Is chlorine safe?

[HarryH3]

The title was link on MSN this morning, to this story:

http://green.msn.com/Articles/article.a ... &GT1=45002

I thought this crowd might find it interesting...

 

[JasonLion]

The title of (EDIT)the link, and to a lesser extent the story,(/EDIT) is misleading. As the article correctly points out it isn't the chlorine that was causing problems, but rather poor sanitation practices which allowed high levels of CC.

 

[HarryH3]

The title of your post is misleading. As the article correctly points out it isn't the chlorine that was causing problems, but rather poor sanitation practices which allowed high levels of CC.

As I stated above, that was the "Title" of the link to the story, on the MSN page. Most here understand the reality, but probably 99.9% of the public that read that story would go away thinking that chlorine in a pool is a bad thing.

 

[JasonLion]

Ah, right. I missed that, sorry. I corrected my comments above.

I know that many many pools are badly managed. But, you would think that a pool where there is going to be a major swim competition would have gotten the CC level down before the competition. And then the media tries to give the story a "hook" by blaming it on chlorine instead of on incompetent pool maintenance people.

 

[chem geek]

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 + NH₃ --> NH₂Cl + H₂O ..... VERY FAST -- happens in seconds with no CYA, in under a minute with CYA
Hypochlorous Acid + Ammonia --> Monochloramine + Water
HOCl + NH₂Cl --> NHCl₂ + H₂O ..... SLOW
Hypochlorous Acid + Monochloramine --> Dichloramine + Water
NHCl₂ + H₂O --> NOH + 2H⁺ + 2Cl^- ..... SLOW
Dichloramine + Water --> Intermediate + Hydrochloric Acid
NOH + NH₂Cl --> N₂ + H₂O + H⁺ + Cl^- ..... FAST
Intermediate + Monochloramine --> Nitrogen Gas + Water + Hydrochloric Acid
HOCl + NHCl₂ --> NCl₃ + H₂O ..... MODERATE
Hypochlorous Acid + Dichloramine --> Nitrogen Trichloride + Water
NHCl₂ + NCl₃ + 2H₂O --> 2HOCl + N₂ + 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 N₂ equivalent to 0.25 ppm Cl₂) 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 Cl₂)
Peak amount of Dichloramine --- 100 seconds (about 1-1/2 minutes) --- 30 ppb (as ppb Cl₂)
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 Cl₂)

4 ppm FC with 20 ppm CYA
90% formation of Monochloramine --- 20 seconds --- 0.23 ppm (as ppm Cl₂)
Peak amount of Dichloramine --- 750 seconds (12-1/2 minutes) --- 24 ppb (as ppb Cl₂)
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 Cl₂)

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 N₂) or 0.12 ppm (as ppm Cl₂)
Monochloramine --- 0.04 ppm (as ppm Cl₂); equilibrium in air is 5.3 ppb
Dichloramine --- 4.5 ppb (as ppm Cl₂); equilibrium in air is 2.2 ppb
Nitrogen Trichloride --- 47 ppb (as ppm Cl₂); equilibrium in air is 6668 ppb (VERY VOLATILE)
Nitrate --- 48 ppb (as ppb NO₃^-) per hour

4 ppm FC with 20 ppm CYA
Ammonia --- 0.24 ppm (as ppm N₂) or 1.2 ppm (as ppm Cl₂)
Monochloramine --- 0.35 ppm (as ppm Cl₂); equilibrium in air is 53 ppb
Dichloramine --- 44 ppb (as ppm Cl₂); equilibrium in air is 21 ppb
Nitrogen Trichloride --- 4.7 ppb (as ppm Cl₂) ; equilibrium in air is 665 ppb (VERY VOLATILE)
Nitrate --- 48 ppb (as ppb NO₃^-) 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 Cl₂ 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

 

[Swimgirl]

Even though the article did point out that the problem with the competition was improper pool maintenance and poor ventilation, it did come off as having an anti-chlorine slant. And then they point out that many pools in Europe no longer use chlorine. Yesterday (or maybe it was the day before) I found an article that mentioned that people who vacation in Europe beware before using pools there; pools tested all through Europe tend to fall short of minimum health standards. Of course, this article didn't mention that.

 

[TomU]

If I understand Chem Geek's post correctly, he's saying that by killing stuff in the water slower, less dangerous byproducts are produced. I'm guessing the slower times with the CYA are still fast enough to be safe. How fast is fast enough?

