When any type of chlorine is added to water, it forms hypochlorous acid (HOCl - the most powerful killing form of chlorine in water) and hypochlorite ion (OCl-), a relatively weak form of chlorine in water. The percentage of HOCl and OCl- is determined by the pH of the water. As the pH goes up, less of the chlorine is in the killing form and more of the chlorine is in the weaker form. The combined total of HOCl and OCl- is the measure of free available chlorine. Free available chlorine is the active, killing type of chlorine that we want in the water.

Chlorine molecules can combine with ammonia and nitrogen compounds in the water to form chloramines, sometimes also called Combined Chlorine. By combining with ammonia and nitrogen, a chloramine loses much of its sanitizing power. Chloramines are 60 to 80 times less effective than an uncombined free chlorine molecule.

Chloramines are formed any time ammonia and nitrogen are in the water. Some of the ammonia and nitrogen compounds are introduced into the water by swimmers and bathers in the form of perspiration, urine, saliva, sputum and fecal matter. An active swimmer sweats one pint per hour, while the average person sweats three pints per hour in a heated spa. Ammonia and nitrogen compounds are also introduced into the water by rain, especially Acid Rain. Each drop of rain has some dissolved nitrogen from our atmosphere and from automobile emissions. Many lawn products contain both nitrogen and ammonia and phosphates, all very bad for pools, can raise your chlorine demand, and encourage algae growth.


Chloramines, or combined chlorine smell bad, they are eye and skin irritants, and they get in the way of free chlorine trying to do it's job. When a pool smells strongly of Chlorine, what smells is not free available chlorine, but Chloramines . When testing for Free and Total Chlorine with a DPD or other capable pool test kit, the level of combined chlorine molecules in your pool can be detected. The formula is quite simple. The difference between Free Chlorine reading and the Total Chlorine reading is the Combined Chlorine reading. Anything over 0.3ppm should be treated to bring the level down by removing the Chloramines from the pool.


Chloramines can be removed from the water by the following three methods:

  1. By adding a mega-dose of chlorine. Usually 3 to 6 times more chlorine than a normal dose is added to the water, or the level of chlorine is raised to 5 to 10 ppm and held there for 4 hours. This is called super-chlorination. To remove chloramines, the ratio of chlorine to ammonia must be at least 7.6 to 1. If this ratio is not obtained, then more chloramines will be produced. A threshold of "breakpoint chlorination" must be reached where total oxidation takes place. This "shock" to the pool will rip apart the molecular combinations, and destroy chloramines. Swimmers and bathers should not enter the water until the level of chlorine has dropped to 3 ppm or less.

  2. By adding a non-chlorine shock to the water. The most common chemical used for this is potassium peroxymonosulfate. This "shocking" requires the addition of one pound of shock for each 10,000 gallons of pool water. The same threshold of breakpoint oxidation must be reached when using non-chlorine shock. So, feel free to add a little extra, just to be sure.

  3. By adding ozone to the water. If an ozone generator is installed on a pool or spa, then oxidation of the ammonia and nitrogen compounds will take place whenever the ozone system is operating. The longer the system operates, the more the ozone can destroy the ammonia and nitrogen. Although most ozone systems operate only when the pool or spa pump is operating, there are 24 -hour systems available which will continuously oxidize ammonia and nitrogen as they enter the water.

Remember, when you smell a strong chlorine odor in a pool - and your eyes are red, it's not because there is too much [free] chlorine in the water, but too much Combined Chlorine. Free chlorine by itself does not smell, nor sting the eyes. The solution to this is to add a whole lot more chlorine, to reach breakpoint chlorination, where the molecular combination of chlorine and nitrogen [or ammonia] will be removed. At least for a while.