Maintaining proper disinfection levels is the core of pool and spa maintenance.

And while a pool and spa pump is often described as the heart of the pool’s circulation system, extending the metaphor, we might think of chemical sanitation as a pool and spa’s immune system.

Because whether it's a backyard pool, hotel spa, splash pad, or public water park, the sanitation system really acts like the facility’s immune system — working around the clock to protect bathers from harmful pathogens.

And despite the many sanitation options available today, chlorine remains the most widely used and trusted disinfectant. Yet keeping chlorine levels stable, effective, and safe can be more complex than it first appears, especially when other variables — like cyanuric acid (CYA), pH, bather load, sunlight, and temperature — come into play.

This issue of Service Industry News outlines best practices for maintaining proper disinfection levels through the use of traditional sanitizers and explores the use of secondary sanitation systems such as UV, ozone, and advanced oxidation processes.

The primary goal of sanitation is to inactivate or kill harmful microorganisms like E. coli, Giardia, Pseudomonas aeruginosa, Legionella, Cryptosporidium, and others. In recreational water environments, these pathogens can spread quickly in improperly maintained water, leading to recreational water illnesses (RWIs) such as diarrhea, skin rashes, ear infections, and respiratory problems.

To prevent RWIs, disinfectant levels must be sufficient not just to kill microbes on contact but also to provide a stable residual that can handle new bather loads and contaminants as they enter the water. This is true for chlorine, bromine, and alternative systems like salt chlorine generators, UV, and ozone.

Let’s start with chlorine because it remains the primary sanitizer used in most commercial and residential settings.

Recommended Free Chlorine Levels

For most swimming pools and spas, the CDC’s Model Aquatic Health Code (MAHC) and local health departments recommend the following free chlorine (FC) levels:

• Pools without cyanuric acid (CYA): 1.0 – 3.0 ppm.

• Pools with cyanuric acid (CYA): 2.0 – 4.0 ppm (minimum, depending on CYA concentration).

• Spas and hot tubs: 3.0 – 5.0 ppm. These values assume properly balanced pH, with most recommendations targeting a range between 7.2 and 7.8. It’s important to note that while maintaining these chlorine levels is essential, they are only effective if the sanitizer is active, meaning it' s not tied up or weakened by other water chemistry factors. Chlorine and Cyanuric Acid

Cyanuric acid (CYA), also known as stabilizer or conditioner, is widely used in outdoor pools to protect chlorine from degradation by the sun’s ultraviolet rays. When used correctly, CYA can extend chlorine life and reduce chemical consumption. But as the industry has learned over the past several decades, there's a tradeoff.

When CYA is present in the water, it bonds with free chlorine, forming a chlorinated cyanurate compound. This compound is far more stable in sunlight (and also less reactive with contaminants) and can be thought of as chlorine held in a “bank,” as a reserve. The downside is that only a fraction of the total chlorine remains in its active, free state as hypochlorous acid (HOCl), the form responsible for killing bacteria and oxidizing contaminants. The upside is that as the hypochlorous acid is used up in the disinfection and oxidation of pool and spa water, hypochlorous acid continues to be released from the cyanuric acid’s “bank” of chlorine where it is held in reserve.

*Note: Free chlorine is not the same as hypochlorous acid. Free chlorine is what we measure with pool and spa test kits. Hypochlorous acid cannot currently be measured directly with pool and spa test kits. However, it is the hypochlorous acid that is directly responsible for sanitation. While hypochlorous acid concentrations cannot be measured in a swimming pool, they can be calculated from the measurements of free chlorine, cyanuric acid, and pH. In pools containing cyanuric acid, the hypochlorous acid concentration is significantly affected by it, while also affected by the pH to a lesser extent.

That’s why to maintain effective sanitation in pools with CYA, pool operators must maintain an appropriate ratio of free chlorine to cyanuric acid.

Further complicating matters, there is some disagreement about what this ratio should be.

The accompanying table shows the calculated hypochlorous acid levels as a function of both the free chlorine and cyanuric acid levels at an average pool pH level (pH = 7.5) and temperatures (Temp = 80 °F).

The colored numbers are calculated hypochlorous acid levels.

The numbers marked red show hypochlorous acid levels that may be too low to be useful in combatting algae and other contaminants.

The numbers marked in blue show hypochlorous acid levels that are probably overkill in combatting algae and other contaminants.

The numbers marked green represent the sweet spot for maintaining swimming pools and spas.

For each colored number (hypochlorous acid concentration), look left to find the corresponding cyanuric acid level, and up to find the corresponding free chlorine level.

These calculated hypochlorous acid concentrations are accurate regardless of whichever chlorine to cyanuric acid ratio you might subscribe to.

The ORP Rule: 650 mV

In a nutshell, this recommendation says the free chlorine should be between 1 to 10 percent of the cyanuric acid concentration.

The World Health Organization (WHO) and the CDC have determined that a minimum of 650 mV (ORP) is required for instantaneous inactivation of most pathogens. That corresponds to about 0.01 ppm hypochlorous acid. Meanwhile, some studies have shown that 0.05 ppm hypochlorous acid kills most algae. So basically, we want a hypochlorous acid concentration between 0.01 and 0.05 ppm.

