What's the Difference Between Chlorine, Bromine, and Saltwater Pools?

Before chemical sanitization, swimming pools were basically cesspools. Nineteenth-century pools—most of which were private and accessible only by the wealthy—were often built on slopes so the dirty water could be drained, and “scum gutters” were installed along the edges to help skim away the slime.

In 1903, Brown University opened its Colgate Hoyt Pool, an expansive space featuring marble-lined walls, and filled the tank with city water clean enough to drink. But as more swimmers took the plunge, the water turned a swampy shade of brown. A mechanical filter was installed, but the water still had to be regularly drained and replaced, a process that took 18 hours. 

In 1909, an enterprising student named John Wymond Miller Bunker—who’d go on to become a professor of physiology and biochemistry at MIT—filled a cheesecloth bag with powdered chlorine and dragged it across the pool’s surface. According to an article by Montclair State University chemist Kevin Olsen, bacteria counts dropped from 700 parts per million to zero in 15 minutes. Bunker reduced the concentration of chlorine and still achieved total sterility in an hour.

More than 100 years later, chlorine remains the go-to product for sanitizing pools. Inexpensive and available in a range of forms, including compressed gas, liquid, granules, and tablets, it can be mixed through the pool’s pumps or dumped in. At proper levels (between one and three parts per million), a few gulps of pool water won’t make you sick, though prolonged exposure to chlorine will make your eyes red and your skin itchy. And the smell? According to Michael Dean, editor of the website Pool Research, that pungent odor wafting out of your swim bag is the result of nitrogen-rich organic compounds interacting with chlorine, producing what’s known as chloramine. The more chloramines present, the more the chlorine is used up, which is why chlorine levels need to be monitored so often. The most abundant organic compound in many pools is urine, so stop peeing in the pool, even if everyone’s doing it.

Chlorine isn’t the only option. Bromine has long been a popular choice among hot tub owners because it breaks down more slowly in warmer conditions. For the same reasons, it’s also a good choice for indoor pools without a lot of sun exposure. Though more expensive than chlorine (Dean says a 50-pound bucket of chlorine costs around $150, while a bucket of bromine runs closer to $300), it doesn’t need to be monitored as closely or replaced as often, which can help balance out the costs. Bromine is also less irritating to your eyes and skin and doesn’t smell quite so bleachy.

Over the past 30 years, advances in chemical technology have led more pools to switch to either salt water or ozone systems. Salt-water chlorinators sterilize pool water through electrolysis, during which electric current passes through iodized salt to create chlorine. In somewhat similar fashion, ozone systems use electric currents or UV to turn oxygen molecules into ozone. Ozone works as an oxidizing agent, chemically reacting to and destroying bacteria and parasites. Harmful to humans in higher concentrations, it can’t be stored or transported the way chlorine and bromine are; rather it must be produced on-site and dissolved through the pool’s filtration system. Salt-water chlorinators and ozonators are expensive systems, running several thousand dollars for smaller, residential pools and up to hundreds of thousands for Olympic-sized facilities. Even then, some chemical sanitization is still needed. Yet both systems dramatically reduce the amount of chlorine or bromine needed to treat the pool, which not only lowers operating costs but makes the water easier on the eyes and skin. As a bonus, ozone cuts the stink since it doesn’t create chloramines.

Your pool is likely cleaned by chlorine, a chemical we’ve long been acquainted with and will continue to experience every time we swim. Although many of us shower to wash away whatever we were swimming in during our workout, the faint aroma that remains, as well as the bleached, straw-like hair, are badges of honor, a swimmer’s calling card.