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by Elaine K Howley

February 5, 2021

A lot of precise calibration goes into keeping pool water safe and swimmable

There’s a lot more to running a swimming pool than simply digging a hole in the ground and filling it with water. Because of the risk of waterborne illnesses, swimming pools must be carefully controlled and maintained so as not to make pool users ill. Sanitizers and other chemicals must be added to the water on a regular schedule to maintain the proper pH balance and disinfection power to inhibit the growth of algae and kill pathogens such as bacteria and viruses.

To do all this, pool operators must maintain the right balance of a mix of chemicals, chief among them is a sanitizing agent. Pool water can be disinfected using a variety of different chemical agents, including:

  • Chlorine. Chlorine is the go-to sanitizing agent for most swimming pools. More technically called hypochlorous acid, chlorine, which is widely used as bleach in industrial settings, is a pH-dependent sterilizing agent that results when chlorine gas is dissolved in water. It easily kills bacteria that can be introduced to the water by swimmers or the environment and also helps control the growth of algae along the pool bottom and walls. Chlorine is also used to sanitize municipal drinking water supplies. Contrary to popular belief, chlorine is the active agent in saltwater pools. In those sanitizing systems, an electrical current is passed through a volume of salt to release chlorine from the sodium chloride molecule. It’s the same sanitizer, just in a smaller, often more skin- and eye-friendly dose.
  • Bromine. Bromine is another halogen sanitizer that may be more frequently used in hot tubs and other higher-temperature bodies of water because bromine can retain sanitizing efficacy after reacting with another molecule. Because pathogen loads may be higher in warmer water, it’s thought that the staying power of bromine may be the better option. However, bromine degrades in UV light, so using it to treat an outdoor pool may not be best.
  • Ozone. Ozone isn’t just the gaseous layer that envelops the earth providing protection from the sun’s harsh rays. It’s also a molecule made up of three oxygen atoms that can be used to remove algae and kill pathogens in pool water without the potential drawbacks of chlorine. It’s generated via a special filter installed in the pool’s circulation system and needs to be used in combination with another sanitizer, such as chlorine or bromine, but it can help reduce the amount of these disinfectants needed to keep pool water clean enough for swimming.

Balancing Chemicals

The Centers for Disease Control and Prevention recommend that public pools and other treated swimming waters be carefully monitored. The pH level—a measurement of how acidic or how basic the water is—should be between 7.2 and 7.8. This is on a scale of 1 to 14, with 1 being very acidic and 14 being very alkaline or basic. A reading of 7 is considered neutral, so the CDC recommends keeping pool water slightly on the basic, or alkaline, side of neutral.

The CDC also offers recommendations for the concentration of sanitizing chemicals, such as chlorine or bromine. Free-chlorine concentration should be at least 1 part per million in swimming pools and at least 3 parts per million in hot tubs or spas. If the pool is being cleaned using bromine, that should measure at least 3 parts per million in pools and at least 4 parts per million in hot tubs or spas.

Recommended ozone levels vary from 0.1 to 0.5 parts per million based on conditions, including water temperature, whether the pool is indoors or outdoors, and covered or uncovered. The number of bathers expected and the type of filtration system being used are also factors that should be taken into consideration.

The CDC offers a handy check list for ensuring that the pool water you’re swimming in is as safe as possible. These suggestions include:

  • Checking inspection scores before heading to any public swimming pool or place with treated water. You can often access the latest inspection results online or once you arrive at the facility.
  • Checking that the drain at the bottom of the deep end is visible and appears secure to the bottom. Clearer water is safer, for a few reasons. First, lifeguards and other pool users can see you so you, helping avoid a collision or disappearing under the surface unnoticed. Cloudy water can also be a sign that the pool’s water filtration system isn’t working correctly or that the pH level is off, both of which should be addressed to bring the water back to a safe balance.
  • Checking that a lifeguard is on duty or that safety equipment such as a rescue ring or pole is available in case of emergency. It’s best to swim under the watchful eye of a trained lifeguard, but if you’re visiting an unguarded pool, use extra caution and never swim alone.


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