If you’ve ever noticed a strong “chlorine smell” around a pool or swimmers complaining about burning eyes, you’re likely dealing with combined chlorine, not free chlorine. Understanding what combined chlorine is, how it forms, and how to manage it effectively is essential for keeping pool and spa water safe, clear, and comfortable.

What Is Combined Chlorine?

Combined chlorine (CC) refers to chlorine that has reacted with nitrogen-based contaminants in the water, such as sweat, urine, body oils, or ammonia. When free chlorine combines with these substances, it forms chloramines — the main component of combined chlorine.

There are three types of chloramines:

• Monochloramine

• Dichloramine

• Trichloramine Each is progressively more irritating and odorous, with trichloramine being the most pungent and volatile: It’s what causes that “chlorine” smell in indoor pools and leads to eye and respiratory irritation.

While free chlorine sanitizes water, combined chlorine is a sign that chlorine is being consumed by contaminants. High levels of combined chlorine indicate that the water is struggling to keep up with the bather load and organic waste.

How Combined Chlorine Forms

Combined chlorine forms when:

• Swimmers introduce nitrogen into the water (via urine, sweat, etc.).

• Chlorine oxidizes these nitrogen compounds.

• The resulting chemical reaction forms chloramines instead of completely breaking down the contaminants.

The more contaminants introduced into the water without sufficient free chlorine to oxidize them, the more combined chlorine accumulates.

Measuring Combined Chlorine

Combined chlorine is not measured directly. Instead, it's calculated: Combined Chlorine = Total Chlorine − Free Chlorine A FAS-DPD test kit (like the Taylor K-2006) is the gold standard for accurately measuring free and total chlorine, allowing pool professionals to determine the level of combined chlorine present.

In general, as per the 2023 MAHC:

• CC should be ≤ 0.2 ppm in properly maintained pools.

• Anything above 0.4 ppm needs immediate attention.

How to Remove Combined Chlorine

The most popularly recommended method to eliminate combined chlorine is breakpoint chlorination — a process that adds a high enough dose of chlorine to completely oxidize chloramines and their precursors.

The Certified Pool Operator (CPO) Handbook, one of the most widely used training manuals in the industry, prescribes the 10x rule as a practical guideline for achieving breakpoint chlorination: “To reach breakpoint chlorination and eliminate combined chlorine, you must add free chlorine equal to 10 times the measured combined chlorine level.”

Using the Breakpoint Chlorination Formula: Breakpoint Dose ≈ 10 × Combined Chlorine So, if your pool has 0.6 ppm combined chlorine, you’ll need about 6 ppm of additional free chlorine to reach breakpoint and eliminate it.

Tips for breakpoint chlorination:

• Use liquid chlorine (sodium hypochlorite) or calcium hypochlorite to reach the target.

• Do it during low-use hours, ideally overnight.

• Circulate water thoroughly for uniform distribution.

• Retest after several hours to confirm success.

Other combined chlorine management strategies include:

• UV or ozone systems, which help oxidize chloramines without chemical addition.

• Good hygiene practices, such as requiring pre-swim showers.

• Dilution via partial draining and refilling.

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