Combined Chlorine (Chloramines)
What is Combined Chlorine (Chloramines)?
Combined chlorine is the portion of available chlorine that has already reacted with ammonia and nitrogen compounds and now sits in the water as chloramines. The 2023 CDC Model Aquatic Health Code defines "Combined Available Chlorine" or "Combined Chlorine" as "the concentration of available chlorine present in the form of inorganic or organic chloramines," notes that it "is less reactive than free available chlorine," and gives the arithmetic as the difference between total chlorine and DPD free chlorine, CC = TC minus DPD-FC (2023 MAHC section 2.0, Glossary of Terms). It is spent sanitizer rather than sanitizing capacity, and it is the usual source of the chlorine smell and eye sting that swimmers blame on too much chlorine. MAHC 5.7.4.4.2A has the operator take action to reduce combined chlorine when the level exceeds 0.4 ppm.
Combined chlorine is a subtraction, not a test of its own. You read total chlorine and free chlorine, and the gap between them is the combined figure, which is why a log sheet with a single “chlorine” column can never show it, and why the commercial pool daily log sheet keeps free and combined in separate columns. On a route the number works as a diagnostic: free chlorine can sit inside target while combined chlorine climbs, and the water still smells wrong and stings. A rising figure usually means bather load or organic material has outrun the feed rate.
The model code treats it as an action item rather than a nuisance. MAHC 5.7.4.4.2A has the operator act once combined chlorine passes 0.4 ppm and lists superchlorination, water exchange and bather hygiene among the options without prescribing one, and 5.7.5.1A puts combined available chlorine in the set tested before opening each day. The MAHC is a model, so the code your health department has adopted is the one that binds the site.
What to do about the reading is a dose question, and doses belong on the tools: the breakpoint chlorination calculator and the pool chlorine dose calculator. Targets and test order sit on the pool water chemistry guide.