How Sunlight Can Affect the Chlorine in Your Swimming Pool

You probably already know chlorine is the most popular chemical when it comes to keeping harmful bacteria in swimming pools at bay. But did you know many things affect a swimming pool’s chlorine levels, including the sun.

Well, if you didn’t and if you’re curious, you’re at the right place. Read on to know all about the role that the sun plays in altering chlorine levels inside a swimming pool. So, irrespective of whether you own a concrete or a fibreglass pool, let’s take a closer look at how chlorine inside a pool is measured.

Measuring chlorine

Once the chlorine solution hits the surface of the pool water and proceeds to make its way inside, it performs two roles. One role involves reacting with the organic matter already present inside the water (bacteria, algae, etc.). Another involves disinfecting the water. That’s why chlorine is measured in terms of free chlorine (the water disinfectant) and combined chlorine (the part that reacts with organic matter). 

While combined chlorine levels show how many pool invaders there are, free chlorine levels show how much chlorine is available to disinfect the water. Free chlorine levels should be at least one part per million. Anything below that means it isn’t such a good idea to swim in the water. So, pool owners must replenish free chlorine from occasionally ensure proper levels of it. Free chlorine is known scientifically as hypochlorite ions – one of several chemicals that chlorine breaks down into once it mixes with water. 

How the sun affects chlorine inside

The sun affects a pool’s free chlorine or hypochlorite ions in two ways – through heat and light. Let’s start with the heat aspect.

On sunny days, particularly during the summer months, the water of your pool will consistently stay warm. It’s in this environment that bacteria thrives. As more and more bacteria make the pool water their home, greater levels of free chlorine are used up. If the depleted free chlorine levels aren’t replenished, bacteria will rapidly take over the pool and render the water unsafe for swimming. 

Sunlight consists of ultraviolet radiation, and the UV rays emitted by the sun breaks down hypochlorite ions. As a result of this breakdown, chlorine gas is formed, which is then released into the atmosphere. The sunniest days only take 2 hours to deplete almost 90 percent of the total chlorine level. Here is an informative guide to help you with the alkalinity in your pool!

What to do to keep the sun from depleting chlorine

The best way to keep free chlorine levels from being depleted by the sun’s heat and light is to use a chemical called cyanuric acid. When cyanuric acid interacts with free chlorine, it forms a stable compound that isn’t significantly affected by sunlight. It can also release free chlorine, which makes it great as a disinfection reservoir that stays unaffected in sunlight’s presence.

There are multiple ways of adding cyanuric acid to swimming pools. You can choose to either add liquid or powder versions of the chemical, or buy stabilized chlorine, which contains cyanuric acid. If you buy chlorine without cyanuric acid, remember to add it manually.

The rule of thumb in terms of cyanuric acid use is to add 1 lb (453.59 g) of the chemical, which raises chlorine levels by 10ppm for every 10,000 gallons. However, different cyanuric acid products are available in different strengths. So, make sure you read the instructions on the packaging carefully before you add it to the pool.

You could simply add chlorine as well. However, it’s best to use cyanuric acid as adding excessive chlorine can disturb the pool chemistry. You should keep a regular check on the free chlorine levels in your pool to know when the addition of the chemical is required.

How to add cyanuric acid

In this section, we’ll tell you all the steps you need to take to add cyanuric acid to your concrete, vinyl liner or fibreglass pool.

  • Liquid cyanuric acid: You can pour liquid cyanuric acid directly into your pool’s filter box. Be careful that you don’t add a lot in at once, as it may lead to a disturbance in the pool chemistry. It’s best to keep adding small amounts from time to time and check the pool water chemistry regularly to see if everything’s at the right levels.
  • Granular cyanuric acid: First, put the granular cyanuric acid in a skimmer sock. Next, use a pole for hanging it in the return jet’s front area. You could also place the powdered chemical in the skimmer box or mix it with warm water and then pour it into the box. However, you should keep in mind that pouring the chemical into the skimmer box can have negative consequences. For starters, the pool floor may become full of granules, which may also damage the pool liners. While you can backwash your pool to remove the granules, you have to give the powder some time (2 – 5 days) to dissolve. If you backwash the pool before this period, you may end up backwashing the chemical as well, rendering it useless.

Irrespective of the form of the chemical you use, finish the adding process by turning on the pool pump’s ‘filter’ or ‘circulate’ setting. If you’re using the chemical’s liquid form, it will only take a few hours for the cyanuric acid to dissolve. However, the granular form of the chemical can take up to 48 hours to dissolve. To make sure that it dissolves entirely, run your pool pump at least 4 hours everyday for a whole week. Feel free to test the water whenever you want and if there’s a lack of cyanuric acid, you can repeat the process.


So, if you have an open pool that’s directly exposed to sunlight, you should use cyanuric acid to reduce the sun’s effects on chlorine. You should also note that if you sanitize your pool using UV and/or ozone instead of chlorine, you won’t need to add cyanuric acid. It’s a product that’s strictly meant for chlorinated pools. With that being said, it’s time for us to sign off, and time for you to take better care of your fibreglass pool.