Free Chlorine Too High - How To Lower It?

Too Much Chlorine In Pool - How To Lower Free Chlorine?

Chlorine is essential for pools and works to keep them clean and sanitary, but it is possible to have too much chlorine in a pool. If free chlorine is too high in your pool, it can put swimmers at risk of health effects and also affect other parts of your pool’s chemistry.

In this article we will talk about ideal chlorine levels, the causes of high chlorine, signs of too much chlorine in a pool, and how to lower chlorine in a pool.

Ideal Chlorine Levels For Pools

Ideal chlorine levels for swimming pools range from 2-4 parts per million (ppm). At these levels, chlorine is capable of killing off potentially threatening bacteria and algae without being harmful to swimmers.

Causes Of High Free Chlorine In Pools

If you have high free chlorine levels in your pool, there are a few possible causes.

You Are Adding Too Much Chlorine

If you run a traditional chlorine pool, then it means you are adding more chlorine than necessary. Be sure to check the dosage instructions for your particular chlorine product and calculate an ideal dosage based on the information provided.

Remember that all pools have different levels of chlorine demand. Even in pools of the same volume, much more chlorine is required by pools that are used frequently or that are fully exposed to direct sunlight during the majority of the day.

Always test your free chlorine levels before adding any more chlorine to your pool. Even if your product’s packaging says to add a certain dose regularly, you should reduce this amount if using it is creating undesirably high chlorine levels in your pool.

Your Chlorinator Is Generating Too Much Chlorine

In a salt water pool, the salt chlorinator is the primary source of chlorine. If your chlorinator is malfunctioning and producing excess chlorine, this will show up as a high free chlorine concentration.

You will need to consult the troubleshooting guide for your unit to see why it might be malfunctioning, but I would start by testing the salt level to see if that has gotten out of balance.

You Have Recently Shocked Your Pool

The use of chlorine shock treatments will cause high chlorine levels in a pool, but it is important to note that these are very temporary, and often do not carry the same side effects as prolonged high chlorine levels.

Chlorine shock treatments are usually added in response to a high load of organic debris or an algae outbreak, so the added chlorine is usually used up quickly, typically in less than 24 hours.

While your pool may show high free chlorine readings after introducing this product, these should slowly return to normal as your pump operates and the chlorine circulates.

Why Too Much Chlorine Is A Bad Thing

High chlorine levels increase the likelihood of many different types of irritation for swimmers, including the emergence of a chlorine rash.

Chlorine rash can be so severe that some people mistake it for an allergic reaction, but it is actually a result of high amounts of skin irritation. You can read more about the effects of chlorine rash here.

If your chlorine levels are especially high, swimmers that come in contact with the water for an extended period of time can also develop symptoms of chlorine poisoning.

Chlorine poisoning can consist of many symptoms, ranging from nausea and vomiting to asthma attacks and other breathing problems.

Abnormally low pH readings can also result from too much chlorine in a pool. Because stabilised forms of pool chlorine contain cyanuric acid, high chlorine levels tend to create acidic water in your pool.

This water can then go on to cause other problems that are associated with low pH, such as corrosion.

If you measure your ORP level, you'll start getting excessively high readings (above the recommended upper limit of 750 mV) if you have excessive free chlorine.

How To Lower Chlorine In A Pool

There are a few different ways that you can lower the amount of chlorine in a pool. Although highly specialised and effective products exist for this purpose, non-chemical solutions to this problem are available, they are just more time consuming.

Allow The Chlorine To Deteriorate

Free chlorine molecules are naturally very fragile. These molecules break down in the sunlight and when exposed to heat, and they are also used up as they work to remove active contaminants from your water.

Because chlorine has these properties that allow it to degrade naturally over time, many pool owners simply wait out their high chlorine levels.

By waiting for your chlorine levels to go down, you can avoid spending money on chemical remedies.

The main downside to this method is that it can be much more time-consuming than other treatments. If you are hoping to use your pool soon, you may want to consider a different option.

Use A Chlorine Neutraliser

Chlorine neutralisers, sometimes called chlorine removers, are composed of a compound known as sodium thiosulphate. This product is capable of quickly lowering the chlorine levels of your pool without other unwanted side effects for your pool’s chemistry.

One main benefit of using sodium thiosulphate is that it is also safe for swimmers. Depending on the concentration of your specific product, swimming may even be possible during treatment time. Always follow instructions on your specific product’s packaging, as variations in safety information do occur.

Lo-Chlor and Zodiac both produce chlorine removers. They are highly potent, with only 50 grams being required to lower chlorine levels by 1 part per million (ppm) in a swimming pool of 50,000 litres.

Regardless of how high your chlorine levels have risen, this product will be highly effective for reducing your levels.

Other Chemical Options For Lowering Chlorine

Some sources also recommend using hydrogen peroxide or ascorbic acid in order to break down the chlorine in your pool faster. Although these methods can work, they are much less reliable than using sodium thiosulphate.

Because hydrogen peroxide and ascorbic acid tend to mess with other chemical levels in your pool, such as the pH and total alkalinity, using them will necessitate the use of pH altering chemicals both before and after use.

This approach is therefore more likely to be a much more expensive and time-consuming option in the long run.

How To Use A Chlorine Neutraliser

Before using a chlorine remover, you will need to test your pool’s current levels of free chlorine. Then, using the information provided on the packaging of your chlorine neutralizer, calculate the correct dose of product for your pool.

Most chlorine removers can be added directly to your pool’s water, and do not require mixing.

You should add this product while the pump and filter is running so that it can circulate fully throughout the pool. After allowing your pool to circulate for 24 hours, you should retest your chlorine levels.

Be sure to note that there are variations among different chlorine neutralising products. Always follow the instructions provided on your product packaging, including abiding by any recommended waiting periods.


Too much chlorine in your pool can definitely be a bad thing. In addition to causing skin irritation and being a potential source of toxicity for swimmers, high chlorine levels can also cause the pH of your pool to drop, creating acidic conditions.

Do you have any questions about having too much chlorine in your pool? Reach out to us in the comment section, we’d love to help!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.