How to Lower Free Chlorine

If you shock the pool too close to activity time and guests are coming but the Free Chlorine residual level is still too high, there are a few things that you can do to reduce it and bring it down faster to a safer level before those guests arrive and want to swim. How do you lower free chlorine in your pool?

  • Let filter run
  • Pull pool tablets from floaters
  • Scrub sides of walls with a long handle brush to agitate pool H2O
  • Add a Chlorine Neutralizer like
    Sodium Thiosulfate or Hydrogen Peroxide
  • Add H2O to dilute the pool
  • CL levels of 6 ppm or higher are unsafe.
  • Repeat until residual is safe to swim in approx. 5 ppm

Pools will naturally gas-off chlorine from the surface, and very high levels can irritate airways and lungs when inhaled for prolonged periods, especially indoor pools where there is no way for that gas to escape. Sometimes Public pools at campsites or Hotels will bump their Chlorine residual up to 5 ppm in the morning in anticipation of expecting a crowd for the day.


Maximum Chlorine Level for Swimming

Normal pool-free chlorine levels are usually around 1.0 ppm to 3.0 ppm, depending on the level of stabilizer in the water. If your pool water is much higher than that and you are spending a lot of time exposed to it, then it could have some effect on you even at these lower concentrations.

Be on the safe side. If you decide to shock your swimming pool then the best time to do that is the night before you are planning to use it. This is for a couple of different reasons. When the Chlorine Residual climbs above 5-6 ppm it can cause skin and eye irritation. Just above the waterline, the chlorine turns to gas from the agitation of the water.

The chlorine gas moves from the pool water to the air. This may cause some breathing problems, so be on the safe side and wait until the Chlorine retest below 5 ppm. It will naturally when:

  1. The Sun starts to burn off Chlorine Residual
  2. Activity in the pool depletes Chlorine Residual

When you super-chlorinate or Shock your pool you are looking to get your chlorinate at high levels of 10 and above and with a normal tester you’ll get an orange or red-colored reading that won’t be readable but you can safely assume the reading is high enough to work.

The same goes for Strip Testers normally 10 is the highest you will see. You can dilute the sample with half pool water and half drinking water and just multiply it by two.


What Happens When You Pee In the Pool

What is Sodium Thiosulfate

After you shock the pool which can bring maximum levels of Chlorine Residual, it will take some time to bring the Chlorine levels back to where it’s safe to swim in. To bring that residual back to a normal reading of 1 to 3 ppm will happen eventually when the sun starts to burn it off. If you don’t have the time then you can do a few things to get the chlorine into a gaseous form to leave the pool water a little faster. Shake it Up!

  • Turn the filter on and start moving the pool water around. This will help agitate the water and release chlorine in gas form
  • Scrub the sides of the walls of the pool as this really helps move the water around and you’ll even smell the Chlorine coming out into the air around the pool area.
  • Pull the pool tablets from the floaters
  • Add a Chlorine Neutralizer like Sodium Thiosulfate
  • Does baking soda neutralize chlorine?  NO- it will raise the alkalinity in your pool water but will not neutralize the high chlorine in your pool.

Chemicals like Sodium Thiosulfate which is sold in Pool Supply stores will deplete the extra Chlorine in a little time.

Initial Dosage: 2 oz. per 10,000 gallons will lower chlorine level by 1 ppm

These inexpensive chlorine neutralizers work so well they bring your water to clarity and make it crystal clear. Follow instructions for the best possible results.

Typically, chlorine neutralizer for swimming pools contains sodium thiosulfate which acts to quickly lower chlorine concentrations. Nothing turns swimmers away faster than the strong chemical effects that are caused by chlorine.

Neutralizers like Sodium Thiosulfate which is the active ingredient will work with Chlorine or Bromine. This neutralizer will work fast and with little of it so don’t overdo it just follow directions. For Sodium Thiosulfate, add about a cup (8 oz) of the dry white crystals per 5,000 gallons of pool water, or about 1 oz per 1000 gallons –

to lower chlorine by 10 ppm. Check your pool water pH and adjust to within label instructions, and add directly to the pool or spa. The Sodium Thiosulfate that you add to the swimming pool will dissipate from most pools usually within a few days.


According to EPA Chlorine and bromine are unique in their ability to provide disinfection. EPA requires all US facilities that treat water to maintain a chlorine residual of no more than 4 parts per million, whether chlorine is used as a primary disinfectant or not. Chlorine gas is a respiratory irritant that the human nose can detect at very low levels (0.2-0.4 parts per million in air. Although Chlorine is necessary it’s also dangerous.



When Should You Shock Your Pool


Symptoms of Chlorine Poisoning

When too much chlorine is added to the water, a safe pool can become a pit of harmful chemicals. In fact, over-chlorinated pools threaten swimmers’ health in two ways. First, contact with the skin can cause irritation similar to a burning sensation.

Also, the gases given off by the evaporation of water oversaturated with chlorine-based cleaning solutions can enter the nose and eyes to cause severe itchiness and discomfort. While cases are usually few and far between, chlorine poisoning has a few telltale signs that you should be on the lookout for:

  • Sudden onset of nausea and vomiting
  • Burning sensation in the throat
  • Itchy eyes
  • Difficulty or shallow breathing
  • Skin redness
  • Dull chest pain.

As is the case for any health event that threatens the health of your lungs and eyes, the “wait and see” method doesn’t hold much water. As soon as you notice the signs of chlorine poisoning, it’s better to be safe than sorry by taking yourself or your kids straight to your nearest CareWell Urgent Care center.

I recommend that parents on vacation check the Ph and Chlorine Residual with a reliable Test kit like this sold on Amazon before using any Public Pool Facility or Indoor-Outdoor Water Park. You have the right to know what your kids will be swimming in all day. So buy a test Strip or Testing Kit and carry it with you. Make sure you are testing the pool water with reliable reagents and equipment.


When using pool chemicals wait for the chemical to work so you don’t add too much or too little-read the article on MyWaterEarth&Sky called How Long Does It Take For Pool Chemicals to Work?




  Urgent Care-Why is too much chlorine in pools so dangerous?


EPARecreational Water Quality Criteria and Methods


JimGalloway Author/Editor



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