How to Lower Chlorine in a Pool

Basic Techniques

Understand "chlorine smell" and stinging eyes. Many people think that a chemical smell or stinging eyes are signs of chlorine. In fact, these usually show up after chlorine has broken down into other chemicals. The right response is usually to raise chlorine with a shock treatment. Better yet, use a test kit to get an accurate read of the chlorine, as described below.

Use a pool test kit. If you haven't already, test the chlorine levels using a test kit from a pool supply store. Make sure the test kit measures both free available chlorine (FAC) and total chlorine. As a general rule, free available chlorine (FAC) should be between 1 and 3 ppm. Total chlorine should be no more than 0.2 ppm higher than FAC. Your local health codes may have different requirements. If your pool also uses ozone or UV disinfection, FAC can be reduced as low as 0.5 ppm.

Remove chlorine sources. If the chlorine levels are only slightly high (about 4–5ppm), chemicals are usually not necessary. Just stop adding chlorine to the pool, and the problem will likely solve itself. To stop adding chlorine, turn off the chlorinator, chlorine feeder, or salt water chlorine generator; take the chlorine tablet out of the pool skimmer; or remove the chlorine floater. If you are not sure which system the pool uses, ask the manager or owner.

Uncover outdoor pools. Ultraviolet light from the sun rapidly breaks down chlorine. A single cloudless afternoon can remove 90% of your pool's chlorine, as long as you have removed all chlorine sources. Ultraviolet lamps are not usually a good replacement for this step. See the UV method below for more information.

Swim while chlorine levels are still at safe levels. Swimming helps lower chlorine, but only try this if chlorine is at slightly high (4 ppm). Experts disagree on how much chlorine is dangerous to swimmers. Public pools often shut down at 10ppm, while some pools use a 5ppm limit to be extra safe. Do not swim if your pool test had additional unexpected results, such as the wrong pH or alkalinity. Do not swim if you smell a strong "chlorine" smell (and the chlorine test gave high results). This smell is actually from irritating substances called chloramines. Chlorine affects the lungs. It is more dangerous in poorly ventilated areas, and if the swimmers have breathing problems.

Replace some of the pool water. This is an expensive, slow option, but it will dilute the chlorine. Drain and replace about ⅓ to ½ of the pool. After refilling, your pool may take a long time to return to normal chlorine levels and pH. If you have a filter has a backwash option, this is the fastest way to (partially) drain your pool.

Test regularly. Repeat the pool test once or twice a day, or every couple hours if the pool is still in use. If chlorine levels do not decrease within a couple days, try one of the methods below. See Tips below for guidelines on other test results, such as pH or cyanuric acid. If your test results fall outside these guidelines and do not correct themselves soon, you may need to hire a professional.

Adding Chemicals to Lower Chlorine

Purchase a chlorine neutralizer from a pool supply store. Ask an employee for help if you are not sure what to choose. Do not use chemicals from other sources. The chemicals sold at the pool supply store are at a specific concentration intended for pools. Sodium thiosulfate is probably the most common chlorine neutralizer, but requires care when handling. Hydrogen peroxide is often the cheapest option, and breaks down into harmless substances. However, this is much less effective if your pool's pH is below 7.0.

Close the pool. Never add chemicals to a pool while swimmers are using it. If other people have access to the pool, put up clear warning signs.

Follow safety precautions. Many pool chemicals can cause injury if they come into contact with lungs, eyes, or skin. Review this safety checklist before you continue: Read the product label carefully for safe handling instructions. Follow all recommendations for safety equipment, and review emergency protocols. Store pool chemicals in a well-ventilated storage area, away from sunlight, heat, and moisture. Do not store acids and chlorine near each other. Do not store dry chemicals next to or underneath liquids. Only have one chemical container open at a time. Close the container and return it to storage before opening another.

Calculate how much you need. Always follow product instructions to determine how to add it to the pool, and how much to use. Many chemicals are available in different forms and in different concentrations, so a general guide cannot cover every option. Generally, when adding sodium thiosulfate, allow for about 0.5 ounces (15mL) per 1,000 gallons (3,800L) water. If you're treating a public pool, go with a more accurate measurement. 2.6 ounces (77mL) sodium thiosulfate will lower chlorine by 1 ppm in 10,000 gallons (37,900L) of water. A pool store employee or an online pool chlorine lowering calculator can help you with this formula.

Add the neutralizer in small doses. Adding too much of the neutralizer can cause major problems: your chlorine can drop to zero, and the unused neutralizer will remain in the pool to destroy the next batch of chlorine as well. Use ⅓ or ½ as much as you calculated.

Wait while testing frequently. Give the pool time to adjust according to label instructions. Test frequently and do not enter the pool until parameters are back to normal. If your measurements stabilize but chlorine is still too high, add another small dose of neutralizer. If your circulation system is slower than average, you may have to wait longer for the neutralizer to take effect.

Raise the pH if necessary. These chemicals usually lower the pH of the pool. Be prepared to raise the pH once chlorine is back to normal. The pH value should be between 7.2 and 7.8, and ideally as close to 7.5 as you can get.

Using Ultraviolet Lamps

Understand UV disinfection. Ultraviolet (UV) lamps designed for pools can neutralize most germs. They cannot keep a pool safe on their own. However, they will let you reduce the amount of free available chlorine (FAC) as low as 1ppm, or even lower under some regional laws. They can also break down some of the irritating or dangerous substances that show up in a chlorinated pool. Finally, although not normally used for this purpose, some types may break down existing high levels of chlorine. Local health codes may have different requirements.

Try a Medium Pressure UV lamp. A "MP" UV lamp is a versatile option with the following benefits: This is the only common lamp that will break down a significant amount of existing chlorine. Even then, you'll need a dose 10–20 times higher than the amount recommended for disinfection. This will probably require multiple lamps. This is the most effective lamp for breaking down chloramines, the substances usually responsible for stinging eyes, irritated skin, and "chlorine" smell. This lamp is fairly good at disinfecting, but not the best option.

Consider a Low Pressure UV lamp. This type of lamp, often called a purifier, has excellent disinfecting ability, though you will still need to use (reduced) amounts of chlorine. This may make it an attractive option for public pools. These lamps also tend to be cheaper and longer-lasting than MP lamps. Advertisements may claim these lamps remove chloramines. This is partially true, but in practice they may or may not reduce the obvious signs, such as stinging eyes.

Evaluate other types. There are a few other kinds of UV lamps, although they are less common. Here's some information to help you figure out what each product does: "Ultraviolet" actually includes a wide range of light with different effects. It is usually divided into UV-A (315–400nm), UV-B (280–315nm), and UV-C (100–280nm). You should be able to find either the light type or a range of wavelengths (such as 245nm) for any product. Only UV-C light helps disinfect pools. Only UV-A light (including UV light from the sun) breaks down significant amounts of chlorine. Even then, it will take an intense amount of light. All three types help break down chloramines.

Test the pool after installation. Hiring a professional to install the UV system is recommended. Once installed according to specifications, very little maintenance is required. Continue to test your pool for chlorine as usual, keeping it at 1ppm or another low level as recommended by your product or local law. To keep the pool in good condition, vacuum it once a week. The plaster cannot tolerate a lot of dust. Vacuuming helps to maintain the color of the pool. When you go for a vacation, the water may become Tahoe blue or gray. But when you vacuum it, it becomes blue once again. This method is not expensive. The reason is that most vacuums do not cost more than $200.