How to reduce chlorine levels in swimming pools

Managing the chemical balance in swimming pools can be a challenge, but lowering high chlorine levels is relatively simple. Indoor pools may present more difficulty in managing this issue, but there are still numerous ways to address it. One option is to use ultraviolet (UV) lighting systems to gradually reduce chlorine levels without contaminating the water.

Understanding the "chlorine smell" and eye irritation. Many people assume that the chemical smell or eye irritation in the pool is due to chlorine. However, these symptoms typically occur when chlorine has broken down into other chemicals. In such cases, you may need to shock the pool to increase the chlorine levels. Alternatively, it is better to use a chemical testing kit to accurately measure the chlorine concentration as described below.

Use a pool chemical test kit. You can purchase a test kit from pool supply stores to measure chlorine levels. Ensure the kit can measure both free available chlorine (FAC) and total chlorine levels.

• According to general standards, free chlorine in water should be between 1 and 3 ppm. The total chlorine level should not exceed the free chlorine level by more than 0.2 ppm. Different regions may have varying standards for these levels.
• If you are using ozone or UV light (ultraviolet) for disinfection, the free chlorine level could be as low as 0.5 ppm.

Eliminate the chlorine source. If the chlorine concentration in the water is only slightly high (around 4–5 ppm), there's no need for chemicals. Simply stop adding chlorine to the pool to resolve the issue.

• To stop adding chlorine to the pool, turn off the chlorine disinfecting system, chlorine feeder, or salt chlorine generator; remove chlorine tablets from the skimmer or floating chlorine dispenser. If you're unsure which chlorine system your pool uses, ask the owner or manager.

Open the outdoor pool's cover. UV rays from sunlight can break down chlorine rapidly. A sunny afternoon can remove up to 90% of chlorine in the pool if all chlorine sources are cut off.

• Using UV lights in the pool is generally not as effective as sunlight in this step. Please refer to the UV method below for more information.

Swim when chlorine levels are safe. Swimming can help lower chlorine levels, but this method should only be used when chlorine levels are not excessively high (around 4 ppm). There is much debate about the chlorine level that poses a danger to swimmers. Public pools typically set the limit at 10 ppm, with some places setting a safer threshold at 5 ppm.

• Do not swim if your chemical test kit shows abnormal results, such as improper pH or alkalinity levels in the water.
• Do not swim if you detect a strong "chlorine" odor (and testing confirms high chlorine levels). This smell is actually caused by chloramines, an irritant chemical.
• Chlorine can affect the lungs, which is more dangerous in poorly ventilated areas and when swimmers have respiratory conditions.

Replace some of the pool water. This method is costly and slow, but it can help reduce chlorine levels. Drain about ⅓ to ½ of the pool water and replace it with fresh water. After refilling, it may take some time for chlorine and pH levels to return to normal.

• If you have a filter with a backwash option, this is the quickest way to remove (some) of the pool water.

Check regularly. Inspect your pool once or twice a day, or every few hours if it is in use. If the chlorine level does not decrease after a few days, try one of the methods listed below.

• See the information below for other chemical test results, such as pH or cyanuric acid levels. If the results are too high or too low and do not return to normal soon, you should consider hiring a professional to treat the pool.

Purchase chlorine neutralizer from a pool supply store. Consult with a sales associate if you're unsure which product to choose, and avoid using chemicals from other sources. Pool supply stores offer chemicals specifically formulated for pools.

• Sodium thiosulfate is probably the most common chlorine neutralizer, but you should use it with caution.
• Hydrogen peroxide is often the most cost-effective option; this chemical breaks down into harmless particles, but it is less effective if the pool's pH is below 7.0.

Temporarily stop using the pool. Be sure not to add chemicals to the pool when it is in use. If the pool is frequently used by others, place a visible warning sign in a noticeable area.

