Chlorine harshness and Cyanuric Acid


Active member
Apr 20, 2014
I’ve heard that Cyanuric Acid reduces the harshness of chlorine in water (from what I read, it’s also called CYA, stabilizer, conditioner, isocyanurate).

Let’s say that I kept the Free Chlorine level at 10% of the Cyanuric Acid level.

For example, 3 ppm Free Chlorine for 30 ppm of Cyanuric Acid.

At what point would the harshness of the 10% match the harshness of:

1 ppm Free Chlorine with 0 ppm Cyanuric Acid?

2 ppm Free Chlorine with 0 ppm Cyanuric Acid?

3 ppm Free Chlorine with 0 ppm Cyanuric Acid?

4 ppm Free Chlorine with 0 ppm Cyanuric Acid?

5 ppm Free Chlorine with 0 ppm Cyanuric Acid?

For example:

100 ppm Free Chlorine with 1,000 ppm Cyanuric Acid?

300 ppm Free Chlorine with 3,000 ppm Cyanuric Acid?



Mod Squad
LifeTime Supporter
TFP Expert
May 19, 2010
Tucson, AZ
Well, first, I think things fall apart a little bit when the levels get ridiculously high like your last couple questions, so I am going to ignore those since they are irrelevant to pools.

You are going to want to take a look at this chart which shows the "active" chlorine level (HOCl or ppm Cl2) as a function of CYA and FC levels. This "active" level is like the harshness.

So, for your example of FC of 3ppm with a CYA of 30ppm (which is a little lower FC level than you would want to maintain BTW according to the FC/CYA Chart), according to the table the active level is 0.042 ppm Cl2.

Now looking at the top row where the CYA is 0, you see that for a FC level 0.1ppm, the active level is still slightly higher and more harsh at 0.048 ppm Cl2.

The problems arise that: 1. It is hard to measure the FC levels that low and 2. Those low FC levels are difficult to maintain since it can be consumed VERY quickly.

Another data point: a FC of 3ppm with 0 CYA (reasonable for a public pool) has an active level of 1.451 ppm Cl2. That is almost 35 times more "harsh" than if you added 30ppm of CYA.

This is why we always recommend CYA for indoor pools as well even though loss to the sun is not a concern.

chem geek

LifeTime Supporter
TFP Expert
Mar 28, 2007
San Rafael, CA USA
All of your examples without CYA are far, far harsher than a 10% FC/CYA ratio. With a 10% FC/CYA ratio the active chlorine level is the same as roughly 0.1 ppm FC with no CYA. So even your 1 ppm FC with no CYA is roughly 10 times harsher. By "harsher", we mean the rate at which chlorine will oxidize swimsuits, skin, hair, pool covers, etc.