Is a Pool Chlorine Level Above 10 PPM Unsafe?

[chem geek]

The active chlorine level that oxidizes swimsuits, skin, hair, corrodes equipment, oxidizes bather waste, creates disinfection by-products, etc. is very low when there is CYA present because its amount is roughly half the FC/CYA ratio. So when we recommend something on the order of an FC that is 10% of the CYA then the active chlorine level is roughly equivalent to around 0.1 ppm FC with no CYA where about half of this is active chlorine. In other words, it's very low and therefore safe.

The ONLY issue with a high FC level on its own, independent of CYA level, is if you were to drink a lot of pool water since that is when this FC number matters because it represents the capacity/reserve of chlorine. It may not react very quickly because of the CYA, but in any given amount that you swallow, there will be a lot of chlorine, assuming the FC level is very high. So the question is how high is high? The EPA limits drinking water to 4 ppm, but that is for drinking 2 quarts of water every day for a lifetime. In practice, 10 ppm FC won't be a problem and since you aren't drinking the pool water even higher levels are not an issue.

So unless you expect your kids to be drinking lots of pool water, then it's safe even at 20 ppm FC assuming you have CYA in the water (even 20 ppm CYA or more would be enough to moderate chlorine's strength in this case). Their getting one gulp by accident is not a problem.

As a point of reference, the LD50 level for hypochlorite is 5800 mg/kg so even a 50 pound child would have to drink 347 gallons of 100 ppm FC pool water to have a 50% chance of dying. The EPA notes the following in this link:

No adverse effects were noted in persons ingesting water containing 50-90 ppm of chlorine (~1.4 to 2.6 mg Cl/kg/day) for a short periods of time (U.S. EPA 1989). Drinking water concentrations of >90 ppm chlorine caused irritation of membranes of throat and mouth (U.S. EPA 1989). Concentrations of chlorine in the drinking water of greater than 25 ppm make the drinking water unpalatable (U.S. EPA 1989).

Even the irritations they note of the throat and mouth would not happen when there is CYA in the water since it would significantly slow down chlorine reaction rates.

[PoolDoc]

As a point of reference, the LD50 level for hypochlorite is 5800 mg/kg so even a 50 pound child would have to drink 347 gallons of 100 ppm FC pool water to have a 50% chance of dying.

Thanks Chem_Geek. And, to put that 347 gallons in perspective, about 20 years ago a lady accidentally drank about 4 oz. of bleach, after setting her juice glass next to a glass which had bleach in it. She then panicked and drank 2 gallons of water to 'dilute' the bleach. The bleach wouldn't have hurt her, but the water killed her, by so diluting the electrolytes in her body.

So, a child COULD NOT drink that much water -- they would die from over hydration long before the chlorine could have any effect.

[JimK]

So it's apparent that chlorine would have to be very high to hurt us, but what about our bathing suits? For example, if my CYA is 80, what would the maximum FC level recommended to avoid destroying my bathing suit?

[PoolDoc]

No good answer to that: the most expensive women's suits are almost completely intolerant of chlorine.

I can tell you that very expensive women's suits are worse than moderate ones, and that suits with elastic (Lycra) are much worse than nylon board shorts or guard suits . . . and that 100% polyester competition suits last forever. But other than that, you have to experiment. However, if you are asking the question, I'd recommend separating your family's suits into "swimming suits" and "showing off suits". If you do that, you'll likely end up with the more chlorine resistant ones in the swimming pile.

I have some OLD (20 year old) data on grades of Lycra used in competition suits . . . and I can tell you that, at least back then, the most common Lycra elastic was sold on the presumption that women would 'slip and dip', not swim. I think the life expectancy was something like 5 hours at 3 ppm - and that the suit would last the summer. They didn't consider or test stabilized chlorine solutions.

[JimK]

No good answer to that: the most expensive women's suits are almost completely intolerant of chlorine.

I can tell you that very expensive women's suits are worse than moderate ones, and that suits with elastic (Lycra) are much worse than nylon board shorts or guard suits . . . and that 100% polyester competition suits last forever. But other than that, you have to experiment. However, if you are asking the question, I'd recommend separating your family's suits into "swimming suits" and "showing off suits". If you do that, you'll likely end up with the more chlorine resistant ones in the swimming pile.

I have some OLD (20 year old) data on grades of Lycra used in competition suits . . . and I can tell you that, at least back then, the most common Lycra elastic was sold on the presumption that women would 'slip and dip', not swim. I think the life expectancy was something like 5 hours at 3 ppm - and that the suit would last the summer. They didn't consider or test stabilized chlorine solutions.

I see. So when shopping for a "pool suit", I should look for nylon or polyester and try to minimize/avoid lycra. We're too cheap to buy expensive suits, so guess we should be fine with the recommended "best guess" levels.

[chem geek]

Just keep in mind that it is the active chlorine (hypochlorous acid) that attacks the swimsuit, not the Free Chlorine (FC) level. So when there is CYA in the water, the rate of attack is orders of magnitude slower. My wife experiences this effect every year when she uses an indoor commercial pool where the elasticity in her swimsuits gets shot after one winter season and her skin is somewhat flaky and hair frizzy until she takes a shower while in our own outdoor residential pool the swimsuits last for years and there aren't the same side effects with skin and hair. The main difference is that the indoor pool has 1-2 ppm FC with no CYA while our outdoor pool has 3-6 ppm FC with 40 ppm CYA which has an active chlorine level similar to around 0.1 ppm FC with no CYA. So the active chlorine level of the indoor pool is 10-20 times higher and oxidizes swimsuits, skin and hair that much faster as well.

The higher FC level does have an effect after you get out of the pool in that there is more chlorine in reserve so that it can keep reacting for longer as the water evaporates, but this effect can be minimized by rinsing the suits and is generally less important that the active chlorine level.

[JimK]

Thanks.

So a related question;

If I remember correctly, with my CYA at 80, the recommended shock level is 20. How much should I allow it to drop before swimming again?