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 immune systems.

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 superchlorinated.

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 trichloride.”

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 pools studied.

“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,” Donohoe said.

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 tablets.

“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 generator.”

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.