Did you know that cyanuric acid is the reason why once-weekly pool service is possible?

It’s true. Without cyanuric acid, chlorine — that most essential pool chemical — would simply disappear within hours.

And we need chlorine, or some kind of disinfectant, to kill pathogens and algae.

Chlorine is an excellent disinfectant, but its one great weakness is that it is sensitive to the sun: Specifically, UV light, which breaks it down and renders it useless.

If chlorine alone is added to pool water, 90-95 percent will be lost within two hours from direct exposure to sunlight. Within four hours, all chlorine could be gone.

But cyanuric acid is like sunscreen for chlorine. It protects chlorine from degradation.

Without cyanuric acid, weekly pool service wouldn’t be feasible, because by the time the service pro returned, the pool would likely be overrun with algae.

Cyanuric acid is a molecule with a ring structure that contains three nitrogen atoms. These nitrogen atoms are capable of loosely binding with chlorine atoms. And in that form, it is resistant to degradation from the sun. There are three different ways that cyanuric acid can be added to swimming pool water.

The first way is to simply add cyanuric acid, which is available in 100-percent granular form: 2.5 pounds of granular cyanuric acid in 10,000 gallons of water equals 30 ppm.

Why is that true? To find out how much cyanuric acid is added to the pool, use the simple formula:

(Cyanuric Acid Content)÷ (Pool Volume x 8.34*) x 1,000,000 = Cyanuric Acid Addition (ppm)

*Note: A gallon of water weighs 8.34 pounds.

Substituting the values from the above example: (2.5 pounds) ÷( 10,000 x 8.34) x 1,000,000 = 30 ppm The other two ways are to add chlorinated isocyanurates, commonly known in the pool industry as trichlor and dichlor, which come as tabs or granular powder.

Trichlor tabs typically contain 54 percent cyanuric acid. The most common tabs are either 1 inch or 3-inch slow-release tabs.

A 3-inch tab weighs about 8 ounces, or 0.5 pounds because there are 16 ounces in a pound. Of that, 0.27 pounds is cyanuric acid (54% x 0.5 pounds = 0.27 pounds).

So how much cyanuric acid is added to 10,000 gallons every time we add a 3-inch tab? It’s about 3 ppm: (0.27 pounds) ÷( 10,000 x 8.34*) x 1,000,000 = 3.24 ppm We can do the same math for dichlor.

Common usage instructions for dichlor shock use recommend adding 1 pound of dichlor to 12,000 gallons of water. Dichlor contains about 57 percent cyanuric acid, which means that 0.57 pounds of that is cyanuric acid (57% x 1 pound = 0.57) so: (0.57 pounds) ÷( 12,000 x 8.34) x 1,000,000 = 5.6 ppm There are a lot of interesting things to know about cyanuric acid, and one of those things is that it is stable. It doesn’t tend to go away once it has been added. That means that if it is continually added to a swimming pool, through regular use of trichlor or dichlor, it can build up over time.

And that matters. That’s because while a little cyanuric acid is a good thing, a lot of cyanuric acid is not. The reason is that while chlorine is a rapid disinfectant and oxidant, it gets slowed down when a lot of cyanuric acid is around. And the more cyanuric acid there is, the slower that chlorine gets. At a certain quantity of cyanuric acid, the same amount of chlorine is performing so slowly that algae and other pathogens can become capable of reproducing faster than the chlorine can kill it.

The Pool and Hot Tub Alliance recommends maintaining cyanuric acid levels in swimming pools between 30 and 50 ppm. The 2024 Model Aquatic Health Code (MAHC) states that cyanuric acid should not exceed 90 ppm.

One of the biggest revelations of recent years is the understanding that the concentration of the killing form of chlorine is actually controlled by how much cyanuric acid is in the water.

In other words, the killing form of chlorine is proportional to the cyanuric acid concentration.

That means that if you have a lot of cyanuric acid in the water, you’ll also need a lot of chlorine to prevent algae, and that’s just a waste. It would be simpler and less wasteful to have less cyanuric acid in the pool, and then you won’t need to add so much chlorine to get the same effect.

Experts have found that keeping free chlorine at 7.5 percent of the cyanuric acid concentration is generally adequate to protect most pools from green algae.

Chlorine = 7.5 × Cyanuric Acid = Kills Green Algae Yellow algae has a higher resistance to chlorine compared to green algae, making it more difficult to eliminate with standard sanitation levels. Experts says you’ll need more chlorine — about 15 percent of the cyanuric acid level. And you’ll also want to lower phosphate levels because phosphates are algae food.

The current Model Aquatic Health Code states that aquatic venues that are using cyanuric acid, which is most outdoor venues, must maintain a minimum DPD free chlorine level of 2 ppm.

To prevent green algae, that corresponds to a maximum cyanuric acid level of nearly 30 ppm.

Meanwhile, the MAHC does not specify a maximum level for chlorine, and instead refers users to the manufacturer’s label, where the maximum level is determined by rules set by the EPA.

Generally, product labels state chlorine should be between 1 and 4 ppm.

The accompanying table may help service pros prevent green algae at various cyanuric acid levels.

As you can see, when cyanuric acid levels exceed the recommended levels (between 30 and 50 ppm), chlorine levels that exceed EPA rules may be needed to prevent algae.

But if you keep your cyanuric acid levels in the target range, you won’t need to add excessive chlorine to maintain proper sanitation and prevent algae growth, keeping the pool water clear for swimmers.