Everyone agrees that adding cyanuric acid to a pool extends the life of the chlorine by protecting it from damage by the sun’s UV rays. Without the addition of cyanuric acid, chlorine concentration is reduced by 90 to 95 percent in two hours in shallow water. At 4.5 feet, the loss after two hours is about 75 percent. Those figures can vary depending on pH and water temperature.
But there are other areas where experts don’t see eye to eye.
CYA works by bonding loosely with the chlorine ions in the pool
water. This bond makes the chlorine less susceptible to being
damaged by UV rays.
However, it also makes the chlorine a bit less effective and
that degradation increases with the concentration of cyanuric acid
in the pool.
The ideal level of cyanuric acid in a pool is between 30 ppm and
50 ppm, with the stabilization effect of the cyanuric acid leveling
off at levels above 50 ppm. The upper limit of cyanuric acid
allowed in commercial pools by most health departments is 100 ppm.
There is no legal standard for its use in residential pools.
The amount of cyanuric acid to be added, and whether it should
be used in indoor pools are questions on which many pool and
aquatics experts don’t agree.
Some professionals say that since indoor pools are not directly
exposed to the sun’s rays, use of cyanuric acid is not
recommended. The concern is that the addition of cyanuric acid to
pool water increases the time required to render certain bacteria,
algae and viruses inactive.
“As soon as you add cyanuric acid, it significantly slows
down the kill rate of the pathogens,” says Steve Donohoe, a
trainer for the National Swimming Pool Foundation based in Colorado
Springs, Colo. “The Centers for Disease Control is very
concerned about one specific pathogen, and that’s
cryptosporidium. We don’t have enough research to know what
the kill rate is. With no cyanuric acid, it takes 12 hours at 20
parts per million. When you introduce cyanuric acid, it extends the
kill rate way out.”
Cryptosporidium is a parasite that is commonly transmitted
through recreational water. Ingesting it can cause diarrhea,
cramping, nausea and fever. It can be fatal in those with weakened
One of the largest outbreaks occurred in 2011 in the Cincinnati/
Northern Kentucky area. Health officials confirmed more than 250
cases of crypto, along with a larger-than-normal number of Shigella
cases (nearly 130). More than a dozen commercial pools were
Yet there are reasons why operators of indoor pools might want
to continue using cyanuric acid.
One is that some indoor pools are still exposed to UV rays. Many
natatoriums have large windows and unless the glass is specifically
made to block UV rays, the light entering the pool area is still
capable of degrading the chlorine in the water.
But that’s not the only rationale. “The primary
reason for using a small amount, say 20 ppm of cyanuric acid, in
indoor pools is to be able to keep a lower active chlorine
concentration with an ample chlorine reserve,” says Robert
Lowry, principal of Lowry Consulting in Jasper, Ga.
Lowry, who has written a number of training manuals on pool
chemistry, adds that, “A 4 ppm free chlorine with 20 ppm
cyanuric acid is equivalent to about 0.2 ppm free chlorine with no
cyanuric acid at 77 degrees F. This equivalence increases with
temperature. Reducing the free chlorine concentration decreases
harshness on swimsuits, skin and hair and produces less nitrogen
Nitrogen trichloride is a chloramine to which some people are
sensitive. It can exist in the air above an indoor pool.
A study performed by the public health unit in Pinellas County,
Fla. on commercial swimming pools showed that the major factor for
determining whether a pool was healthy enough to swim in was the
amount free chlorine present in the pool. The amount of cyanuric
acid did not significantly affect the amount of bacteria in the
“If you’re not really concerned about
cryptosporidium, go ahead and use a little bit of cyanuric acid in
indoor pools. We do know that for pretty much everything except
crypto, it doesn’t take much chlorine to kill it and adding
cyanuric acid is good, because it keeps chlorine in the pool. The
danger always comes when you don’t have chlorine,”
One method of chlorinating smaller commercial pools, such as
those in apartment complexes and motels, is to use granules or
tablets of trichlorisocyanuric acid, commonly known as trichlor.
Trichlor is a combination of chlorine and cyanuric acid. The
tablets are inserted into a feeder which adds the chemical to pool
water. That makes it convenient for sites without a dedicated pool
service, but it can cause cyanuric acid levels to increase to the
point where it’s harder for chlorine to do its job. Almost 60
percent of the pools in the Pinellas County study used trichlor
“Higher CYA requires higher FC, but not many people are
aware of this,” Lowry said. “We need to make people
aware of the free chlorine-cyanuric acid relationship and the need
for free chlorine to be 7.5 to 10 percent of cyanuric acid. And in
order for the free chlorine level not to be too high, we need to
keep the cyanuric acid level no more than 50 ppm or perhaps 80 ppm
for very sunny areas like Arizona or pools with a chlorine
Cyanuric acid does not evaporate from a pool, and the only way to
greatly lower a pool’s concentration of CYA is to partly
drain the pool and refill it with fresh water.
For Donohoe, it comes down to how the pool is used. “How
many people are you going to have in your pool and how are you
going to keep chlorine in there?” he asked. “Adding
cyanuric acid slows down the kill rate of all pathogens. So if you
have a heavy bather load, you don’t want cyanuric acid in
that water. You want chlorine to kill the pathogens. In most
commercial pools, if they’re heavily used, even if
they’re outside, you don’t want cyanuric acid.”
Pools such as those are typically on a chlorine feeder.
So, there’s no clear consensus on whether to use cyanuric
acid. It boils down to one thing, however: “Just make sure
the pool’s healthy,” Donohoe said. That’s
something everyone can agree on.