teapot said:
With chlorinated Isocyanurates under the presence of a UV source (the sun) there is also production of Cyanogen Chloride, which our own bodies can metabolise into cyanide ions by haemoglobin and glutathione. There is research by the WHO into the effects of this gas and Scoliosis on young children.
I just feel its safer to let the levels reduce especially for a pregnant woman.
Cyanogen Chloride is a disinfection by-product from chlorine oxidation of L-Histidine and in a recent study where such levels were measured in high bather load commercial/public pools, it ranged mostly in the 1-10 ppb range. The American Conference of Governmental Industrial Hygenists (ACGIH) aqueous Threshold Limit Value (TLV) for CNCl is 0.3 mg/L (so 300 ppb). However, the Danish paper you had linked to from the Dryden site indicated samples measuring > 100 ppb on many occasions. As was noted in that paper, "The obtained data indicate a relationship between high TOC concentrations and elevated concentrations of disinfection by-products." Also, "Based on actual observations of temporal variations of by-products, it can be concluded that the concentration levels exhibit a pronounced temporal variation, in some cases from 10 to over 160 µg/L during the same day."
TOC is the total organic carbon, but the above temporal variation of the DPBs with TOC would indicate that this comes from bather load since the amount of chlorinated cyanurates will be fairly constant throughout the day and night (though in the Danish study there may not have been any CYA at all in those pools). I have not seen any scientific literature showing the chlorinated cyanurates contributing to any significant amount of disinfection by-products. The fact that CYA levels remain fairly constant over long periods of time would go against them as a source of DBPs, at least in any significant amount. Specifically, a CYA drop of 2 ppm per month would be 3*61.46/129.07/30 = 48 ppb per day if the CYA completely converted to cyanogen chloride which, of course, it doesn't. The breakdown of CYA by oxidation from chlorine is described in
this post where the end products for the primary reaction are nitrogen and carbon dioxide gasses.
There is no scientific literature I could find regarding the UV breakdown of chlorinated cyanurates. There is UV absorption, just as there is with CYA itself (and this is described in the 1974 O'Brien paper, though not in the range of UV in sunlight), but the quantum yield in both cases appears to be very small (i.e. most photons that are absorbed do not result in degradation, but rather just in thermal heating possibly with re-emission at longer wavelengths). If it were high, then one would notice significant reductions in CYA in pools exposed to sunlight, after accounting for dilution effects and direct chlorine oxidation independent of sunlight.
The primary effect of UV in pools is in the breakdown of hypochlorous acid and especially hypochlorite ion as this initially produces free radicals of OH•, Cl• and O
-•. These mostly terminate in reactions that result in oxygen gas (O
2) and chloride ion (Cl
-) and possibly some amount of chlorate, ClO
3-. One does notice a reduction in Free Chlorine that is greater than predicted by hypochlorous acid and hypochlorite ion alone, but the end result may again be that of chloride ion and oxygen gas with the CYA not degraded.
So I don't see where the chlorinated cyanurates directly contribute to cyanogen chloride or to other DBPs in any significant quantity. Do you have any sources demonstrating this link?
Richard