I have looked at virtually every case of asthma, respiratory illness, sore eyes, etc. I could find and every single one was either an indoor pool that most likely did not have Cyanuric Acid (CYA) or was a pool with improperly balanced chemistry (usually too low chlorine or the pH was way off or an indoor pool with >> 100 ppm CYA). As Jason points out, the problem is not chlorine itself, but the disinfection by-products (DBPs) that occur when chlorine combines with ammonia and organics.
The irony is that no one in this industry is using their heads and looking at the chemistry of chlorine with CYA to realize that any chlorinated pool without CYA (such as most indoor pools) are essentially being vastly over-chlorinated. Everyone focuses on Free Chlorine (FC) as if it means something about the active chlorine concentration -- it doesn't. It is simply a measure of the combination of the chlorine IN RESERVE plus the active chlorine. When CYA is present, it mostly measures the chlorine in reserve which, at an FC of 3 ppm with 30 ppm CYA, is 98.5% of the chlorine -- only 1.5% of the chlorine is active (i.e. hypochlorous acid).
The rate of ANY chemical reaction with chlorine (hypochlorous acid) is proportional to the active chlorine concentration, NOT to the FC level.
An indoor pool with 1-2 ppm FC and no CYA has 10-20 TIMES the amount of active chlorine compared to a pool with 3.5 ppm FC and 30 ppm CYA (both pools at a pH of 7.5). What is amazing to me is how the industry can be so focussed on the pH dependence on active chlorine concentration (especially when CYA is not present) and yet ignore the much greater effect of CYA. The rates of formation of DBPs in typical pools with no CYA will be 10-20 times faster and in some cases the end result amount of DBPs will be 10-20 times higher. A specific example for this concerns the breakpoint chlorination of ammonia which has been well researched with the definitive chemical model being that of Jafvert & Valentine 1992.
I have a spreadsheet with that model (and earlier models from Selleck & Saunier and Wei & Morris) that shows that in conditions where there is an excess of FC to oxidize ammonia, which is what one normally wants so as not to run out of chlorine, that without CYA the amount of dichloramine is higher and the amount of nitrogen trichloride produced during the breakpoint reaction is 10-20 times higher during the breakpoint and that the final end result concentration of nitrogen trichloride is 10-20 times higher. The reason for this is due to the following sequence of reactions (this is a subset of all the reactions, but is what dominates):
HOCl + NH
3 --> NH
2Cl + H
2O ..... VERY FAST -- happens in seconds with no CYA, in under a minute with CYA
Hypochlorous Acid + Ammonia --> Monochloramine + Water
HOCl + NH
2Cl --> NHCl
2 + H
2O ..... SLOW
Hypochlorous Acid + Monochloramine --> Dichloramine + Water
NHCl
2 + H
2O --> NOH + 2H
+ + 2Cl
- ..... SLOW
Dichloramine + Water --> Intermediate + Hydrochloric Acid
NOH + NH
2Cl --> N
2 + H
2O + H
+ + Cl
- ..... FAST
Intermediate + Monochloramine --> Nitrogen Gas + Water + Hydrochloric Acid
HOCl + NHCl
2 --> NCl
3 + H
2O ..... MODERATE
Hypochlorous Acid + Dichloramine --> Nitrogen Trichloride + Water
NHCl
2 + NCl
3 + 2H
2O --> 2HOCl + N
2 + 3H
+ + 3Cl
- ..... FAIRLY FAST
Dichloramine + Nitrogen Trichloride + Water --> Hypochlorous Acid + Nitrogen Gas + Hydrochloric Acid
The "NOH" is an unknown intermediate (in the Jafvert & Valentine model it is designated as "I"). Basically what happens in the breakpoint reaction is that hypochlorous acid very quickly combines with ammonia to form monochloramine. Then more hypochlorous acid combines with monochloramine to form dichloramine and this builds up until its formation equals the rate at which dichloramine forms an intermediate product that then degrades to nitrogen gas. After the peak of dichloramine, its formation becomes slower than its degradation and its concentration drops. You can also see that more hypochlorous acid can combine with dichloramine to form nitrogen trichloride and that this can break down with dichloramine.
Remember what CYA does to chlorine in water. It binds to most of it making the actual hypochlorous acid concentration very low. This means that the reactions above involving hypochlorous acid will be much slower. The formation of monochloramine will be slower, but since the reaction is so fast this doesn't matter very much. It does matter for the formation of dichloramine since it makes the second reaction above (the first SLOW reaction) much slower then the third reaction above (the second SLOW reaction) which results in lower peak amounts of dichloramine because the peak amount is when the two reaction rates are equal -- essentially if you are filling a container at a slower rate then if the rate of it getting emptied is proportional to the amount in the container, this amount will be lower. Also, because the rate of formation if nitrogen trichloride is proportional to the hypochlorous acid concentration, there is less produced. The fact that it takes longer doesn't matter since there is a relatively fast reaction breaking down nitrogen trichloride with dichloramine. The entire breakpoint reaction takes longer, but proportionately so the net result is less nitrogen trichloride that is produced. The original Wei & Morris model predicted proportionately smaller dichloramine amounts at lower chlorine concentrations, but this was not seen in actual experiments and the Jafvert & Valentine model much more accurately predicts what is actually seen.
