But you don't put your head in the washing machine.

Your wife would probably be safer if she were swimming under the water as the highest concentration of chlorine and byproducts is at and just above the surface of the water, just where your wife will have her nose and mouth. I would wait a couple of days until the level has dropped.

The shock levels in the Chlorine / CYA Chart have a Free Chlorine (FC) level that is roughly 40% of the Cyanuric Acid (CYA) level. At 77ºF and a pH of 7.5, the amount of active chlorine (hypochlorous acid) at this shock level is the same as found in a pool with 0.6 ppm FC and no CYA. At 88ºF, this may be equivalent to a pool with 1.1 ppm FC and no CYA. This is generally less than found in many indoor pools that often have 1-2 ppm FC with no CYA. Now, that said, I believe such indoor pools are essentially being over-chlorinated because the chlorine/CYA relationship is not understood by many in the industry.

The minimum FC in that same Chlorine/CYA chart is around 7.5% of the CYA level. At 77ºF, this is roughly equivalent to around 0.06 ppm FC with no CYA. At 88ºF, this may be equivalent to around 0.14 ppm FC with no CYA. So shock levels of chlorine have from 7.8 to 10 times the amount of active chlorine as compared to the minimum chlorine levels for manually dosed pools. This should result in faster oxidation of swimsuits, skin and hair, faster outgassing of hypochlorous acid and greater production of irritating and volatile nitrogen trichloride, but faster oxidation and lower intermediate levels of monochloramine and dichloramine. The short-term effect on other disinfection by-products (DBPs) may be a temporary increase in their production though the amount of such DBPs in residential pools may be very low.

So there is a spectrum of risk that is relative and there is no question that swimming at shock levels is exposing one to more active chlorine with all that this implies, but still at levels that are lower than most commercial/public indoor pools. Also note that the higher FC is relevant if one directly ingests the water since that is more about a total chlorine intake. There is always the possibility that in spite of all the scientific papers showing minimal killing and oxidation effects from chlorine bound to CYA and minimal skin absorption, that there is something else going on that could increase risk.

If it were my wife, I'd much rather have her in our own pool at shock level compared to a commercial/public pool whose maintenance was questionable and that had much, much higher bather load. However, that's not really the question here. Since we're talking about a one-time exposure and not a continual exposure at these levels for an entire swim season, I'd rather have my wife happily cooled off, but that's just me. She'd likely complain about the faint chlorine smell as she is very sensitive so she'd be the judge as to whether it was something she wanted to do. As with most of these sorts of decisions, discuss it together and come to a conclusion that you are both comfortable with (especially her).

Don't forget you always have the option of adding some dechlorinator (at extra cost) to rapidly lower the FC level. Just don't overdo it since you still want adequate chlorine levels in the pool.

Richard

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.

teapot, what is the point of bringing this up? Swimming isn't completely safe. But the risks you bring up, as they apply to an outdoor pool maintained the way we recommend, are minuscule when compared to say the risk of driving somewhere, or skiing, or any of a dozen other things that lots of people do all the time.

Nm.

Agreed Jason, but we are talking about a pregnant woman and why take the risk at shock level when the risk will reduce to negligible in a day or two.

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 (O₂) and chloride ion (Cl^-) and possibly some amount of chlorate, ClO₃^-. 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