Science of "safe to swim up to SLAM level"

PointeTaken

Well-known member
Aug 28, 2019
57
Hoover, AL
First of all, let me begin by saying this isn't an indictment or skepticism of this community or the TFP methodology. This is a genuine open-ended request for more detailed info mostly out of curiosity, and also due to the fact that a lot of the concepts and principles on this site challenge what I learned as a pool pro once upon a time. When rules/concepts/etc get engrained in you, it's hard to accept anything that challenges them without pause. One thing I have never come across on this forum is a post declaring that any method or process developed by TFP has put swimmers or property at risk, so it's clear to me that everything here has essentially been vetted through the experience of hundreds of thousands of people. The intent of this post is not to cast doubt.

I've read the Pool Water Chemistry thread initially posted by @chem geek, and the chemistry behind the Chlorine/CYA relationship made sense to me, even with my 10th grade-level understanding of chemistry. It even got a nod of approval from my usually skeptical father-in-law (also a pool owner), who has a PhD in chemistry and was a chemist for Dow and 3M over his 40 year career.

There's one thing that wasn't clear to him, isn't clear to me, got an alarmed eyebrow-raise from my wife, and got a full on "stop what you're doing right now" response from my own father, a man with a doctorate degree in public health who did research for the NIH and CDC for many years. That thing is the assertion that it is safe to swim in water chlorinated at concentrations below SLAM level. To get into my mindset, let's break down what I learned about chlorine 15 years ago when I was getting my certificate to manage and operate a public pool:

The health department required a minimum of 1 ppm free chlorine (2 ppm when using dichlor or trichlor) with an ideal target of 3 ppm free chlorine. The maximum allowed by the health department for a sustained period while bathers were in the water was 5 ppm. If FC levels above 7 ppm were detected, the pool had to be closed to swimmers immediately. Any FC levels 10 ppm and above constituted an "immediate hazard" to the health of swimmers; if such levels were detected while swimmers were in the water, the health department had to be notified immediately (as with readings under 1 ppm). The 10 ppm upper hard limit was partially due to the fact that ph is no longer accurately measurable above 10 ppm FC, and some health departments almost treat ph with greater reverence than FC. But, it was also clearly explained that allowing bathers in water with a FC concentration above the 10 ppm mark could cause significant dermal distress or respiratory distress.​
Now of course, we're talking about a public facility with a wide range of swimmers, some of whom could have respiratory illnesses, etc. There's also the concern of lawsuits and the like. But still, when it gets beat into your head that anything above 10 ppm is an immediate hazard to health, and then you're introduced to a concept that says swimming in water with concentrations below SLAM level - even if that level may be 20+ ppm - is safe, it's a hard idea to grasp. I've looked all over this forum and haven't found anything that scientifically and concretely explains why it's safe to swim in water as long as it's below the SLAM level, and I'm hoping someone can either explain it to me or point me to a thread that does. I get that it has to do with the relationship of chlorine to CYA, but I'm interested in knowing what specifically makes (for example) an FC of 18 unsafe at CYA levels of 40, but safe at CYA levels of 50.
 

thetekgeek

Bronze Supporter
Jun 8, 2018
166
Nebraska
I don't have the answer you are looking for, but I am a sucker for threads like these because I love reading about the science behind the methods. Replying so I can check back when the experts chime in.
 
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Donldson

TFP Expert
LifeTime Supporter
In The Industry
Jun 12, 2009
3,941
NW Ohio
As we are approaching this from an industry standpoint, perhaps an article in an industry publication would be a good place to start:

Aqua Magazine: CYA and Chlorine in Plain Language

IMO, the most important quote from that article is
With a CYA:FC ratio of 20:1, the concentration of HOCl stays pretty constant at 0.02 ppm.
Which means that 20 ppm CYA and 1 ppm FC is virtually the same as 200 ppm CYA and 10 ppm FC. With 3 ppm FC and zero CYA (perfectly safe and ideal according to the CDC) a body of water with pH of 7.5 at 80 degrees F would have an HOCl of 1.451. A pool with 2.5:1 CYA:FC concentration (SLAM Level) would have roughly 0.3 ppm HOCl. So a pool at SLAM level has only 1/5th the active chlorine (HOCl) as a pool that is within CDC guidance for safe and ideal. More chlorine can be less harsh, one of the most difficult things for anybody new to TFP to buy in to until it is experienced.

