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But what about… salt-water pools?

MIT Medical answers your COVID-19 questions — and we also answer your follow-up questions! Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer.

Thank you for the article on whether indoor swimming pools are safe. At my gym, they’re limiting swimmers to one per lane with a maximum of four swimmers in the pool at a time. They’re also leaving 15 minutes between pool-lane reservations for members to exit the pool and for the pool area to be cleaned. Good safety protocols, I think. But this is a saltwater pool that is not chlorinated. Is it safe to swim in that type of pool?

Illustration of a partial view of a swimming pool with one person swimming in a lane and one person ascending an exit ladder at the side of the pool

It sounds like your gym has instituted very good safety protocols. And you don’t need to worry about the safety of the water. That’s because your gym’s saltwater pool also uses chlorine. The difference has to do with how the chlorine ends up in the water.

With a chlorine pool, someone has to add chlorine-based disinfectants to the water on a regular basis. The chemicals in those disinfectants — usually hypochlorous acid and hypochlorite — kill potentially harmful microorganisms in the water. (And, in the process, these chemicals interact with human sweat, body oil, and urine to create compounds called chloramines, which produce that distinctive “chlorine smell.”)

But in a saltwater pool, chlorine is generated directly from the salt.

It starts making sense once you dredge up a few repressed memories from high-school chemistry. Pool salt is the same thing that is in your saltshaker. That’s NaCl or sodium chloride — sodium plus chloride. The chlorine generator in the pool’s filtration system consists of an electrified metal plate. When saltwater flows over that plate, it undergoes simple electrolysis, splitting the sodium and chloride ions. This transforms the chloride portion of the sodium chloride into free chlorine, which efficiently disinfects the pool water. Three cheers for chemistry!

So, there’s no reason not to get in some laps as soon as possible. Just remember to be careful when it comes to any pool-related activity that might bring you into close contact with other people, particularly in relatively small, closed spaces. In other words, while you don’t need to worry about the safety of the pool water, you should do what you can to get in and out of the locker room quickly.

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