Follow safety warnings. Many pool chemicals can cause harm when they come into contact with the lungs, eyes, or skin. Be sure to review the following guidelines before proceeding:

• Carefully read the product instructions to understand safe usage, necessary protective equipment, and emergency procedures.
• Store pool chemicals in a cool, dry place, away from sunlight, high humidity, and temperature extremes. Never store acids and chlorine together, and avoid placing dry chemicals near or beneath liquid chemicals.
• Open only one chemical container at a time, close it, and store it properly before opening another container.

Calculate the required amount of chemicals. Always follow the product instructions to determine the necessary dosage and add it to the pool properly. Different chemicals come in various forms and concentrations, so general guidelines may not apply to all situations.

• Generally, you can add 15 ml of sodium thiosulfate for every 3,800L of water.
• For public pool treatment, more precise calculations are necessary. 77 ml of sodium thiosulfate will reduce chlorine levels by 1 ppm in 37,900 L of water. Pool supply store staff can assist you with calculations, or you can use an online chlorine reduction calculator.

Add a small amount of neutralizer. Adding too much neutralizer can cause significant problems: chlorine levels could drop to zero and excess neutralizer will remain in the pool and continue breaking down the added chlorine. Only use about ⅓ or ½ of the originally calculated chemical amount.

Wait and check regularly. Allow the chemicals in the pool to adjust according to the instructions. You will need to check frequently and avoid swimming until the levels return to normal. If the readings stabilize but the chlorine levels are still too high, continue adding small amounts of neutralizer.

• If the pool’s water circulation is slower than usual, you will need to wait longer for the neutralizer to take effect.

Increase pH if necessary. Neutralizers typically lower the pool’s pH. Be prepared to raise the pH once the chlorine level has returned to normal. The pH should be maintained between 7.2 and 7.8, with an ideal level around 7.5.

Understand UV (Ultraviolet) Sterilization. UV lamps used in swimming pools are effective at neutralizing most types of bacteria. While they cannot independently ensure pool safety, they help lower chlorine levels to as little as 1 ppm or even less, depending on local regulations. These devices can also break down various compounds that may cause irritation or danger, commonly found in chlorine-sanitized pools. Finally, although rarely used for this purpose, certain UV lamps can reduce high chlorine concentrations in the pool.

• Local areas may have different chemical concentration regulations.

Consider using a UV lamp with medium pressure. A "medium-pressure" UV lamp is a smart choice due to the following advantages:

• This is the most common type of lamp and can decompose a substantial amount of chlorine in the pool. However, for water sterilization, you will need 10-20 times more pressure and multiple lamps to achieve this.
• This type of light is the most effective at breaking down chloramines, which are chemicals that irritate the eyes, skin, and give the water a "chlorine" smell.
• This lamp provides a reasonable sterilization effect, though it is not the best option available.

Consider using a low-pressure UV lamp. This type of lamp is also known as a filtration device because it has excellent sterilization capabilities, though you'll still need to use a small amount of chlorine. It’s ideal for public pools.

• This lamp is more affordable and durable compared to medium-pressure UV lights.
• While marketed as capable of eliminating chloramines, it may not entirely remove the visible signs of chloramine presence, such as eye irritation.

Evaluate Other Types of UV Lamps. There are several other UV lights available on the market that are less commonly used. Below is some useful information to better understand these devices:

• "Ultraviolet light" encompasses a broad spectrum of light with various applications. UV light is typically divided into UV-A (315-400nm), UV-B (280-315nm), and UV-C (100-280nm). You can find information about the type of light or the wavelength (e.g., 245nm) on each device.
• Only UV-C light has a sterilizing effect on pools.
• UV-A light (including sunlight) breaks down a significant amount of chlorine. However, this requires a substantial amount of light.
• All three types of UV light help break down chloramine.

Test the Pool After Installing a UV Light. It's recommended to hire a professional for installation. Once properly installed, the device typically requires minimal maintenance. Continue monitoring chlorine levels regularly, maintaining them at 1 ppm or at the low level recommended by the device or in accordance with local regulations.