Combined chlorine is chlorine that has already reacted with ammonia, sweat, urine, or other nitrogen-containing contaminants in water. It forms compounds called chloramines, which are far less effective at sanitizing than the free chlorine you originally added. Combined chlorine is also responsible for the harsh “pool smell” most people mistakenly attribute to too much chlorine, when in reality it signals the opposite: the sanitizer has been partially used up.
How Combined Chlorine Forms
When you add chlorine to water, it produces hypochlorous acid, the active disinfecting agent. This is called free chlorine. As free chlorine encounters nitrogen-based contaminants (ammonia from sweat, urine, body oils, sunscreen, or environmental debris), it reacts with them and transforms into chloramines. There are three types, formed in sequence as more chlorine reacts with ammonia: monochloramine, dichloramine, and trichloramine.
Monochloramine still has some mild sanitizing ability, but dichloramine and trichloramine are essentially spent. Trichloramine is the most problematic because it’s a volatile gas. It escapes from the water surface and lingers in the air, particularly in indoor pools where ventilation is poor. Organic nitrogen compounds like amino acids can also react with chlorine to form organic chloramines, adding to the total combined chlorine reading.
Free, Combined, and Total Chlorine
These three measurements are connected by a simple equation: total chlorine equals free chlorine plus combined chlorine. Free chlorine is the portion still available to kill bacteria and oxidize contaminants. Combined chlorine is the portion that has already been consumed. Total chlorine is everything together. If your total chlorine reads higher than your free chlorine, the difference is combined chlorine sitting in the water doing very little useful work.
For example, if you test 3.0 ppm total chlorine and 2.5 ppm free chlorine, your combined chlorine level is 0.5 ppm. That number tells you how much chlorine has been “locked up” by contaminants.
Why It Matters for Health
Combined chlorine is more than a water chemistry nuisance. Chloramines in pool water irritate the skin, eyes, and respiratory tract. The effects are worse when chloramines off-gas into the air above the water’s surface. Because these gases are heavier than air, they settle right at the waterline where swimmers breathe, and they accumulate in enclosed spaces. Indoor pool facilities with poor air handling can develop concentrations high enough to cause coughing, nasal irritation, and red, stinging eyes in both swimmers and spectators standing nearby.
The Pool Water Treatment Advisory Group (PWTAG) recommends that combined chlorine should not exceed 1.0 mg/L (ppm). Readings at or above that level indicate a problem requiring immediate investigation and corrective action. High combined chlorine in drinking water systems is less common because utilities carefully control the chloramine formation process, but in pools and spas, bather load makes the chemistry far less predictable.
How to Test for Combined Chlorine
The standard method uses DPD (diethyl-p-phenylenediamine) reagents in a two-step process. The first reagent reacts only with free chlorine, turning the water sample a magenta or pink color. You read that color intensity against a scale to get your free chlorine value. Then a second reagent containing potassium iodide is added to the same sample. This allows chloramines to also react with the indicator dye, deepening the color. The second reading gives you total chlorine. Subtract free from total, and the remainder is your combined chlorine level.
Most pool test kits and digital colorimeters follow this same sequence. Test strips that measure both free and total chlorine let you calculate the difference, though liquid reagent kits tend to be more precise. Testing regularly is the only reliable way to catch rising combined chlorine levels before they become a problem, since you can’t judge chloramine concentration by smell alone.
How to Lower Combined Chlorine
The primary method for eliminating combined chlorine is breakpoint chlorination, commonly called “shocking” the pool. This involves adding enough free chlorine to chemically destroy the chloramines through oxidation. The required dose is roughly 10 times the current combined chlorine level. So if your combined chlorine reads 0.8 ppm, you need to raise the free chlorine by about 8 ppm to reach the breakpoint where chloramines are fully broken apart.
The chemistry behind this ratio comes from the chlorine-to-ammonia reaction. It takes a 7.6-to-1 ratio of chlorine atoms to ammonia atoms to reach the breakpoint, and since other contaminants like bacteria and algae also consume chlorine during the process, the practical recommendation rounds up to 10-to-1. Adding less than this amount can actually increase dichloramine and trichloramine production temporarily, making the smell and irritation worse before it gets better. You need to push past the breakpoint completely.
Dilution is the other effective strategy. Draining a portion of the pool water and replacing it with fresh water physically removes chloramines and the nitrogen-based contaminants that created them. PWTAG recommends this approach when combined chlorine levels remain stubbornly high, particularly when the source is organic chloramines that resist breakpoint oxidation. In practice, most pool operators use a combination of regular shocking and periodic water replacement to keep combined chlorine well under 1.0 ppm.
Preventing Chloramine Buildup
Since combined chlorine forms when free chlorine meets nitrogen-based contaminants, reducing those contaminants is the most direct prevention. Showering before entering a pool removes sweat, body oils, and residual cosmetics that react with chlorine. Keeping young children on regular bathroom breaks and responding immediately to fecal incidents reduces ammonia and urea in the water.
Maintaining adequate free chlorine levels at all times helps too. When free chlorine drops low relative to bather load, the ratio of combined to free chlorine climbs quickly. Pools with high traffic, warm temperatures, or outdoor exposure to organic debris need more aggressive chlorine management and more frequent testing. Good ventilation in indoor facilities won’t prevent chloramines from forming in the water, but it does clear trichloramine gas from the air and significantly reduces respiratory symptoms for everyone in the building.