I will allow the rest of the article to speak for itself. Please keep in mind that the article, while following the same science TFP has been promoting for well over 10 years, was not written by anybody at TFP and may differ in some of the conclusions.
 
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JamesW

TFP Expert
Mar 2, 2011
16,488
It really comes down to equilibrium. At any instant, most of the chlorine is combined with cyanuric acid. Only a very small amount is free hypochlorite or hypochlorous acid.

This greatly reduces the activity of the chlorine.

Also, note that some doctors suggest adding bleach to bathwater for certain skin conditions where the fc is up to about 50 ppm with no cyanuric acid to moderate the chlorine's strength.


So, you ask for science to back up the recommendations. I would ask that you post the science that backs up the recommendations that you are citing.

The problem with the government recommendations is that there is no current science that backs up their levels.

As far as I can tell, the numbers were pulled out of thin air decades ago by someone and then blindly followed by everyone else.

Are you contacting the people who write the government recommendations and challenging them to provide scientific evidence or logic that supports their recommendations?
 
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PointeTaken

Well-known member
Aug 28, 2019
57
Hoover, AL
So, you ask for science to back up the recommendations. I would ask that you post the science that backs up the recommendations that you are citing.

The problem with the government recommendations is that there is no current science that backs up their levels.

As far as I can tell, the numbers were pulled out of thin air decades ago by someone and then blindly followed by everyone else.

Are you contacting the people who write the government recommendations and challenging them to provide scientific evidence or logic that supports their recommendations?
I'm not challenging anybody or any conventions, I'm merely trying to understand the concept better because it's so counterintuitive to what I was taught. I'm not saying what I was taught was right or can even be backed by science. I wouldn't be surprised if they can't. Back then as a 22 year old I could have cared less about the science, I just wanted to get my certificate out of the way so I could get back out to the pool. As someone pushing 40, I'm very interested in the science when someone is giving me a recommendation, especially when I have a vested interest in the form of property and my family. Like I said before, I don't doubt any of the recommendations; I haven't seen any reports of people breaking out in rashes from head to toe because they swam in a pool with 15 ppm of chlorine. I've bought in to the concepts, I'm just curious in the "why" and "how" especially since it's radically different compared to what I was previously taught (and blindly accepted).
 
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PointeTaken

Well-known member
Aug 28, 2019
57
Hoover, AL
As we are approaching this from an industry standpoint, perhaps an article in an industry publication would be a good place to start:

Aqua Magazine: CYA and Chlorine in Plain Language
Thank you! That article and your comment really made me understand things a lot better. That was kind of the missing link for me in fully understanding the CYA and Chlorine relationship.
 

JamesW

TFP Expert
Mar 2, 2011
16,488
The key is the equilibrium that was figured out by O'Brien. If your dad is a chemistry expert, equilibrium should be a very familiar topic.

You, or your dad, will have to do the research as there are many references that cover the subject. See the following reference.

J. E. O'Brien, J. C. Morris and J. N. Butler, “Equilibria in Aqueous Solutions of Chlorinated Isocyanurate”, Chapter 14 in Alan J. Rubin, ed., Chemistry of Water Supply, Treatment and Distribution, 1973 Symposium, (published 1974), Ann Arbor Science Publishers, Ann Arbor, MI, pp. 333-358. ISBN 0-250-40036-7
 

frogabog

Gold Supporter
LifeTime Supporter
Jul 16, 2010
2,833
Portland, Oregon
My father had MRSA in his spine for years. He had 14 surgeries, and a hole in his spine the size of a small sweet potato. My two boys and eventually my ex husband all developed the skin lesion version of MRSA. My dad inquired to his physician about the kids when they had these infections and his Dr. told him to tell us to bathe weekly in a bath of tub water and a cup of bleach. Amongst washing all bedding and clothes in bleach.

The bleach bath is the prevention.

Anecdotal at best, but worth noting.
 
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ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,413
Northern NJ