Why 97% of Pool Chlorine Can Be Inactive (Free Chlorine vs CYA)
Many pool owners are surprised to learn that having chlorine in the water does not always mean the pool is properly sanitized.
The reason often lies in the chemical interaction between free chlorine and cyanuric acid (CYA).
Free Chlorine vs Bound Chlorine
Free chlorine (hypochlorous acid, HOCl) is the form of chlorine responsible for killing bacteria, viruses, and algae.
When cyanuric acid is present, most free chlorine forms a reversible bond with CYA. While this protects chlorine from sunlight, it significantly reduces its immediate sanitizing strength.
Under typical pool conditions, up to 97% of chlorine can remain bound at any given moment.
Why This Matters for Pool Safety
Bound chlorine is not completely inactive, but it works much more slowly than unbound free chlorine.
- Disinfection response time increases
- Bacteria and algae survive longer
- Water may appear clear but remain unsafe
The FC/CYA Ratio Rule
Effective sanitation requires maintaining free chlorine at approximately 7.5% of the cyanuric acid level.
| CYA (ppm) | Minimum Free Chlorine (ppm) |
|---|---|
| 30 | 2.2 |
| 50 | 3.8 |
| 100 | 7.5 |
| 150 | 11.3 |
If you rely on basic testing methods, read: Why Pool Test Strips Fail When Cyanuric Acid Is High
Why Accurate Testing Is Essential
Managing the FC/CYA ratio is impossible without precise measurement. Test strips and visual color-matching kits often fail to detect small but critical differences in free chlorine.
Pool professionals rely on digital photometers such as the:
Pool & Hot Tub Digital Water Tester
Accurate chlorine and CYA data allow you to maintain safe, effective, and predictable pool water conditions.
Frequently Asked Questions About Chlorine and CYA
Why is chlorine ineffective even when levels seem normal?
When cyanuric acid levels are high, most free chlorine becomes chemically bound, reducing its ability to disinfect the water quickly.
What is bound chlorine?
Bound chlorine is chlorine that has formed a reversible bond with cyanuric acid, making it far less effective for immediate sanitation.
What is the FC/CYA ratio?
The FC/CYA ratio describes the relationship between free chlorine and cyanuric acid. Effective sanitation typically requires free chlorine at about 7.5% of the CYA level.
Can test strips detect inactive chlorine?
No. Most test strips measure total chlorine and cannot determine how much chlorine is actively sanitizing the water.
