Combined chlorine (CC) forms when free chlorine reacts with nitrogen compounds from bather waste. Once formed, combined chlorine is a weak disinfectant and a strong irritant. Eliminating it requires breakpoint chlorination.
Key Facts
- Combined chlorine above 0.5 ppm is the action threshold — shock treatment is needed.
- The "pool smell" is chloramines, not free chlorine — the problem is too little effective chlorine, not too much.
- Combined chlorine is calculated as: CC = Total Chlorine minus Free Chlorine.
- Shocking to breakpoint destroys combined chlorine by converting it back to inert compounds.
How Combined Chlorine Forms
Every bather who enters a pool introduces nitrogen-containing compounds: ammonia in sweat and urine, amino acids from skin, and nitrogenous compounds in sunscreen and cosmetics. Free chlorine reacts with these compounds to form chloramines (combined chlorine). The most common pool chloramine is monochloramine (NH2Cl), followed by dichloramine (NHCl2) and nitrogen trichloride (NCl3). All are far less effective as disinfectants than free chlorine, and dichloramine and nitrogen trichloride are the primary sources of the irritating "pool smell."
Measuring Combined Chlorine
Combined chlorine is not measured directly — it is calculated. A DPD-1 test measures free chlorine; a DPD-3 test (or using DPD-1 plus DPD-3 reagent) measures total chlorine. Combined chlorine equals total chlorine minus free chlorine. If your free chlorine reads 2.0 ppm and your total chlorine reads 2.7 ppm, combined chlorine is 0.7 ppm — above the 0.5 ppm threshold that indicates a shock is needed. Test both free and total chlorine at least once a week during peak swimming season.
Eliminating Combined Chlorine
The only way to destroy combined chlorine in a pool is breakpoint chlorination — adding enough free chlorine to drive the total to at least 10 times the combined chlorine level. At this concentration, chloramines are chemically destroyed. Once breakpoint is reached, combined chlorine drops to near zero and the pool smells clean again. Shock treatments are formulated to reach breakpoint. Always shock in the evening to prevent UV from destroying the concentrated chlorine before it can do its work.
Examples
After a pool party, the water smells strongly and guests report eye irritation. A test shows FC 2.0 ppm and TC 3.2 ppm, giving CC of 1.2 ppm — well above the 0.5 ppm threshold. The shock dose needed is at least 10 x 1.2 = 12 ppm of free chlorine added to the pool. Using the pool shock calculator for a 15,000-gallon pool, that translates to approximately 6 lbs of calcium hypochlorite shock added after dark.
Common Mistakes to Avoid
- Reducing chlorine when the pool smells bad — the problem is combined chlorine (too little active chlorine), not too much.
- Only measuring free chlorine and never measuring total chlorine, so combined chlorine goes undetected.
- Shocking during the day, when sunlight destroys the high chlorine dose before it can reach breakpoint.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- CDC — Healthy Swimming Guidelines