Does Chlorine Evaporate From Pool Water?

Does Chlorine Evaporate From Pool Water

Swimming in a pool is a fun and relaxing way to spend your summer days. The only reason we are able to safely swim in pools is thanks to chlorine, a chemical that can disinfect the bacteria and pathogens in the water so that it won’t spread.

Pool owners must perform constant maintenance to ensure that the amount of chlorine and other chemicals in the pool are within a safe range. If you’re not paying attention, the free chlorine in the pool can drop to unsafe levels where it is no longer able to disinfect the pool and keep it sanitary.

Chlorine naturally gets used up over time, and some factors that can speed up this process are exposure to sunlight, hours of use, and bacteria being introduced from the outside. On average, chlorine can last around four and a half days before more needs to be added. 

You can increase how long chlorine lasts by requiring guests shower before entering, using a pool cover when the pool is not in use, adding a chlorine stabilizer, and not peeing in the pool.

In this article, we’ll discuss in more detail the factors that cause chlorine to be used up more quickly and how you can ensure the chlorine in the pool lasts longer.

Why is chlorine so important?

To better understand why chlorine evaporating from the pool or getting used up quickly is such a serious issue, we need to discuss what chlorine does for us in the first place.

To be blunt, without chlorine, the pool will become a cesspool of bacteria and algae within 24-48 hours. Seriously, the water will turn green within that period of time.

If you have a small kiddie pool, then you probably don’t care enough to add chemicals to the pool since you can just dump the water out and refill it easily each time. For pool owners with a 20,000 gallon pool, this is not as feasible.

Rather than dumping out the water after each use, pool owners must disinfect the water with chemicals, and chlorine does that job fantastically. Chlorine reacts with the cell walls of bacteria and algae, rupturing them, and killing these unwanted organisms.

Swimming in a pool without chlorine is like swimming in somebody’s used bath water. Actually, considering that many people like to pee in the pool, and that the average person has 0.14 g of poop on their body, a better comparison is that it’s like swimming in toilet water.

Chlorine is one of the chemicals that water treatment plants use to treat dirty water so that it is safe to drink. You can trust it to make your swimming pool safe to swim in. That is why you must keep the free chlorine levels around 1-3 ppm to ensure it is sanitizing the water effectively.

How fast does chlorine evaporate from a pool?

As mentioned, chlorine can completely evaporate from a pool in four and a half days on average, possibly sooner or longer, based on the following factors:


One of the fastest ways to naturally deplete chlorine levels is to expose it to sunlight. Chlorine reacts to the ultraviolet radiation produced by the sun, causing it to fall apart on a molecular level. It turns into a gas that is then released into the atmosphere.

The degradation caused by sunlight is incredibly high. After just a couple hours of exposure, 90% of the chlorine can evaporate from the pool. That is why at outdoor public pools, you will notice service technicians adding chlorine and stabilizers frequently.

So if you ever find yourself in a situation where you accidentally added too much chlorine, just leave your pool exposed to the sun for an hour or so and the chlorine levels will be reduced significantly.

If you aren’t using the pool, it’s a good idea to cover it up so that sunlight won’t evaporate all of the chlorine.


In a similar vein, high temperatures also indirectly break down chlorine. Since bacteria multiplies more rapidly in warm water, there will be an increased number of bacteria that the chlorine needs to eliminate.

When the free chlorine in the pool eliminates bacteria, it becomes combined chlorine, meaning it is effectively used up. That is why we care so much about free chlorine – the chlorine that hasn’t been used up and will continue to sanitize the pool.

You should strive to keep the free chlorine level higher than the combined chlorine. You can determine the combined chlorine level by taking the total chlorine and subtracting it by the free chlorine.

Anyways, when it’s warm outside, there is a general rule pool owners follow. You should add double the amount of chlorine you normally would for every 10 degrees Fahrenheit the temperature climbs above 80 degrees.

Frequency of pool use

Another factor that can cause chlorine to be used up quickly is how often the pool is being used. The more often a pool is used, the more bacteria and other contaminants enter the pool and cause the free chlorine to be used up.

Introduction of contaminants

Expanding on the above point, chlorine reacts with all substances it comes into contact with. Therefore, if a swimmer is wearing lots of lotion, sunscreen, hair products, oils, and so on, then these substances will use up the free chlorine in the pool.

Since swimming is exercise, you will naturally sweat a lot. Some people, when they feel the urge to pee, simply pee in the pool. These all introduce bacteria that chlorine must clean up.

Furthermore, the presence of leaves, insects, and dirt in the pool can also exhaust the free chlorine in the pool. Basically, free chlorine is used up when anything happens.

Why you must keep chlorine levels high

Knowing how quickly chlorine can be used up in the pool, pool owners must be ready to check the chlorine levels at least 2-3 times a week to ensure the free chlorine level is between 1-3 ppm.

Once the free chlorine drops below that, it’s time to add some chlorine. You can easily calculate how much chlorine you need to add by using an online pool calculator.

After adding chlorine, you should wait at least 2-4 hours before swimming in the pool. Read the label on your brand of chlorine to see what your requirements are.

How to help chlorine last longer

Now that you know the most common ways that chlorine is used up, you have a better understanding of how to prevent most of these issues. Here is how you can keep chlorine from depleting so quickly:

  • Shower before entering the pool. This not only applies to you, but the rest of the family and friends that want to swim in your pool. This gets rid of contaminants that would deplete your chlorine.
  • Use a pool cover. The sun can rapidly deplete chlorine, so always cover it up when it’s not in use.
  • Use a chlorine stabilizer. The best time to swim is when the sun’s out, so you need to add cyanuric acid, a chlorine stabilizer, to decrease the sun’s impact on chlorine. The stabilizer level should be kept around 30-150 ppm.
  • Use the bathroom before entering the pool. Same idea as showering before entering the pool. Most people are too lazy to get out of the pool to pee, so they just go in the pool. That’s unsanitary and uses up chlorine. Pee before you enter the pool instead.
  • Wear a swimming cap. Hair products degrade chlorine, but chlorine can also degrade your hair’s health. So why not protect both by wearing a swim cap to keep water from touching your hair, and your hair from touching the water.

The bottom line

The average pool owner adds chlorine to their pool every 3-7 days, though you should be testing the water 2-3 times in a week to ensure the water is still safe to swim in.

There are numerous factors that can speed up how quickly the chlorine is used up in the pool, such as UV exposure, heat, frequency of use, and introduction of contaminants. However, once you’re aware of these factors, you can work to reduce their harmful effects.

By showering and using the bathroom before entering the pool, adding cyanuric acid to decrease the sun’s effects on chlorine, wearing a swimming cap, and covering the pool up when it’s not in use, you can ensure the pool water lasts on the higher end of the 3-7 day range.