How to Raise Free Chlorine in Pool

Chlorine is commonly used as a chemical ingredient in a variety of products, because of its capabilities to disinfect and bleach. It is effective in killing bacteria and microorganisms in water.

Without chlorine added, your swimming pool would be full of bacteria and algae. The water would be unhealthy and the water may appear cloudy with a greenish color.

If you own a pool or work where you must maintain pools, it is important to understand how chlorine works, so that you can calculate and make use of the right amount, all the time.

Too little, then your pool becomes laden with bacteria and other unwanted contaminants.

Too much, and your skin and eyes are most likely going to become very irritated.

Raising the level of free chlorine in a pool may be required when it begins to drop.  There are two main processes that you can use to increase the free chlorine level: shocking with chlorine and using a non-chlorine oxidizer.

The fascination of chlorine is in a simple chemical reaction. Immediately the chlorine solution is mixed into your pool water, it breaks down into two critical molecules: hypochlorous acid and hypochlorite ion.

How to Raise Free Chlorine in Pool

These two chemicals work by attacking and breaking down the lipids in the cell walls of microorganisms, killing them and rendering them harmless. It takes less than an hour for the process to complete. Both chemicals are vital to the hygiene of your pool.

Types of chlorine

The free chlorine

This is the amount of chlorine in the water that is available to sanitise contaminants.

The combined chlorine

Also known as chloramines, this is the chlorine that has combined directly with the contaminants.

The total chlorine

This is the sum of the free and combined chlorine.

Total chlorine = Free chlorine + Combined chlorine

In this article, we will focus on free chlorine.

Free chlorine

Free chlorine is of great importance for monitoring pool water. It is the amount of chlorine measured in parts per million (ppm) that is available to eliminate harmful microbes and neutralize contaminants.

If there is not enough free chlorine, it could be impossible to get rid of the contaminants that have taken over the water.

What happens if free chlorine is low?

You might be familiar with at least one of the common side-effects of swimming: sore eyes, skin irritations, fading swimming costumes, and that odd smell that lingers until we take a shower. Those are some of the effects of low levels of free chlorine in the water.

When free chlorine levels are low, the little amount left in the pool water will be used up quickly by fast-growing microorganisms like bacteria, and this makes the pool unsafe for swimming.

What causes free chlorine levels in pools to drop?

Free chlorine gets low in the pool due to excessive sunlight, high bather load, improper water chemistry, the running time of the pool, and the filtration system used in the pool.

 Excessive sunlight

Sunlight and heat affect pool chlorine. Ultraviolet rays can reduce chlorine in pool water as it heats the water. Warmer water tends to breed more bacteria, and so the pool’s chlorine gets used up faster and must be replenished more frequently

It is safe to say that chlorine reacts when it’s hit by sunlight and by scorching summer temperatures.

How fast does chlorine evaporate from a pool? 

Just two hours of sunlight on a beautiful day can reduce the chlorine level in your pool by as much as 90 percent. That’s why you see pool service technicians adding chlorine and stabilizers regularly. 

High bather loads

Bather load in the swimming pool industry is a term that describes the number of people ‘bathing’ in the pool during a given amount of time. A rough rule-of-thumb is that the chlorine demand from bather load in swimming pools is around 4 grams of chlorine per person-hour.

If the population of the ‘bathers’ increases, it may cause the amount of available free chlorine to drop quickly.

Improper water chemistry

An imbalance in the water chemistry of a pool can affect its level of free chlorine. When you carry out testing and find out that your free chlorine reading is lower than the total chlorine level, there is a problem. There are contaminants in the pool.

Any oddity in the ideal measures of the chemical levels of pH, alkalinity, and calcium hardness of the pool water will affect the level of available free chlorine.

The running time of the pool

When you do not run the pool pump for at least 8 hours, every 24 hours, the pool water will not properly circulate and keep clean. Circulation helps to distribute free chlorine and then filter out unwanted materials from the water. 

The pool’s filtration system

Proper water circulation and filtration are paramount to creating a healthy environment in the pool. When the pool’s filter is faulty, it can affect water circulation which may lead to a buildup as the skimmer will not be effective in removing debris. This in turn causes the free chlorine level to drop.

How to increase the level of good free chlorine in pools

Shocking your pool to reach breakpoint chlorination

Shocking is the process of adding chemicals, usually, chlorine, to pool water. This will break apart chloramines (combined chlorine), raise chlorine levels quickly and kill algae, bacteria or other harmful pathogens. 

To reach the breakpoint, sufficient chlorine must be added to the pool to raise the free available chlorine level. The goal is to raise the level to roughly 10 times the amount of combined chlorine level.

When shocking is done wrongly and you do not reach the breakpoint, it may lead to even more combined chlorine (chloramines), in the pool.

It is preferable to shock the pool at dusk or nighttime because doing so during the day will cause the sun to burn off unstabilized chlorine.

Making use of a non-chlorine oxidizer

Traditionally, chlorine is used in large amounts when you need to shock a pool. When chlorine levels in the pool get low, chlorinated pool shock treatment is added to restore the balance.

Alternatively, we have non-chlorine pool shock (aka Potassium Monopersulfate, or MPS).  Non-chlorine shock contains a combination of oxygen and potassium. It is an ideal shock alternative for chlorine-free pools that do not make use of chlorine as their sanitizer. 

A non-chlorine shock does not disinfect the water. It is mainly intended to oxidize the water and help remove the contaminants, which then clears the cloudy appearance of the pool water. The power of the active oxygen generated by the non-chlorine shock creates free chlorine, allowing it to kill bacteria and stay effective longer.

How do you raise free chlorine without raising total chlorine?

We can raise the level of free chlorine, without raising the total chlorine level as well, by simply shocking the pool to reach breakpoint chlorination.

Frequently Asked Questions (FAQs)

Is continuous exposure to chlorine in pool water damaging to the eyes?

When your eyes come in contact with the chlorine in pool water, the tear film that defends your cornea is washed away, consequently making your eyes less protected from dirt or bacteria that may exist in the pool. So, swimmers may develop eye infections. It would be best to wear goggles when you swim.

What is the difference between free chlorine and shock?

Free chlorine is the standard chlorine that you use to sanitize your pool. When you shock a pool, you are making use of more amount or dose of chlorine to raise the free chlorine level in the water, quickly. Free chlorine keeps the water clean, whereas shocking is the process of applying chemicals to purify the water in the pool.


Every pool is unique – and not just in shape and size, but in how much chlorine is needed to keep the water clean. Keeping your free chlorine at the proper level will help your pool water be safer to swim in, keep it clear and make it pleasant to look at, as well.

Chlorine helps keep the pool water clean by preventing the growth of algae, attacking and neutralizing harmful bacteria and microorganisms.

To maintain a proper water balance, both the free chlorine and combined chlorine should be measured moderately. If the free chlorine level is too low, the pool is not safe to swim in. A high reading level of combined chlorine indicates that the pool is flooded with too many unwanted materials.

Increasing the level of free chlorine can help you check the prevalence of contaminants in the pool water, and it is possible to achieve this either through a shock or by adding non-chlorine oxidizers.

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