Safe swimming with elevated chlorine

[chem geek]

Ben and Carl,

Since some of the Disinfection By-Products (DBPs), including Trihalomehtanes (THMs), are known to be a problem with long-term exposure at levels of 0.08 ppm, the issue is not whether chlorine disinfection poses risk, but rather whether the risk is low relative to the risk of infection (Ben's point in earlier posts).

The rate of reaction for any chemical process is dependent on the concentration of each of the reactants as well as on temperature (and if catalysts are present, etc.). In a CYA pool, the concentration of the chlorine (HOCl and OCl-) that reacts with organics to produce DBPs is exceptionally low, typically around 0.1 ppm for HOCl and OCl- combined, so the rate of production of DBPs will be very low as well. In a non-CYA pool, as found in indoor pools, the level of chlorine (all of which is in the form of HOCl and OCl-) is usually at least 2 ppm, or 20 times higher. This higher level of chlorine is closer to the chlorine disinfection rates used in water treatment where the concern of DBPs (and the switch to using chloramines) has taken place (and remember that the concerns were with drinking the water, not just bathing in it). With the same organic loads, this means that the rate of production of DBPs will be 20 times higher. Spas have a higher temperature so 104 degrees vs. 80 degrees might have a higher rate of DBP production of 1.5x-4x (I couldn't find the rate constants and their temperature dependence for DBP production, but generally 10 degree Celsius higher temperature increases the rate by the range I indicated).

Ben had noted that asthmatics genearlly have problems only with indoor pools. I suspect that the lack of CYA use in indoor pools is at least part of the problem. Poor air circulation and the lack of sunlight which might breakdown the DBPs are other factors. This is part of the reason why I believe a small amount of CYA should be used for indoor pools though this would pose problems for easily shocking the pool (non-chlorine shock would probably need to be used).

My own personal experience (actually that of my wife's) is that the rubber in swimsuits breaks down rapidly (over one winter) when used in an indoor pool that I presume is not using CYA while there is no noticeable breakdown when used in our outdoor pool with CYA over the summer. Though rubber is not a typical organic from living things, it still gives credence to the principles outlined above. One of these days, I'm going to take my test kit down to the indoor pool my wife uses in the winter and will see what's really in that pool!

Richard (aka "chem geek")