Combined Chlorine

It is free, available chlorine that does the hard work of killing bacteria and oxidizing contaminants. When the free chlorine combines with these contaminants, such as ammonia, soap and other nitrogen-containing organic compounds, we call it combined chlorine, or chloramines. In pool and spa water, this form of chlorine has very little sanitizing ability, and no oxidizing ability. You can think of combined chlorine as a spent bullet.

Water treatment plants often use chloramines to treat drinking water because chloramines are more stable than free chlorine, and they last longer. Those chloramines are specially developed for treating drinking water, and as a result they display very little odor.

However, free chlorine has a much bigger job in swimming pools and spas because of all the nasty things that can make their way into the water. When free chlorine combines with different contaminants like human waste, the resulting chloramines put out a foul odor. Combined chlorine can also irritate the skin and eyes of bathers and swimmers. Therefore, when the combined chlorine concentration is greater than 0.2 ppm (parts per million), you need to shock (superchlorinate) the pool or spa. Superchlorinating will oxidize the combined chlorine and get rid of the contaminants for good.

There is no convenient, direct testing method that measures combined chlorine. Instead, we often have to start with total chlorine, which we can easily measure. Total chlorine is just the sum of combined chlorine and free chlorine. In other words,

(combined chlorine) = (total chlorine) minus (free chlorine)

For example, a test that yields a total chlorine level of 3 ppm and a free chlorine level of 1 ppm indicates a combined chlorine level of 2 ppm (3 ppm total - 1 ppm free = 2 ppm combined chlorine). In the immortal words of Dr. H. Tueau, "Time to shock that sucker!"

There are other indicators that suggest the presence of combined chlorine. Your nose is a good place to start. As we mentioned above, combined chlorine can emit an odor bad enough to make your eyes water. This odor is often misinterpreted. Many swimmers and pool operators assume that the "strong chlorine smell" means there is too much chlorine in the water. In reality, that smell probably indicates that the pool needs more active chlorine, not less. Consider the smell of household bleach, which contains a greater concentration of chlorine than any swimming pool or spa should have. The bleach still smells fresh and not so strong. On the other hand, the odor of chloramine is so strong and unpleasant that your nose will complain many yards away from an average swimming pool.

In addition to the awful smell, your body may detect the presence of combined chlorine in the form of skin and eye irritation. Some people who have experienced this irritation have thought they were allergic to chlorine. Instead, it is more likely that they were sensitive to the buildup of chloramines. There was probably not enough free chlorine present to sanitize the water and oxidize the combined chlorine. This is especially true in spas and hot tubs where the sanitizer residual is used up quickly.