Pool shock is a large dose of chlorine or a non-chlorine oxidizer that breaks down combined chlorine (chloramines) and other contaminants in the water, and chlorine shock also kills bacteria and algae. Shock when your test shows combined chlorine building up, after algae appears, and before or after a heavy bather load. With a chlorine shock, the goal is breakpoint chlorination: raising free chlorine to 10 times the combined chlorine level, according to CDC.
To keep your pool clean, safe, and healthy for the best possible swimming experience there are specific tasks that must be completed before diving in. To ensure crisp, freshwater one of those tasks to check off the list is shocking your pool. Shocking your swimming pool or spa has always been good practice for recreational water management. Regardless of the type of sanitation system you’ve chosen, shocking will typically be required.
Why Do I Need To Shock My Pool?
Pools are shocked to keep the water healthy and clean by removing algae and bacteria after heavy bather use or contamination events. In addition, pool shocking is used to eliminate harmful combined chlorine in the water.
Also known as chloramines, combined chlorine can cause “pool” odor, red eyes, irritated skin, and health issues. Combined chlorine is a disinfection by-product (DBP) that forms when chlorine reacts with contaminants, like lotions, sweat, and urine in the pool water.
You may want to shock your pool in certain situations, such as:
- After a new pool construction is completed and there is a fresh fill from any source of water.
- Opening your pool for the season after it has been closed for the winter.
- After heavy bather loading from pool parties.
- When pool or spa water temperatures are consistently high from sun exposure or heating the water regularly.
- After heavy rainfalls that can add organic loading to the water.
What Is Pool Shocking?
The term shocking means to add enough chlorine to the pool or spa water to destroy all combined chlorine present in the water. In detail, the process of pool shocking means adding chlorine or non-chlorine shock to your pool in order to raise the “free chlorine” level. The goal is to raise “free chlorine” levels to a point that destroys unwanted contaminants, such as algae, chloramines, and bacteria.
A common term for using a pool shock to destroy combined chlorine is called “breakpoint chlorination.” Usually, reaching “breakpoint chlorination” means raising free chlorine to 10 times the combined chlorine level, according to CDC’s chloramines and pool operation guidance (2025). For example, if your test shows 0.5 ppm combined chlorine, the breakpoint target is about 5 ppm free chlorine.
Here are some things to keep in mind before using pool shock:
- No matter what type of pool shock you use, it’s important that you follow the instructions on the shock label very carefully.
- Make sure that the pool shock chemicals are stored in a cool and dry place.
- Always apply pool shock chemicals directly to pool or spa water.
When Should I Shock My Pool?
The best time of day to shock your pool is in the evening after sunset so that the sun’s UV rays will not evaporate the shock from the water and will allow the shock to oxidize the water for a longer length of time. You’ll want to choose a time when your swimmers are finished and out of the pool for the day because you don’t want anyone in the pool during this time.
Make sure to use protection when handling shock such as eyewear and gloves. Prepare the shock by following the manufacturer’s recommendations. After you shock the pool, keep the pump running so the shock mixes through the water, and test before anyone swims. Pool shock products are pesticides registered with the EPA (UF/IFAS Extension, 2014), so follow the label: one EPA-registered chlorine shock label, for example, says swimming may resume when the chlorine residual is between 1 and 4 ppm (EPA-accepted label, 2013). Make sure to recheck your pool chemistry the next day and balance accordingly.
What Type of Pool Shock Should I Use?
The type of shock you should use will depend on your water chemistry and the reason you’re shocking.
If the water is green, you should use a cal-hypo shock. However, if you’re shocking as part of weekly maintenance and your pool looks clean, you can use sodium dichlor, potassium monopersulfate or sodium hypochlorite.
Shock when your test shows combined chlorine building up, after algae appears, and before or after a heavy bather load.
Here are four main types of pool shock:
1. Calcium Hypochlorite
Calcium Hypochlorite, or cal-hypo, is the most powerful and fast-acting shock available. This popular shock method dissolves quickly and is unstabilized. Unstabilized shocks burn off from the sun’s UV rays and does not increase cyanuric acid levels.
Cal-hypo has an alkaline pH and adds calcium, so it will also slightly raise your pool’s pH and calcium hardness, according to the Los Angeles County Department of Public Health’s pool service technician study guide (revised 2014). To avoid damage to your pool surface, be sure to always pre-dissolve a cal-hypo shock before putting it in your pool. Additionally, cal-hypo shocks should be added in the evening when the sun goes down. You can enjoy your swimming pool after it returns to it’s typical chlorine levels.
2. Sodium Dichlor
Sodium dichlor, also known as dichloro-s-triazinetrione or dichloroisocyanuric acid, is a stabilized, slow-dissolving shock method. Unlike cal-hypo, dichlor will dissolve slowly and raise cyanuric acid levels. Sodium dichlor is close to neutral (the LA County study guide lists its pH as 6.8), so it does not raise pool pH, and the guide notes that regular use can push pH down. In addition to a shock treatment, dichlor can be used as a sanitizer and oxidizer. When using dichlor as a shock method, use 1 pound per 10,000 gallons of water. Similarly to cal-hypo, a dichlor shock should be added after the sun goes down.
