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Combined Chlorine: What It Is & How to Remove Chloramines

Quick Reference

  • What it is: Chlorine that has bonded with contaminants (chloramines)
  • Target level: 0.0 ppm (as close to zero as possible)
  • Problems: Eye/skin irritation, strong chlorine odor, poor sanitization
  • Solution: Shock treatment to break chlorine bonds
  • Formula: Combined Chlorine = Total Chlorine - Free Chlorine

Table of Contents

What is Combined Chlorine?

Combined chlorine refers to chlorine molecules that have bonded with organic contaminants in your pool water, forming compounds called chloramines. Unlike free chlorine, which actively sanitizes your pool, combined chlorine has lost most of its disinfecting power and creates water quality problems.

Think of combined chlorine as "used up" chlorine - it's still chemically present in your water, but it's no longer doing its job of killing bacteria and algae.

The Three Types of Chlorine

Understanding combined chlorine requires knowing how it relates to the other two types of chlorine in your pool:

Chlorine Type Description Target Level
Free Chlorine Active, available chlorine that sanitizes water 1.0-3.0 ppm
Combined Chlorine Used chlorine bonded with contaminants 0.0 ppm (as low as possible)
Total Chlorine Free Chlorine + Combined Chlorine Should equal Free Chlorine
Pro Tip: In a healthy pool, total chlorine and free chlorine readings should be nearly identical, meaning combined chlorine is close to zero.

Why Combined Chlorine Forms

Combined chlorine forms when free chlorine reacts with organic contaminants in your pool:

  • Body oils and lotions from swimmers
  • Sweat and urine (the main culprits)
  • Dead skin cells and hair
  • Environmental debris like leaves and pollen
  • Cosmetics and personal care products

When chlorine bonds with these nitrogen-containing compounds, it forms chloramines - the technical name for combined chlorine. The most common types are:

  • Monochloramine (NH2Cl)
  • Dichloramine (NHCl2)
  • Trichloramine (NCl3) - the most problematic

Problems with Combined Chlorine

High levels of combined chlorine create several water quality issues:

Health and Comfort Problems

  • Eye and skin irritation - chloramines are more irritating than free chlorine
  • Respiratory irritation - especially problematic in indoor pools
  • Strong chemical odor - that overwhelming "chlorine smell" actually indicates chloramines, not high chlorine

Water Quality Issues

  • Poor sanitization - combined chlorine is significantly less effective than free chlorine
  • Cloudy water - chloramines can contribute to hazy conditions
  • Algae growth - insufficient active chlorine allows algae to develop
Important: That strong "chlorine smell" at pools actually indicates too much combined chlorine, not too much free chlorine. A properly balanced pool has little to no odor.

Testing for Combined Chlorine

Most pool test kits measure free chlorine and total chlorine. To find combined chlorine, use this simple formula:

Combined Chlorine = Total Chlorine - Free Chlorine

Example Readings

  • Total Chlorine: 3.0 ppm
  • Free Chlorine: 2.0 ppm
  • Combined Chlorine: 1.0 ppm (too high!)

When to Take Action

  • 0.0-0.5 ppm: Normal range - ignore unless persistent with symptoms
  • 0.5+ ppm: Investigate and consider treatment
  • 1.0+ ppm: Treatment necessary

Accurate Testing

For diagnosing combined chlorine issues, FAS-DPD drop testing (like Taylor K-2006) provides the most accurate results. Test strips are fine for quick sanity checks, but they're not precise enough for troubleshooting chloramine problems.

MPS Interference: If you've used non-chlorine shock (potassium monopersulfate), it can create false combined chlorine readings on DPD tests. Wait 24+ hours after MPS treatment or use an interference remover for accurate results.

Recommended Test Kits

For accurate chlorine testing, FAS-DPD kits like the Taylor K-2006 are the gold standard for DIY pool owners. Browse our pool testing supplies for reliable options.

How to Remove Combined Chlorine

The primary method for eliminating combined chlorine is shock treatment (also called breakpoint chlorination).

Breakpoint Chlorination Process

Traditional pool operator guidance suggests adding 10 times the combined chlorine level in shock. This rule applies mainly to inorganic chloramines (like ammonia-based ones) but doesn't always solve organic contamination issues.

For residential pools with cyanuric acid (CYA), a more reliable approach is the SLAM process (Shock Level And Maintain):

  • Raise free chlorine to shock level based on your CYA
  • Maintain that level until chlorine holds overnight and water clears
  • This addresses both chloramines and underlying organic contamination
  • Learn more about SLAM from TroubleFreePool, the champions of this method

Simple Shock Treatment Steps

  1. Test water to determine combined chlorine level
  2. Choose your shock method (breakpoint rule or SLAM process)
  3. Add shock in evening when sun won't break it down
  4. Run pump during treatment (continuously for SLAM, normal cycle for simple breakpoint)
  5. Retest after 8-12 hours
  6. Repeat if combined chlorine still present

Shock Product Selection

  • Liquid chlorine: Cleanest option with no side effects
  • Calcium hypochlorite (cal-hypo): Effective, but adds calcium to water
  • Stabilized shock (dichlor/trichlor): These work chemically, but they add CYA. Use only if you intentionally want to raise cyanuric acid levels
Pro Tip: All chlorine types ultimately deliver the same sanitizing chemical. The differences are in side effects - liquid chlorine is usually the cleanest default choice.

Alternative Methods

  • UV systems - Break down chloramines continuously
  • Ozone generators - Oxidize contaminants before they form chloramines
  • Enzyme products - Break down organic waste that feeds chloramine formation

Prevention Tips

Preventing combined chlorine is easier than removing it:

Reduce Contaminant Load

  • Shower before swimming - removes oils, lotions, and sweat
  • Rinse swimwear in fresh water before pool use
  • Limit cosmetics and personal care products
  • Regular skimming to remove organic debris

Maintain Proper Chemistry

  • Consistent free chlorine levels appropriate for your CYA
  • Proper pH (7.2-7.6) for optimal chlorine effectiveness
  • Adequate circulation - run pump as needed for your pool size and bather load
  • Shock only when needed - for algae, persistent CC >0.5 ppm, cloudy water, or failed overnight chlorine loss test, not on a calendar schedule
DIY Philosophy: If you maintain proper free chlorine levels for your CYA, routine shocking isn't necessary. Shock when there's a specific problem to solve, not because it's "weekly shock day."

Hot Tub Considerations

Hot tubs face unique combined chlorine challenges:

  • Higher temperatures accelerate chloramine formation
  • Higher body-to-water ratio means more contaminants per gallon
  • Smaller water volume makes chemistry swings more dramatic
  • More frequent testing needed - every use or daily during heavy use
  • More frequent shocking may be necessary compared to pools

The same principles apply, but spa owners should be more proactive with testing and treatment due to the challenging environment.

Related Terms

Learn more about related chlorine concepts:

  • Free Chlorine - The active sanitizer in your pool (coming soon)
  • Total Chlorine - The sum of free and combined chlorine (coming soon)