High Combined Chlorine in a Commercial Pool: What It Means

High combined chlorine means a lot of your chlorine has reacted with sweat, urine and other swimmer waste and is no longer doing much disinfecting. CDC's Model Aquatic Health Code (MAHC, 2024 edition) says to take action above 0.4 ppm (your state's adopted code sets the actual limit). Record the numbers and get your trained operator involved.

What is combined chlorine?

CDC's Chloramines and Pool Operation page explains that chlorine binds to the body waste swimmers bring into the pool. This forms chloramines, which are a type of combined chlorine. Chloramines can irritate skin, eyes and the nose and throat when they turn into gas above the water, especially indoors.

The MAHC glossary calls combined chlorine the chlorine tied up as chloramines, and says it is less reactive than free chlorine. Total chlorine is the two added together.

Give free, combined and total each its own labeled box on your log. A box marked only "chlorine" does not tell the next shift which test was done. For the free chlorine limits, see our commercial pool chlorine range guide.

How do you calculate combined chlorine?

CDC gives three steps: measure total chlorine, measure free chlorine, then subtract.

Combined chlorine = total chlorine − free chlorine

Use both readings from the same test round, in the same unit, done the way your kit's instructions say.

An illustrative example: a lifeguard reads total chlorine at 2.6 ppm and free chlorine at 2.0 ppm. Combined chlorine is 2.6 − 2.0 = 0.6 ppm. That is above the 0.4 ppm action level in the MAHC, so it gets reported to the trained operator right away.

Write down both readings as well as the answer so a supervisor can check the math later.

What makes a combined chlorine result too high?

Two public sources use the same number:

• CDC's chloramines page says most city, county and state health departments limit combined chlorine to 0.4 ppm or less.
• MAHC section 5.7.4.4.2 says the facility must take action to lower combined chlorine when it is above 0.4 ppm.

Write your site's limit and where it came from next to the testing instructions. If an old wall chart and the current plan disagree, report it to your manager before following either one.

What should you do when the number is high?

Use this decision rule as a starting point, then match it to your own written plan.

What you see	What to do	Who to tell
Combined chlorine at or below your local limit	Record it and keep testing on schedule	No one extra
Combined chlorine above your local limit	Retest if your procedure says to, record both rounds, follow your site plan	Trained operator, same shift
Above the limit plus eye, skin or breathing complaints	Follow your plan for closing the pool, record complaints	Trained operator and manager
Someone has trouble breathing or a chemical spill	Follow the emergency plan	Emergency services first

When you report it, send:

• The pool name and the actual test time.
• Free, total and calculated combined chlorine.
• The pH from the same test round. See our commercial pool pH guide.
• The test kit used and any trouble reading it.
• What staff noticed, such as a strong smell and where. Describe it; do not guess the cause.
• Whether the pool is open or closed.

How do operators bring combined chlorine down?

This is the trained operator's job, not a lifeguard's. The MAHC lists superchlorination, swapping out some of the water, and getting swimmers to follow good hygiene. CDC also describes ultraviolet light or ozone systems.

CDC describes superchlorination (also called breakpoint chlorination) this way: close the pool, make sure the area has good airflow, and raise free chlorine to about 10 times the combined chlorine level. In the example above, that is about 6 ppm (10 × 0.6). Then free chlorine is brought back down to the normal range before swimmers return. Only staff trained in pool chemical safety should handle pool chemicals, according to CDC's pool chemical safety guidance. Follow the product label and your health department.

Prevention helps too. CDC suggests asking swimmers to shower before swimming, use the toilet first and wear a swim cap, and posting signs about it in the locker room.

Why does the indoor air matter?

CDC says chloramine gas is heavier than air, so it sits right above the water where swimmers breathe. CDC says the heating and air system should bring in fresh outdoor air, move it across the water, and send chloramine-filled air outside. CDC notes this often goes wrong in winter, when sites cut fresh air to save on heating.

The MAHC (section 5.6.2.8) asks the operator of an indoor pool to have a written plan to keep combined chlorine low. Pass air complaints to the manager and the operator together with the water results. If a building specialist needs to check the air system, keep that job open until someone writes down what they found. If the water is also cloudy, see our guide to cloudy commercial pool water.

Poolean keeps water readings, test photos, closures and recorded reopening steps on each pool's record, so the report and the follow-up stay together. See the water testing features or book a 30-minute call.

The short version

• Combined chlorine = total chlorine − free chlorine, from the same test round.
• CDC's MAHC says to act above 0.4 ppm.
• Record both readings, the result, pH and any complaints, and tell the trained operator.
• Only trained staff should treat the water; follow the label and health department.
• Indoors, fresh air moving across the water and out of the building matters.