Shocking a pool, also known as super-chlorination, is a maintenance process where a highly concentrated dose of chlorine-based chemical is added to the water. The primary purpose is to quickly raise the Free Available Chlorine (FAC) level to break down contaminants that regular chlorination cannot handle effectively. This powerful treatment sanitizes the water by destroying organic matter, algae, and bacteria, eliminating irritating compounds called chloramines.
The Chemistry Behind the Wait
Shocking the pool intentionally raises the Free Available Chlorine (FAC) concentration in the water far above the level considered safe for swimming. This process aims to reach “breakpoint chlorination,” which is the level required to fully oxidize all combined chlorine and other organic waste. Chloramines are the compounds that form when chlorine reacts with nitrogen and ammonia found in sweat, oils, and urine, and they are responsible for the unpleasant, strong “chlorine smell” and the irritation of eyes and skin.
Swimming in water with extremely high FAC levels can cause immediate physical discomfort. High chlorine concentrations strip the natural oils from skin and hair, leading to dryness, irritation, and redness. The elevated chemical concentration can also irritate the mucous membranes of the eyes and respiratory system, leading to stinging, redness, and coughing. Therefore, the waiting period is not for the chemicals to simply dissolve, but for the highly concentrated FAC to dissipate to a safe residual level.
Determining Safe Entry Time
The definitive answer to when you can swim is not a set number of hours, but a chemical measurement of the water itself. While a general timeframe of 8 to 24 hours is often cited, this is merely an estimate that depends heavily on environmental conditions and the initial dose of the shock.
For safe swimming, the FAC level should be between 1 and 3 parts per million (ppm), and no higher than 5 ppm. To confirm this level, a reliable water testing kit, such as a DPD or FAS-DPD kit, must be used to get an accurate reading. You must continue to test the water until the FAC level falls within the acceptable range. Relying solely on the passage of time is a safety risk, as factors like water temperature, sunlight, and the amount of stabilizer (Cyanuric Acid) in the water can significantly slow or speed up the rate of chlorine dissipation.
Accelerating the Process
Actions can be taken to help reduce the waiting period by encouraging the highly concentrated chlorine to dissipate more quickly. Maximizing water circulation is one of the most effective methods, so the pool pump should be run continuously after shocking. This action ensures the shock is thoroughly distributed and helps the volatile chlorine gas off the surface more efficiently.
Aeration, which involves moving water at the surface, also helps speed up the process of chlorine dissipation. If the pool has features such as a waterfall, fountain, or jets, running them can increase the surface area contact between the water and the air, encouraging the release of chlorine gas. Additionally, UV rays from the sun are highly effective at breaking down chlorine. Removing any pool covers and exposing the water to direct sunlight will naturally accelerate the reduction of the FAC level.