How to Lower the Free Chlorine in a Pool

When chlorine in pool water reacts with nitrogen-containing substances, such as swimmers' sweat or urine, compounds called chloramines form. Chloramines produce the distinctive odor often associated with chlorinated pools, and they also may cause skin and eye irritation and respiratory problems in swimmers. The effects of high chloramine levels are often misinterpreted as a sign of too much chlorine in the water, but in fact, the opposite is true; adding chlorine will reduce the level of chloramines in the pool.

Free Chlorine and Chloramines

When chlorine is first added to pool or spa water, it is called free chlorine, and in this form, it is most effective as a disinfectant. However, as free chlorine reacts with organic matter to form chloramines, also called combined chlorine, the proportion of free chlorine in the water drops. The sum of the levels of combined and free chlorine in the water is called the total chlorine level.

Adding more free chlorine to the water, in a process called superchlorination or shocking, reduces the chloramine level because free chlorine breaks the chemical bonds of the chloramine compounds, converting them to nitrogen gas. When all of the chloramines are gone, any leftover chlorine is referred to as the free chlorine residual.

Testing and Levels

To determine the levels of free and combined chlorine in the water, first test to determine the level of free chlorine, then test to determine the level of total chlorine. The difference between the two levels is the level of combined chlorine.

To maintain adequate disinfection capability, the level of free chlorine in the water should not be below 1 part per million. In general, the free chlorine level should remain between 1.5 and 2.5 parts per million. Combined chlorine levels should not be above 0.5 parts per million, and swimmers are likely to be more comfortable if the level is below 0.2 parts per million.

Raising Free Chlorine Level

If the water's free chlorine level is too low and its combined chlorine level is too high, the goal is to reach the point, called breakpoint chlorination, at which there is enough free chlorine in the water to neutralize all the chloramines in the water.

To achieve this level, the water needs to contain 10 times as much free chlorine as it does combined chlorine. Using the levels determined through testing, multiply the combined chlorine level by 10 and then subtract the current free chlorine level to determine how much chlorine you'll have to add to reach breakpoint chlorination.

Lowering Free Chlorine

Low free chlorine levels are more likely to be a concern than high levels, but excessive free chlorine in pool or spa water can cause problems, too, including damage to equipment and bleaching of hair and swimwear.

Free chlorine degrades when exposed to ultraviolet light, so halting the addition of chlorine to your pool and leaving the pool uncovered and unused on a sunny day will significantly lower the water's chlorine level in a matter of hours.

Chlorine neutralizing chemicals such as sodium thiosulfate or sodium sulfite will lower chlorine levels more quickly, but they will also significantly alter the pool's pH level and will neutralize all of the chlorine in the water if they're overused.

Lowering chlorine levels by partially draining and refilling the pool is potentially the most time-consuming, labor-intensive and costly option.