When the concentration of chlorine disinfectant in pool water is outside of its ideal range, the pool is vulnerable to the growth of potentially harmful microorganisms and algae. In situations where the chlorine level is too low, you may need to shock the pool to make the water safe again.

The process of shocking a pool or spa involves temporarily elevating the free chlorine level with a chlorine-based disinfectant product several times above recommended levels to restore the proper balance of disinfectant in the water, to sanitize the water or to kill algae.

Chlorine and Chloramines

Chlorine-containing compounds are the most commonly used pool disinfectants. When chlorine is first introduced into the water, it effectively kills microorganisms and algae and keeps the water clear and safe for swimmers. But as chlorine reacts with organic compounds in the water, it is converted from its effective form -- called free chlorine -- into chemical compounds called chloramines. Chloramines, also called combined chlorine, are not effective disinfectants and potentially harmful to swimmers.

The recommended free chlorine level is generally between 1 and 3 parts per million; combined chlorine levels should not reach above .5 ppm. From 0.3 to 0.5 ppm, swimmers may start to complain about the water. Over time, the proportion of free chlorine to combined chlorine can shift out of the proper balance; shocking the pool can restore the levels to where they should be.

When to Shock

Shocking is necessary when the pool is initially filled, when free chlorine levels drop to zero or when the combined chlorine level rises above .5 ppm. Shocking the pool can also eliminate algae build up in the water or on pool surfaces. Spas should be shocked when they're refilled, after a period of disuse or after periods of heavy use.

Shocking and pH

During the shocking process, high levels of chlorine will interfere with pH testing and make it difficult to get an accurate pH reading, so it's important to test the pH of the water and adjust the pH level as necessary before you shock.

Shocking Process

You must test the water first to determine how much chlorine you'll need to shock it.

Step 1

Assess the quality of the water in the swimming pool with a test kit to determine the level of cyanuric acid stabilizer in the water. Cyanuric acid helps to prevent chlorine from breaking down in the presence of sunlight, but it also reduces the effectiveness of chlorine as a disinfectant, so the more stabilizer there is in the water, the more chlorine you'll need for an effective shock. The level of stabilizer will determine the target free chlorine level for the shocking process, which will typically be between 10 and 39 ppm.

Step 2

Add a chlorine disinfectant product after you've determined the target level to raise the free chlorine to that level.

The amount of chlorine you'll need to add depends on the type of product you're using and the amount of change you need to make. For example, 2 ounces of 67 percent calcium hypochlorite will raise the free chlorine level of 10,000 gallons of water by 1 ppm, but it takes 10.7 ounces of 12 percent sodium hypochlorite to make the same change.

Step 3

Retest the free chlorine and combined chlorine levels at least twice a day and add more chlorine as needed to maintain the target level for the pool's water.

Step 4

Test the water and adjust as necessary until the shocking process is complete. The shocking process is complete when the combined chlorine level is less than .5 ppm, the water is losing less than 1 ppm of free chlorine overnight and the water remains clear. At this point, stop adding chlorine and allow the free chlorine levels to drop naturally to their everyday recommended levels for swimming.

Bromine and Salt Water Pools

Bromine, a disinfectant often used in spas and sometimes in pools, is not subject to breakdown into harmful compounds the same way that chlorine is, so in a bromine-sanitized pool, the build up of chloramines is not a trigger for the shocking process. However, chlorine shocking is still an option for sanitizing or killing algae in a pool or spa in which bromine is used.

Salt water pools use a chlorine generator to introduce disinfectant into the water instead of requiring the manual addition of chlorine, but the shocking process and the reasons for doing it are the same for these pools as they are for fresh water pools, although the target free chlorine level will likely be lower.