The two most popular hot tub sainitisers (disinfectants) are chlorine and bromine, with chlorine being the most popular. But with many people judging chlorine as harsh, bromine, with its "softer" touch on the skin, meets the demands of spa uses that disapprove of chlorine.
Chlorine is by far the most popular sanitiser among hot tub and pool users worldwide. Chlorine was historically unrivaled until the growing popularity of bromine in the 1980's, due its fast-acting effectiveness in disinfecting spas and pools. It also has the added benefit of being cheaper Per KG than bromine, suiting smaller budgets. However despite effectiveness in sanitation and cost, chlorine has been labelled by many as "harsh" on the skin, eyes and lungs. This mainly comes down to association with "eye sting" and that sharp smell at public pools. The reason for this is the chlorine when it combines with organic compounds in spa water, so, in more simple terms, the dirtier the pool or spa, the more the chlorine will smell, hence the odour at public pools.
Bromine first became popular with pool and spa users in the 1980's, as scientists saw it as a more effective water disinfectant. The clear benefit was that, when bromine combined with organic compounds, the bromamines, unlike chlorine, helped to continually sanitise the water rather than cause odour. Many see bromine as "softer on the skin", but in my personal opinion, i believe this is more down to a distaste to chlorine, rather than bromine actually being less harsh. However, like all developments, issues with this bromine became clearer over time. At first, bromine was so stable, that chlorine had to be added to it, to speed up its sanitation of water. Even though developments have speeded up bromine sanitation, it cannot keep up with the Usain Bolt of chemicals, which is chlorine. Some strains of algae, such as black fungi, have been resistant to bromine, but affected by chlorine, limiting the effectiveness of bromine further.
In my opinion, chlorine is the safest and most effective hot tub sanitiser. But with bromine constantly being adapted and becoming increasingly effective, Its inevitable that the contest will go on. With more recent "non-chlorine" systems also being developed, such as AquaGarde, new competitors will join the race to be the top disinfectant.
Happy Hot Tubbing !