Shocking a swimming pool serves a few key purposes that are vital for keeping the water clean and safe for swimming. The main reasons to shock a pool are:
To Kill Bacteria
One of the primary reasons for shocking a pool is to kill bacteria. As swimmers use a pool, their bodies shed bacteria and other contaminants into the water. This bacteria can build up over time and reach unsafe levels if the pool is not properly shocked on a regular basis.
Chlorine is added to pool water to kill bacteria. However, the chlorine level can become depleted after heavy bather loads. Shocking the pool raises the chlorine level quickly to oxidize contaminants and kill bacteria.
Some of the most common bacteria found in swimming pools without proper shocking include:
- E. coli
- Pseudomonas aeruginosa
- Legionella
These bacteria can cause recreational water illnesses (RWIs) if swimmers ingest contaminated water. RWIs include gastrointestinal illnesses, skin rashes, ear infections, and upper respiratory infections.
Shocking the pool water with chlorine or other oxidizing chemicals helps destroy these harmful germs and prevents illness.
To Get Rid of Chloramines
Another key reason to shock a swimming pool is to get rid of chloramines. Chloramines are chemical compounds that form when the chlorine in a pool combines with nitrogen-containing contaminants brought in by swimmers.
Some common sources of nitrogen compounds are:
- Urine
- Sweat
- Mucus
- Skin cells
- Hair
- Lotions
- Cosmetics
Chloramines cause several issues when they accumulate in a swimming pool:
- Reduced chlorine effectiveness – When chlorine is bonded to nitrogen, it is not available to kill germs.
- Eye and skin irritation – Chloramines can irritate the eyes, nose, and skin of swimmers.
- Respiratory issues – Chloramines can cause breathing issues for some people when present at high levels.
- Cloudy water – Chloramines make the water appear cloudy and hazy.
- Chlorine smell – The strong “chlorine odor” at pools is actually caused by chloramines, not free chlorine.
Shocking breaks apart the chloramines, freeing up the chlorine to work effectively again. It also immediately lowers the chloramine level, reducing negative impacts on swimmers.
To Oxidize Contaminants
Shocking a pool oxidizes or breaks down contaminants in the water using chemicals like potassium monopersulfate, sodium dichlor, or potassium peroxymonosulfate. Some common contaminants oxidized by shock include:
- Sweat
- Urine
- Mucus
- Sunscreens
- Lotions
- Makeup
- Organic debris
- Algae
If these contaminants are not oxidized and removed, they will consume chlorine rapidly and make maintaining proper disinfection more difficult. They can also lead to swimmer discomfort, cloudy water, irritated eyes and skin, and unpleasant odors.
Shocking the pool water periodically helps destroy a buildup of contaminants so the sanitizer can work on killing germs rather than reacting with nitrogenous waste and organic matter.
To Maintain Proper Chlorine Level
Shocking raises the chlorine level in pool water quickly so that proper disinfection levels can be maintained. The CDC recommends maintaining free chlorine levels of:
- 1-3 ppm for residential pools
- 3-5 ppm for public pools
Without periodic shocking, the chlorine level would drop too low between standard chlorine additions and lead to poor kill-off of germs. The chlorine level can decline quickly with heavy bather load, very warm weather, rain or wind, or other factors.
Shocking gives the chlorine level a quick boost to keep it in the recommended range for proper sanitization. This helps prevent swimmers from being exposed to contaminated water between regular chlorine disinfections.
To Improve Water Clarity
Shocking a cloudy pool water helps clear up the water so it is more appealing for swimming and aesthetics. Some reasons pool water can become cloudy include:
- High chloramine concentration
- High bather load
- Buildup of nitrogenous contaminants
- Buildup of algae
- High pH allowing metals or minerals to precipitate
Shocking helps burn off chloramines, lower nitrogen compound level, oxidize algae and organic waste, and destroy metals and minerals in the water. This allows the particles making the water appear cloudy to be filter out more easily or destroyed.
