I'd like to hear a chemist chime in here, but my understanding is the concentration of elemental chlorine gas in a pool's atmosphere is going to be pretty close to nil. Chlorine compounds, yes, but chlorine, no. It's so reactive it doesn't stay in its elemental form for long. Now I also know that some tiny fraction of molecules are going to crack apart in water - even tap water has some free H and OH ions in it. Whether chlorine ions get produced in the pool and if so how long they remain as ions when the reach the air is what I'm not sure of.
The thread title is misleading. The potential danger is not from chlorine gas directly. Chlorination can be done either using a hypochlorite salt or by elemental chlorine, which as you note dissolves and reacts pretty quickly to form hypochlorite (there will be some small dissolved chlorine gas remaining but not very much). Pool pH control is essential to have the right balance of hypochlorite (OCl-) & hypochlorous acid (HOCl); it is the acidic that is more effective at killing pathogens. I believe hypochlorite salts are generally used for (chlorinated) swimming pools, while chlorine gas tends to be used for drinking water and sewage disinfection.
It shouldn't be all that surprising that things that are meant to kill bacteria etc might have health effects on nontarget organisms, but most of the risk is not from HOCl & OCl- directly but from reactions involving them. Chloramines are produced by the reaction with ammonia and organic amines (including urea). A number of disinfection byproducts (eg chloroform and other trihalomethanes, THMs) are also produced by the reaction with other organics.
Of course the risk from these things is less than the risk posed by disease if the water is not disinfected. But there are potentially better ways of sanitizing the water. The facility I swim at has two pools, one of which uses uv disinfection and the other uses ozone. The reason they went to these systems was that, with chlorination, the air quality was so bad that most of the kids were hacking and some were having more serious respiratory problems. The pool is great now; (light) chlorination is used as a backup when necessary.
Most civil or environmental engineering or wastewater treatment references will tell you more than you ever want to know about chlorination chemistry.
I'd like to hear a chemist chime in here, but my understanding is the concentration of elemental chlorine gas in a pool's atmosphere is going to be pretty close to nil. Chlorine compounds, yes, but chlorine, no. It's so reactive it doesn't stay in its elemental form for long. Now I also know that some tiny fraction of molecules are going to crack apart in water - even tap water has some free H and OH ions in it. Whether chlorine ions get produced in the pool and if so how long they remain as ions when the reach the air is what I'm not sure of.
The thread title is misleading. The potential danger is not from chlorine gas directly. Chlorination can be done either using a hypochlorite salt or by elemental chlorine, which as you note dissolves and reacts pretty quickly to form hypochlorite (there will be some small dissolved chlorine gas remaining but not very much). Pool pH control is essential to have the right balance of hypochlorite (OCl-) & hypochlorous acid (HOCl); it is the acidic that is more effective at killing pathogens. I believe hypochlorite salts are generally used for (chlorinated) swimming pools, while chlorine gas tends to be used for drinking water and sewage disinfection.
It shouldn't be all that surprising that things that are meant to kill bacteria etc might have health effects on nontarget organisms, but most of the risk is not from HOCl & OCl- directly but from reactions involving them. Chloramines are produced by the reaction with ammonia and organic amines (including urea). A number of disinfection byproducts (eg chloroform and other trihalomethanes, THMs) are also produced by the reaction with other organics.
Of course the risk from these things is less than the risk posed by disease if the water is not disinfected. But there are potentially better ways of sanitizing the water. The facility I swim at has two pools, one of which uses uv disinfection and the other uses ozone. The reason they went to these systems was that, with chlorination, the air quality was so bad that most of the kids were hacking and some were having more serious respiratory problems. The pool is great now; (light) chlorination is used as a backup when necessary.
Most civil or environmental engineering or wastewater treatment references will tell you more than you ever want to know about chlorination chemistry.
