure conditions"
Bromine or combination
Bromoform (CHBr
3) -- EPA lists this as a "probable" carcinogen
Dibromochloromethane (CHBr
2Cl) -- EPA lists this as a "possible" carcinogen
Dichlorobromomethane (CHBrCl
2) -- EPA lists this as a "probable" carcinogen
Very roughly speaking, at the same concentrations, bromoform is more than 30 times as carcinogenic as chloroform while the dibromochloromethane and dichlorobromomethane are about 10 times more carcinogenic than bromoform. One in 1 million lifetime cancer risks for drinking 2 quarts of water every day are at 130 ppb for chloroform, 4 ppb for bromoform, 0.4 ppb for dibromochloromethane and 0.6 ppb for dichlorobromomethane.
This is why sanitation systems for drinking water try and remove organic particulate matter prior to chlorination to reduce the disinfection by-products and why they use chlorine and not bromine for disinfection (chlorine is also less expensive).
This paper measured levels of the disinfection products (in many pools, but we're not interested in that here and) in one commercial spa at 16.2 ppb for chloroform, 1.12 ppb for bromoform, 0.605 ppb for dichlorobromomethane, and below detection limit for dibromochloromethane, but this was for a chlorinated spa and it would be expected that a brominated one would have higher levels of the brominated compounds. Since the amount of disinfection by-products is roughly proportional to bather load, the amounts will probably be somewhat lower in a residential spa. The actual route of exposure is primarily inhalation since you aren't drinking the spa water, but there aren't measurements for concentrations of these volatile organics in air above the spa nor in cancer risk limits for that route (except for chloroform at 0.04 µg/m
3 for 1 in a million lifetime cancer risk for constant exposure).
To minimize risk, you can start your soak with a minimal amount of sanitizer (1-2 ppm FC) and add more after your soak in sufficient quantity to oxidize bather waste. This is easy to do when using the Dichlor-then-bleach method (as described in
Using Chlorine in a Spa). One should also uncover the spa for at least 10 minutes prior to soaking in order to air out any buildup of gasses accumulating under the cover. Note that if you do not soak with sufficient sanitizer then there is risk of person-to-person transmission of disease but if it's just you or your family in the tub then the risk is low.
There are also two non-halogen (i.e. no chlorine nor bromine) approaches that are more expensive: Baquacil and Nature2 with MPS. The latter is less problematic and uses chlorine "as needed" to keep the water clear, sometimes once a week, while non-chlorine shock (MPS) is used as the primary oxidizer/sanitizer when used in conjunction with silver ions from Nature2 (this only works to be sanitary at hot spa temperatures).
too many times until finding this 
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