Cloudy water is visually unappealing and can indicate problems with your pool chemistry or filtration that should be addressed. Shocking is a quick solution that can restore water clarity quickly so swimmers can enjoy crystal clear water.
To Prevent Algae Growth
Shocking your pool helps prevent and control algae growth. Algae can grow quickly in warm, nutrient-rich pool water and make the water unsafe and unattractive for swimming.
Some common types of swimming pool algae are:
- Green algae – Appears as green floating patches or green water.
- Black algae – Also called black spot algae, appears as black dots on walls.
- Mustard algae – Makes water appear mustard yellow.
- Pink algae – Pink-colored spots on walls or floors.
Shocking raises the oxidizer level quickly to destroy algae spores and cells before they can reproduce and spread. It also helps get rid of nutrients in the water that algae need to thrive.
Regular shocking as part of your pool maintenance routine will prevent algae from taking over your pool. Shocking should be used along with algaecides if you already have visible algae in the water.
To Allow Higher Bather Loads
Shocking allows swimming pools to handle higher bather loads safely. As more swimmers use a pool in a given period, more bacteria, chemicals, and contaminants are brought into the water. This includes:
- Skin particles
- Cosmetics
- Lotions
- Sunscreens
- Urine and fecal matter
- Mucus
- Respiratory secretions
Shocking helps destroy these contaminants quickly and let the chlorine or bromine work against pathogens. Without shocking, high bather loads would quickly deplete the sanitizer level and make the water unsafe.
For pools designed for higher use like hotel pools or waterparks, shocking allows the water to withstand more usage between chemical additions. Public pools often require shocking each night after closing when bather load is highest.
To Prep Pool Before Addition of Chemicals
Shocking is recommended before adding certain balancing or sanitizing chemicals to the pool. The oxidation process helps use up contaminants that could react with the chemicals being added.
Some examples where shocking as prep is useful include:
- Before adding chlorine tablets to a chlorinator – Prevents cloudy water, staining, and scale buildup on pool surfaces and equipment from metals in the source water.
- Before adding liquid chlorine – Removes chloramines and prevents formation of toxic chlorine gas.
- Before adding pH adjusting chemicals – Prevents pH from drifting after addition. Metals oxidized by shock will not affect pH change.
- Before adding stabilizer or conditioner – Prevents cyanuric acid buildup from adding too much conditioner.
Shocking to prep the pool ensures the added chemicals work as intended and are not depleted or derailed by high contaminant levels in the water.
To Improve Efficiency of Chlorine
Shocking improves the efficiency and performance of chlorine added to the pool. When contaminants like ammonia and nitrogen compounds are present, they use up chlorine rapidly through reactions.
Less free chlorine is available to kill germs when much of the chlorine reacts with nitrogenous waste.
By burning off these contaminants with shock, more of the subsequently added chlorine will go toward disinfecting the water rather than reacting with chemical waste.
Think of shocking as paving the way so the majority of the chlorine you add can be used for its intended purpose of destroying pathogens.
To Remove Excess Organic Matter
Shocking helps remove excess organic matter in pool water that contributes to chlorine demand issues. Bathers introduce many forms of organic matter into the water including:
- Mucus
- Saliva
- Sweat
- Skin particles
- Hair and fur
- Lotion residue
- Personal care products
- Leaf debris
- Insect droppings
- Dirt
- Pollen
Organic matter reacts with chlorine rapidly. Removing it through oxidation allows the chlorine to work more efficiently and reduces total chlorine demand.
Less total chlorine needs to be added if the organic matter level is kept in check with regular shocking.
To Prevent Clogged Filters and Pumps
Shocking helps prevent clogged filters, pumps, and other pool equipment by oxidizing contaminants. Some reasons pool equipment can clog include:
- Buildup of skin, hair, and particles in the filter and pump
- Algae growth coating filters and pumps
- Scale formation from metals coating equipment
- Oils from swimmers accumulating
Shock oxidizes many contaminants and particles, allowing the filter to remove them from the water so they don’t stick to the filter media over time.