Note: Dichlor adds cyanuric acid every time you use it, so it is not our recommended shock for pools that already have enough stabilizer. See “How does cyanuric acid affect pool shock?” below.
3. Potassium Monopersulfate (Non-Chlorine Shock)
Potassium monopersulfate, or MPS, is a non-chlorine, oxygen-based shock.
MPS does not sanitize or kill algae on its own, so it must be used with an EPA-registered sanitizer such as chlorine, according to the Pool & Hot Tub Alliance’s recreational water quality committee (2020). A MPS shock can be used to oxidize, destroy contaminants and help chlorine operate more efficiently. Unlike both cal-hypo and dichlor, MPS won’t change your pool’s cyanuric acid levels. Known as the “shock and swim” method, MPS can be used at any time of the day. You can start swimming nearly 15 minutes after using this shock.
4. Sodium Hypochlorite (Bleach or Liquid Shock)
Sodium hypochlorite, also known as bleach or liquid shock, is a “heavy-duty” type of pool shock that isn’t typically used in home pools. Sodium hypochlorite is quick-dissolving and, unlike cal-hypo, does not need to be pre-dissolved before putting it in your pool. Sodium hypochlorite shock is often delivered in bulk by truck. It is often used in commercial pools, such as hotels, condos and health club pools. This commercial strength liquid chlorine is also used by laundry and dry cleaners and is strong: the LA County study guide lists liquid chlorine at about 10% to 15% available chlorine.
By contrast, CDC describes household bleach as 5.25% to 6.15% sodium hypochlorite (CDC disinfection guideline, 2023), so it takes more household bleach to shock the same pool. Never mix bleach or liquid chlorine with other pool chemicals (CDC). If you buy liquid bleach for your pool, be sure to get unscented and unaltered, regular bleach.
How does cyanuric acid affect pool shock?
Cyanuric acid is a chlorine stabilizer, and it slows how fast chlorine kills germs. CDC’s home pool and hot tub water treatment guidance (2024) says disinfection times are longer when cyanuric acid is present and recommends at least 2 ppm free chlorine in pools that use it, instead of at least 1 ppm. CDC recommends not using cyanuric acid or stabilized chlorine products in hot tubs.
Stabilized shocks such as dichlor add cyanuric acid each time you use them, according to the Oregon Health Authority’s cyanuric acid fact sheet (2025). If your test shows the pool already has enough stabilizer, choose an unstabilized shock such as cal-hypo or liquid chlorine. For public pools, CDC’s 2024 Model Aquatic Health Code cross-reference guide lists closing a pool when the ratio of cyanuric acid to free chlorine exceeds 45:1, and the Oregon fact sheet explains that past that ratio the chlorine is unable to kill pathogens in the water.
How Does Clear Comfort’s AOP Affect Pool Shocking?
Clear Comfort’s hydroxyl-based AOP (advanced oxidation process) is a supplemental system: it works alongside a low chlorine residual, with typical free chlorine of 0.5 to 1.5 ppm, so you still test the water and shock when it needs it. Hydroxyl-based AOP breaks down the sweat, oils and other contaminants that combine with chlorine to form combined chlorine, which can mean fewer shock treatments. Maintenance is an annual cartridge exchange that takes approximately 1 to 5 minutes per year.
Learn how hydroxyl radicals work on our hydroxyl-based AOP technology page. Clear Comfort makes systems for home pools and spas and for commercial aquatic facilities.
Frequently asked questions
How often should you shock a pool?
Shock a pool when testing shows combined chlorine building up, when algae appears, or after heavy use. CDC notes that most health departments limit combined chlorine in public pools to 0.4 ppm or less, a common benchmark for when to shock.
How much shock does it take to reach breakpoint chlorination?
Raise free chlorine to 10 times the combined chlorine level, according to CDC. A pool with 0.5 ppm combined chlorine needs about 5 ppm free chlorine to reach breakpoint; use the product label to work out the dose for your pool’s volume.
How long after shocking a pool can you swim?
Test the water first and follow the product label, because pool shock products are EPA-registered pesticides. One EPA-registered chlorine shock label says swimming may resume when the chlorine residual is between 1 and 4 ppm.
Does pool shock change pH?
It depends on the type. Cal-hypo has an alkaline pH and slightly raises pH and calcium hardness, while dichlor is close to neutral (pH 6.8 in LA County’s pool technician study guide) and adds cyanuric acid. The Pool & Hot Tub Alliance notes that MPS may reduce alkalinity and pH over time.
Is non-chlorine shock a sanitizer?
No. MPS is an oxygen-based oxidizer that does not sanitize or kill algae, so it must be used with an EPA-registered sanitizer such as chlorine. Leftover MPS can also read as combined chlorine on a standard DPD test unless the test kit includes a neutralizer reagent.
Do you need to shock a pool that has a Clear Comfort system?
Yes, when testing shows combined chlorine building up or after a contamination event. Clear Comfort’s hydroxyl-based AOP is supplemental and works alongside a low chlorine residual (typical free chlorine of 0.5 to 1.5 ppm), so regular testing and shocking when needed still apply.
To see how a Clear Comfort hydroxyl-based AOP system works with a low chlorine residual in a home pool or spa, visit our residential page.
Louie Warco
Customer Success Manager