Here are some specific numerical examples comparing what happens over time in two scenarios -- a pool with 2 ppm FC and no CYA (a typical indoor pool) compared to a pool with 4 ppm FC and 20 ppm CYA (which is what I would recommend for an indoor pool). Note that the difference in concentration of hypochlorous acid in these two situations is roughly a factor of 10. I assume a temperature of 77F (so higher temperatures have everything go faster) and a pH of 7.5. I assume an introduction of 0.05 ppm ammonia (ppm N
2 equivalent to 0.25 ppm Cl
2) so that there is plenty of chlorine in both scenarios to oxidize it.
2 ppm FC with no CYA
90% formation of Monochloramine --- 2 seconds --- 0.23 ppm (as ppm Cl
2)
Peak amount of Dichloramine --- 100 seconds (about 1-1/2 minutes) --- 30 ppb (as ppb Cl
2)
50% Breakpoint (half of ammonia fully oxidized) --- 268 seconds (about 4-1/2 minutes)
90% Breakpoint --- 795 seconds (13-1/4 minutes)
Peak and near final amount of Nitrogen Trichloride ---
36 ppb (as ppb Cl
2)
4 ppm FC with 20 ppm CYA
90% formation of Monochloramine --- 20 seconds --- 0.23 ppm (as ppm Cl
2)
Peak amount of Dichloramine --- 750 seconds (12-1/2 minutes) --- 24 ppb (as ppb Cl
2)
50% Breakpoint (half of ammonia fully oxidized) --- 2278 seconds (about 38 minutes)
90% Breakpoint --- 6760 seconds (113 minutes; almost 2 hours)
Peak and near final amount of Nitrogen Trichloride ---
4.3 ppb (as ppb Cl
2)
The same conclusion for nitrogen trichloride is reached if I use a continuous introduction model to keep the FC constant (say, through continuous or monitored chlorine addition) and the rate of introduced ammonia constant (through bather load). If I assume an introduction of 0.1 ppm ammonia (ppm Nitrogen) per hour which is roughly a chlorine demand of 1 ppm FC per hour for somewhat high bather load, then this results in the following steady state concentrations:
2 ppm FC with no CYA
Ammonia --- 0.023 ppm (as ppm N
2) or 0.12 ppm (as ppm Cl
2)
Monochloramine --- 0.04 ppm (as ppm Cl
2); equilibrium in air is 5.3 ppb
Dichloramine --- 4.5 ppb (as ppm Cl
2); equilibrium in air is 2.2 ppb
Nitrogen Trichloride --- 47 ppb (as ppm Cl
2); equilibrium in air is 6668 ppb (VERY VOLATILE)
Nitrate --- 48 ppb (as ppb NO
3-) per hour
4 ppm FC with 20 ppm CYA
Ammonia --- 0.24 ppm (as ppm N
2) or 1.2 ppm (as ppm Cl
2)
Monochloramine --- 0.35 ppm (as ppm Cl
2); equilibrium in air is 53 ppb
Dichloramine --- 44 ppb (as ppm Cl
2); equilibrium in air is 21 ppb
Nitrogen Trichloride --- 4.7 ppb (as ppm Cl
2) ; equilibrium in air is 665 ppb (VERY VOLATILE)
Nitrate --- 48 ppb (as ppb NO
3-) per hour
So with CYA in the water, the steady state has more ammonia, monochloramine and dichloramine but less nitrogen trichloride, all by a rough factor of 10. The primary irritant is Nitrogen Trichloride which is extremely volatile. In pools without CYA, essentially the breakpoint reaction happens too quickly so produces more Nitrogen Trichloride. It is well known in the water treatment industry that to reduce DBP formation one uses lower amounts of chlorine over a longer period of time (same CT value) rather than high levels of chlorine over shorter periods of time. Nitrogen Trichlorde levels in the air on the order of 21-36 ppb (12-21 ppb Cl
2 equivalent) were shown in studies to trigger asthma. Basically,
it would take 10 times the amount of airflow in the steady state to make the no CYA pool have the same nitrogen trichloride air quality as the CYA pool.
The problem is that though there is a lot of ammonia in sweat and urine, it is not the largest component. Urea is the largest component by far in sweat and urine. Urea composes 68% of the Nitrogen in sweat and 84% of the Nitrogen in urine. Ammonia composes 18% of the Nitrogen in sweat and 5% of the Nitrogen in urine. So what happens with urea is critically important yet there is no model determined for the oxidiation of urea by chlorine (though there are speculated reactions). The National Swimming Pool Foundation (
NSPF) is funding a study by
Ernest "Chip" Blatchley III of Purdue University (referred to
here) to identify the volatile DBPs and as part of that work he will hopefully develop a model for chlorine oxidation of urea. This will let us know if, in theory, CYA will have a moderating effect on Nitrogen Trichloride production in this case as well.
I suspect that the mechanism with urea may be such that it will slowly build up in concentration in the water so that the average Nitrogen Trichloride concentration may not change much. However, CYA should smooth out the peaks of such concentration such as during a swim team competition since it slows down the reaction process. Since air exchange in a facility is a fixed quantity, smoothing out such blips would still be very helpful, allowing for the bulk of the production and removal of nitrogen trichloride to be done overnight with a lower average air concentration during the pool usage in the day. We'll see...
In case anyone thinks that the industry isn't aware of these effects, consider
patent 5591692 where Table VIII shows a 40.9% reduction in chloroform (a trihalomethane, THM) when using 50 ppm CYA and greater reductions when also using glycoluril which is even more powerful than CYA at binding to chlorine.
Richard