A strong chlorine smell in your pool almost never means there’s too much chlorine. It usually means there’s too little effective chlorine. That sharp “pool smell,” along with red eyes and itchy skin, comes from combined chlorine, a byproduct that builds up as free chlorine gets used up. The fix is a process called breakpoint chlorination, which adds enough chlorine at once to destroy the combined chlorine and get your water properly sanitized again.
Free chlorine vs. combined chlorine
Free chlorine is the active sanitizer in your pool. It kills bacteria, keeps algae from taking hold, and breaks down contaminants like sweat, body oils, sunscreen, and ammonia. As it does that work, some of it turns into combined chlorine, which is essentially spent chlorine with very little sanitizing power left. Because combined chlorine is so much weaker than free chlorine, a pool can test positive for chlorine and still be poorly sanitized. When combined chlorine builds up, the water develops that strong, irritating odor. A clean, properly balanced pool has very little smell at all.
Signs your pool has a combined chlorine problem
- A strong chemical odor around the pool, especially on still, humid days
- Red or burning eyes after swimming
- Itchy or dry skin
- Water that looks dull or won’t stay clear
- Algae that keeps coming back even though you’re adding chlorine regularly
If several of these sound familiar, there may be more going on than just your chlorine level. Our post on 5 signs your pool needs a service call covers other warning signs worth watching.
How to test for combined chlorine
You need a test kit that reads both free chlorine and total chlorine, which a DPD test kit does. An OTO test kit only measures total chlorine, so it can’t tell you whether your chlorine is actually working. Once you have both readings, subtract free chlorine from total chlorine. The difference is your combined chlorine. For example, if total chlorine reads 3.0 ppm and free chlorine reads 1.5 ppm, you have 1.5 ppm of combined chlorine. Any time combined chlorine reaches about 0.5 ppm or higher, the pool should be super-chlorinated.
How breakpoint chlorination fixes it
Breakpoint chlorination means raising free chlorine high enough to completely break down the combined chlorine in the water. The general process looks like this:
- Keep swimmers out of the pool until the treatment is complete
- Test free and total chlorine with a DPD kit
- Subtract free chlorine from total chlorine to find your combined chlorine level
- Multiply that number by 10 to find how many ppm of chlorine you need to add (1.5 ppm of combined chlorine means adding 15 ppm)
- Calculate how much product your pool needs based on its volume, then add it
- Retest over the next 24 to 48 hours until free and total chlorine read the same or nearly the same
Don’t cut corners on the dose. Adding too little chlorine won’t reach breakpoint, and it can actually increase combined chlorine levels, leaving the pool worse off than before. Before anyone swims again, make sure free chlorine has come back down to your pool’s normal recommended range.
How much chlorine your pool actually needs
To figure out how many ppm one pound of your chlorine product adds, multiply the weight of active chlorine in one pound of product by 120,000, then divide that number by your pool’s volume in gallons. For example, calcium hypochlorite shock that’s 65% available chlorine contains 0.65 pounds of active chlorine per pound. Multiply 0.65 by 120,000 and you get 78,000. In a 20,000-gallon pool, 78,000 divided by 20,000 equals 3.9, so each pound adds roughly 3.9 ppm. To reach 15 ppm, you’d need about 4 pounds. Always follow the product label, add chemicals to water rather than water to chemicals, and never mix different pool chemicals together.
Preventing combined chlorine buildup
The easiest way to avoid this problem is consistency. Test both free and total chlorine regularly, keep free chlorine in the proper range instead of letting it run low between treatments, and shock after heavy use, pool parties, or big Central Indiana storms. Having swimmers rinse off before getting in and keeping your filter clean help too. Our guide to keeping your pool crystal clear walks through a simple weekly routine, and our weekly maintenance service can take testing and balancing off your plate entirely.
For pools that see heavy use or keep running into chlorine odor, a UV or ozone system can make a big difference. Both are supplemental sanitation systems installed on your pool’s circulation line. As water passes through, UV light or ozone breaks down combined chlorine (chloramines) and destroys many contaminants before they ever have a chance to build up. The result is less chlorine smell, less eye and skin irritation, and less work for your chlorine. These systems don’t replace chlorine entirely, since your pool still needs a free chlorine level to protect the water between passes through the system, but they can reduce how much chlorine you use and how often you need to shock. Our team can help you decide whether a UV or ozone system is a good fit for your pool.
Why this matters before you close your pool
If you’re getting ready to close for the season, the water you close with is the water you’ll find under the cover next spring. Closing a pool with high combined chlorine or weak sanitizer gives algae a head start over the Indiana winter. Our team checks and balances water chemistry as part of every pool closing, so your pool goes into the off-season in good shape.
FAQ
Does a strong chlorine smell mean there’s too much chlorine in my pool?
Usually not. The smell comes from combined chlorine, which builds up when there isn’t enough free chlorine to keep up with contaminants.
What’s the difference between free chlorine and total chlorine?
Free chlorine is the active sanitizer. Total chlorine is free chlorine plus combined chlorine. The gap between the two tells you how much spent chlorine is in your water.
Can I use an OTO test kit to check for combined chlorine?
No. OTO kits only measure total chlorine. You need a DPD kit that reads both free and total chlorine.
How long does breakpoint chlorination take?
Plan on 24 to 48 hours. Keep testing until free and total chlorine read the same or nearly the same.
What happens if I don’t add enough chlorine?
The pool won’t reach breakpoint, and combined chlorine levels can actually rise. It’s important to calculate and add the full dose.
Is there equipment that helps prevent combined chlorine?
Yes. UV and ozone systems work alongside chlorine to break down combined chlorine and contaminants as water circulates, cutting down on odor and irritation. Your pool still needs free chlorine, but these systems can reduce how much it takes to keep the water clean.
When should I call a professional?
If the smell keeps coming back, your water won’t stay clear, or you’d rather not handle chemicals yourself, it’s time to bring in help. Repeated chemistry problems can also point to circulation or equipment issues our repair team can diagnose.
Still dealing with chlorine odor, cloudy water, or a pool that just won’t stay balanced? Barton Pool Company has been caring for pools across the greater Indianapolis area since 2007. Contact Barton Pool Company or call 317-735-2323 and we’ll help get your water back on track.
