If you manage a public spa, HOA hot tub, or commercial aquatic feature, you’ve probably encountered this scenario: You can smell chlorine. But your free chlorine reading is low.
So what’s really happening?
Understanding the interplay between free chlorine, total chlorine, and pH is critical not just for water clarity — but for Legionella risk management and public health protection. In warm, aerosolized environments like spas, chemistry matters.
What Is Free Chlorine?
Free chlorine (FC) is the portion of chlorine available to actively disinfect water.
It exists primarily in two chemical forms:
• Hypochlorous acid (HOCl) – highly effective disinfectant
• Hypochlorite ion (OCl⁻) – weaker disinfectant
The balance between these two forms is controlled by pH, which we’ll discuss shortly.
For public spas, recommended operational levels are:
Free Chlorine: 2–4 ppm
Anything below this range reduces the margin of safety — particularly in warm water (80–104°F), where bacterial growth accelerates and disinfectant demand increases. If your spa is testing at 0.7 ppm free chlorine, that level is considered low for effective pathogen control.
What Is Total Chlorine?
Total chlorine (TC) equals: Free chlorine + Combined chlorine
Combined chlorine consists of chloramines, which form when chlorine reacts with:
• Sweat
• Body oils
• Urine
• Nitrogen compounds
• Organic debris
Here’s the key misconception:
A strong “chlorine smell” usually does NOT mean there is too much chlorine. It often means there is not enough free chlorine, and chloramines have accumulated.
Combined chlorine consists of chloramines, which form when chlorine reacts with:
• Sweat
• Body oils
• Urine
• Nitrogen compounds
• Organic debris
Here’s the key misconception:
A strong “chlorine smell” usually does NOT mean there is too much chlorine. It often means there is not enough free chlorine, and chloramines have accumulated.
The Chlorine Smell Explained
That sharp pool or spa odor is typically caused by chloramines — not free chlorine.
Chloramines:
• Reduce disinfection efficiency
• Cause eye and skin irritation
• Create odor
• Indicate organic contamination
• Signal increased oxidant demand
To determine combined chlorine: Total Chlorine – Free Chlorine = Combined Chlorine
Combined chlorine should ideally remain: Below 0.2 ppm
If it exceeds this level, oxidation (shock treatment) is typically required.
Why pH Directly Controls Disinfection Power
pH does not just affect comfort — it controls chlorine effectiveness. At lower pH levels, more chlorine exists as hypochlorous acid (HOCl), the stronger disinfectant. At higher pH levels, more chlorine converts to hypochlorite ion (OCl⁻), which is significantly less effective.
For spas:
Ideal pH Range: 7.2–7.6
Acceptable range: 7.2–7.8
At pH 7.2, chlorine efficiency is relatively high.
At pH 8.0, chlorine effectiveness drops dramatically.
This means: Even if chlorine levels appear adequate, elevated pH can reduce microbial kill efficiency — including against Legionella.
Why This Matters for Legionella Risk
Warm water spas create ideal growth conditions for Legionella bacteria:
• Temperatures between 77–113°F
• Aerosolization from jets
• Biofilm formation in piping
• High organic loading
Legionella control depends on:
• Adequate disinfectant residual
• Proper pH balance
• Circulation and filtration
• Biofilm control
• Routine monitoring
Low free chlorine in warm water increases vulnerability — even if the water looks clear. Maintaining proper chemistry is one of the primary control measures in Legionella prevention programs.
How to Correct Chloramine Buildup
If you smell chlorine and free chlorine is low, the spa may require oxidation. Two common options:
1. Chlorine Shock (Superchlorination)
Raises free chlorine temporarily to destroy chloramines through breakpoint chlorination.
After treatment:
• Retest free chlorine
• Confirm combined chlorine is below 0.2 ppm
• Adjust pH if necessary
• Allow levels to return to operational range before reopening
2. Non-Chlorine Shock (Potassium Monopersulfate)
Oxidizes organics without significantly raising chlorine levels.
Useful for maintenance but does not replace proper disinfectant residual.
Best Practice Monitoring for Public Spas
For HOA and commercial facilities:
• Test free chlorine daily
• Test total chlorine regularly
• Calculate combined chlorine
• Shock if combined chlorine exceeds 0.2 ppm
• Maintain pH between 7.2–7.6
• Document readings
• Perform periodic microbiological testing where warranted
For line clearance verification or post-maintenance checks, free chlorine is the primary compliance parameter — but total chlorine reveals overall water quality performance.
Final Thoughts: Chemistry Is Risk Management
Free chlorine disinfects.
Total chlorine tells you what has already reacted.
pH determines disinfectant power.
When all three are aligned, spas remain safe, clear, and compliant. When they drift, odor, irritation, and microbial risk follow.
For HOA communities and public facilities, proper water chemistry is not just maintenance — it is public health protection.