Pumps and other components stay clearer longer when shocking is performed consistently to remove gunk before it can build up.
To Restore Water After Environmental Issues
Shocking can help restore pool water after environmental issues that negatively impact the chemistry and appearance. Situations where shocking may be needed include:
- After heavy wind or rainstorms that carry in debris
- Following dust storms or wildfires that can contaminate the water with smoke, sediment, and ash
- After long periods of neglect where algae or an unsafe chlorine level developed
- Following nearby construction work that loosened soil into the water
Shocking helps burn off the contaminants, particles, and dissolved compounds to restore water quality after these external issues.
To Remove Yellowing and Discoloration
Shock can help remove yellowing, green coloring, or other discoloration issues in pool water. Some potential causes include:
- Low pH allowing metals like iron, copper, or manganese to precipitate and stain
- Algae growth
- Too much stabilizer leading to chlorine being unable to sanitize properly
- High calcium hardness leading to scale formation
- Imbalanced water chemistry corroding metal components
Shocking helps destroy algae, oxidize dissolved metals so they can be filtered out, and burn off excess stabilizer and contaminants that deplete chlorine. Removing the root cause of the discoloration through shock allows the water to return to a clear blue.
To Remove Odors
Shock is useful for removing unpleasant odors coming from pool water. Some potential sources of odors include:
- Chloramines from the combination of chlorine and contaminants
- Buildup of nitrogenous compounds like urine, sweat, and mucus
- Algae growth
- Rotting organic matter
- Improperly stored chemicals
Shocking the pool water helps destroy the source of the odor so swimmers are not bothered by bad smells:
- Oxidizes chloramines to remove “chlorine odor”
- Oxidizes urine, sweat, and other nitrogen compounds
- Kills algae so it does not give off smell as it decomposes
- Destroys rotting matter
Regular shocking prevents odor issues by burning out contaminants before they have a chance to break down and smell.
To Prevent Growth of Biofilms
Shocking helps prevent growth of biofilms in pools. Biofilms are slimy films of bacteria and algae that coat pool surfaces. They provide protection for disease causing germs and create areas resistant to chlorine disinfection.
Biofilms lead to:
- Bacteria reservoirs even when rest of pool water is properly sanitized
- Corrosion and damage to pool surfaces and equipment
- Cloudy water
- Increased need for scrubbing and manual removal
Shocking produces radical oxidizers that penetrate and destroy biofilm layers before they create health hazards. Regular shocking as part of your maintenance program minimizes biofilm formation.
To Prevent Swimmer Discomfort
Shocking helps prevent swimmer discomfort issues like itchy skin and eyes, and illness. Problems swimmers may face in a poorly maintained pool include:
- Itchy, red eyes from chloramines and irritation
- Dry, irritated skin
- Itchy skin rashes
- Coughing or nasal congestion from breathing irritants
- Gastrointestinal illness from ingesting pathogens
- Ear infections
Shocking destroys contaminants that cause irritation and illnesses, leading to a more comfortable and healthy swim.
Conclusion
There are many important reasons to shock a swimming pool on a regular schedule. Shocking kills bacteria and viruses to destroy disease-causing pathogens. It removes chloramines and nitrogen compounds that deplete chlorine sanitizer. Shock oxidizes organic contaminants, preventing them from overwhelming the chlorine and forcing high consumption. It also burns off algae and prevents growth and spreading throughout the pool. Other benefits include maintaining proper chlorine levels, improving filtration, preventing equipment clogs, enhancing water clarity, removing odors and discoloration, and preventing uncomfortable conditions for swimmers. By continually removing bather wastes and other contaminants through oxidation shocks, the pool water stays properly sanitized, clear, and ideal for swimming.