Next, we need to figure out how that range corresponds to our measured chlorine and cyanuric acid. Unfortunately, the exact math to get there is tricky, and involves solving a bunch of equilibrium equations.

Fortunately, someone else has done the work for us.

In the early 2000s, water chemistry expert Richard Falk solved those equations to determine the amount of cyanuric acid and chlorine that taken together result in a given hypochlorous acid concentration. See accompanying graphic.

To obtain a hypochlorous acid concentration of between 0.01 and 0.05 ppm, the free chlorine should be between 1 to 10 percent of the cyanuric acid concentration.

MAHC Rule:

As of the 2023 edition, the Model Aquatic Health Code (MAHC) states that CYA should not exceed 45 times the amount of FAC, and sets an absolute maximum of 300 ppm CYA, at which point remediation is required.

For consistency, this corresponds to maintaining the chlorine at 2.2% of the cyanuric acid level.

So, if CYA is 90 ppm, free chlorine should be at least 2.0 ppm (because 90 × 2.2% ≈ 2.0).

Falling below this ratio is considered an “imminent health hazard” under the MAHC.

The CDC and technical committees behind the MAHC aimed to set a limit where there would still be a detectable and minimally effective concentration of hypochlorous acid (HOCl) — the active disinfectant form of chlorine — even in the presence of CYA.

• When CYA is present, most chlorine binds to it, forming lesseffective chlorinated isocyanurates.

• Only a very small fraction of FAC remains as hypochlorous acid.

• At a 45:1 ratio, the CDC deemed that the remaining HOCl was just enough to satisfy a baseline level of disinfection, sufficient for nonoutbreak conditions. This decision was influenced by:

• Modeling studies on HOCl availability at different CYA: chlorine ratios.

• Recognition that public pool operators often already used stabilized chlorine and had relatively high CYA levels.

• The need to define a clear, enforceable threshold (i.e., something youcantestandciteduringinspections).

Our Favorite: The 7.5% Rule: To control green algae, (and most other pathogens and contaminants) the most popular recommendation is to maintain the free chlorine levels at 7.5% of the CYA level to ensure a sufficient concentration of active hypochlorous acid.

Example: If CYA = 40 ppm Then Free Chlorine should be: 0.075 × 40 = 3.0 ppm At this ratio, the pool maintains a reasonable level of disinfecting power while still benefiting from UV protection.

At a 7.5% FAC-to-CYA ratio, the amount of hypochlorous acid (the active disinfecting form of chlorine) is comparable to what you'd have in a pool with 1–3 ppm chlorine and no CYA. (See accompanying table).

Risks of High CYA

When CYA levels get too high, several problems can occur:

• Reduced chlorine effectiveness: Sanitizer loses power to kill pathogens, especially fast-spreading viruses and parasites.

• Increased risk of biofilm formation: Microbial colonies can thrive on surfaces where chlorine is rendered less effective.

•Chlorinedemandskyrockets:More chlorine is needed to maintain even a marginally effective FC:CYA ratio.

• Algae outbreaks: Persistent, unexplained algae growth is often tied to over-stabilized water.

Best Practices for Managing Chlorine and CYA

Test Frequently Regular testing of both free chlorine and CYA levels is essential. CYA should be tested at least weekly in outdoor pools, or more often if using stabilized chlorine. Free chlorine should be checked daily, and more frequently in commercial or high-bather-load environments.

Maintain the Proper Ratio

Strive for a free chlorine level that is 7.5% of the CYA level. In public pools, where safety and liability are paramount, aim for even higher ratios.

Avoid Over-Stabilizing

Keep CYA levels between 30 and 50 ppm and avoid exceeding 60 ppm unless dictated by special circumstances (e.g., extreme sun exposure and heavy use).

Use Non-Stabilized Chlorine

In pools already high in CYA, switch to sodium hypochlorite (liquid bleach), calcium hypochlorite, or a salt chlorine generator to avoid adding more stabilizer.

Dilute High CYA Water

If CYA has built up over time, among the most practical methods is to lower it is by draining and diluting the pool water. Partial water replacement is often the most economical solution.

Educate Clients, Operators

Many pool owners and even some professionals misunderstand the role of CYA. Help your team and customers understand CYA improves compliance and reduces complaints.

Indoor Pools and Spas

Many experts say indoor pools generally do not require CYA, as there’s no UV exposure to degrade chlorine. They believe adding CYA in these environments reduces chlorine’s effectiveness unnecessarily and is generally discouraged.

That said, other experts nonetheless recommend using cyanuric acid even in indoor pools because it changes the concentrations of various types of harmful disinfection byproducts and because the chlorine is less damaging to bathers’ swimsuits.

Spas, meanwhile, whether indoor or outdoor, often operate with bromine or chlorine without CYA, relying on high turnover and temperature to maintain clarity and sanitation.

Conclusion

Maintaining proper chlorine levels is both an art and a science. It requires not only frequent testing and diligent adjustment but also a deep understanding of the chemical relationships at play.

The chlorine-to-CYA ratio is among the most important — and frequently overlooked — factors affecting sanitation in outdoor pools.