If the sun did not destroy chlorine, with an outdoor, open-air pool, would it be better to run the higher active chlorine concentration simply to kill things faster? Since air quality shouldn't be an issue, it seems like it'd be safer.

 

[frustratedpoolmom]

Even though the article did point out that the problem with the competition was improper pool maintenance and poor ventilation, it did come off as having an anti-chlorine slant. And then they point out that many pools in Europe no longer use chlorine. Yesterday (or maybe it was the day before) I found an article that mentioned that people who vacation in Europe beware before using pools there; pools tested all through Europe tend to fall short of minimum health standards. Of course, this article didn't mention that.

mmmmmhhhhmmmmm.....typical media slant. They portray the story the way that sounds the juiciest to their editor/producer. They make the froggy sound downright useful...

 

[chem geek]

If I understand Chem Geek's post correctly, he's saying that by killing stuff in the water slower, less dangerous byproducts are produced. I'm guessing the slower times with the CYA are still fast enough to be safe. How fast is fast enough?

If the sun did not destroy chlorine, with an outdoor, open-air pool, would it be better to run the higher active chlorine concentration simply to kill things faster? Since air quality shouldn't be an issue, it seems like it'd be safer.

You understand the post correctly. As for the kill times of chlorine with CYA, they are fast enough to be safe for everything except for the protozoan oocysts of Cryptosporidium (and to a lesser extent, Giardia) that can't be inactivated quickly with chlorine alone anyway unless you use very high shock levels. Fortunately, it takes a very low level of chlorine to kill most bacteria and viruses. The "Min" chlorine recommendation in the chlorine/CYA charts is approximately equivalent to 0.06 ppm FC with no CYA and does a 99% kill of most bacteria in about 80 seconds (corresponding to a 99% or 2-log CT value of 0.08). Even heartier bacteria have 99% kill rates of around 2-4 minutes. Such fast kill rates not only completely prevent uncontrolled bacteria growth, but they also significantly reduce the risk of person-to-person transmission. Bacteria in biofilms can take more chlorine to kill, but having a residual in the water at all times prevents such biofilms from getting started (regular brushing of pool surfaces also helps prevent this). More technical detail about chlorine and rates of disinfection is found in this post at The Pool Forum.

It takes a higher level of chlorine to kill or prevent algae growth than it does to kill bacteria. The levels in the chlorine/CYA table were designed by Ben Powell to prevent the growth of green algae for nearly all pools. Preventing mustard/yellow algae in some pools may require higher levels closer to Ben's original "Max" column.

On the other hand, the minimum standard in the industry for public/commercial pools of 1 ppm FC with 100 ppm CYA kills bacteria around 6 times more slowly so increases the risk of person-to-person transmission, though is still fast enough to prevent uncontrolled bacteria growth since bacteria take 15-60 minutes to double in population (so a 50% kill in 15 minutes would just barely keep up -- a 99% kill in that time is much faster than the bacterial growth rate). One member of the industry in an influential position on the APAP-11 standards committee takes the position that the 1 ppm FC with 100 ppm CYA is fine unless one can demonstrate actual outbreaks or illness reports at this level. I disagree with this since there is a spectrum of risk and individual reports from infrequent person-to-person transmission are unlikely to get reported. I'd personally rather draw the line closer to 1-2 minute 99% kill times than a 15 minute kill time and you get the advantage of preventing algae growth without the need for supplemental algaecide.

Part of the reason there isn't an air quality issue with outdoor pools is the fact that there is sunlight since the UV rays of sunlight break down nitrogen trichloride fairly quickly. In addition, air circulation is usually much better unless there is no wind. You are correct that not using CYA in outdoor pools would result in faster kill times, but there are downsides to the higher chlorine levels beyond higher levels of disinfection by-products. It also oxidizes faster so will be more corrosive (to metal, for example) and will wear out swimsuits (especially their elasticity) faster, will make skin flakier and hair frizzier. My wife experiences this every winter season when she uses an indoor community center pool with around 2 ppm FC and no CYA. She has to buy new swimsuits every winter while duplicate swimsuits used for summer have lasted nearly 5 seasons in our own outdoor pool with 3-4 ppm FC and 30 ppm CYA with minimal signs of wear. There is also a noticeable difference in her skin and hair after swimming in these two situations. I believe the difference is due to the indoor pool having 20 times the amount of active chlorine (hypochlorous acid).

Richard