Hot Tub Chemistry: Bromine vs Chlorine and Why Your Hot Tub Isn’t Just a Small Pool

People have been sitting in hot water, on purpose, for a very long time. The Romans built enormous bathing complexes with furnaces to heat the water and pipes to carry it where it needed to go. In Japan, soaking baths have been popular for more than 1,300 years. Hot water feels wonderful and it always has. For most of history though, that water belonged to a place, like a bathhouse or a natural spring, or a resort. Then the 1950s came around, and the Jacuzzi family built a portable pump to help a family member with arthritis. Californians in wine country started filling old wine baths with hot water and soaking in them, and by the 1960s, wooden tubs were showing up all over the state.

In 1968, Roy Jacuzzi launched a dedicated whirlpool bath and named it the Roman, which seems to fit. By the 1970s, Jacuzzi and other manufacturers were building fiberglass and acrylic shells with the pump, the heater, and the filter all inside. By the 1980s, they were in backyards all over the country. And that's where the story changes. The water moved from a place to go to a few hundred gallons on your deck that was shared by a handful of people, covered most of the time. The person that bought it was the one that maintained it. The pump made the trip, the heater made the trip, the jets made the trip.

The chemistry did not. So let's talk about what's in that water.

Real Chemistry, Real Answers. Every Tuesday.

You find them wherever heat from deep in the earth reaches the surface. Japan, Iceland, New Zealand, Hungary, Turkey, and right here in the United States from Colorado to Arkansas. People travel for them. Japan alone has more than 3,000 hot spring resorts, and in Hot Springs, Arkansas, eight grand bathhouses went up along a single street, mostly in the early 1900s. Typically, no chemistry is involved. It's whatever the earth sends up, so the minerals are the earth's and the risks are too. Then there's the hydrotherapy spa, a kind used for hydrotherapy and recovery. People use those because they're dealing with something like an injury or rehabilitation of some kind.

And that raises the stakes on the water quality. Somebody with a weak immune system or an open wound isn't the person you want sitting in bad chemistry. The in-ground spa is the one built right into the pool, or sitting next to it with its own plumbing. Some of those are finished in plaster, like the pool they're attached to, and that matters more than most people realize. Then the portable tub, the standalone one, an acrylic or fiberglass shell wrapped in insulation with its own pump, heater, filter, and a cover. Most of the ones people own are this kind. And then there's the inflatable, which heats its water with a small heater built into the pump unit and does the whole job for a fraction of the price.

One more thing before we go further. Everything I say about numbers in this episode is for a backyard tub. A commercial spa at a hotel or a gym answers to its local health department. If your regulator gives you a number, use that number. They're the one who decides whether you pass or fail an inspection. What all five of these have in common is simple a lot of heat and not very much water. A hot tub isn't a small pool. It's a completely different animal, and the reason comes down to three things stacked on top of each other. The first is people per gallon. Take four people in a 400-gallon tub, or about 1,500 liters.

That's the same number of bathers per gallon as 250 people in a 25,000 gallon pool, or about 95,000 liters. Nobody would send 250 people into a pool that size and then test the water once a week. In a tub, that load is a normal Saturday. The second is heat. Hot tubs are capped at 104 degrees Fahrenheit or 40 Celsius. And that number is about the bather, not the water. In December of 1979, the Consumer Product Safety Commission warned that hot tub water should never exceed 104 degrees Fahrenheit or 40 Celsius after 10 hot tub deaths that year. Three of them were alcohol-related drownings in a water of around 110 degrees Fahrenheit or 43 Celsius.

And at 106 degrees or 41 Celsius, soaking can push body temperature to the point of a heat stroke. So 104 is where comfort ends and danger starts. But that same heat is hard on the water. A tub at 104 sits right in the range the Legionella pathogen likes best, and the heat makes disinfectant break down faster, so the sanitizer is disappearing quicker at exactly the moment the bacteria are happiest. The third is that there's nowhere for anything to go. A pool has enough water to dilute what people leave behind. A tub has a few hundred gallons, and every drop of sweat, lotion, soap, whatever, stays right where it landed until somebody drains it.

Put those all together and you have the most demanding water in the whole backyard, in the smallest container. The good news is that it's small enough to fix quickly when you catch a problem. The bad news is that it changes quickly too, so a tub that's fine on Tuesday can be a completely different tub by Saturday. From what I've seen, most hot tubs keep the cover closed more than 90% of the time. That means a tub isn't really an open body of water. It's a sealed container of warm, chemical water, and that changes things in a few ways. First, the air. Everything that comes off that water collects in the space under the lid.

If you run chlorine and if you lift the cover to a sharp chlorine smell, that's combined chlorine, the same thing we went through in episode 12. It's the tub asking for help. The vapor also settles on the underside of the cover, and cover manufacturers will tell you that's why covers turn chalky and stiff and why zippers and stitching corrode. It's a smart thing to leave the cover off for 15 to 30 minutes after you add chlorine or shock. Second, heat. The cover holds heat in, which is the whole point. But it also means the water sits at that high temperature around the clock, and the temperature variable in the LSI never gets a break.

We'll do that math later on. And the cover itself takes a beating. The foam core soaks up water even when it's sealed, and a cover that is turned heavy is usually telling you that already happened. So the cover isn't just a lid. It decides what the air does and what the temperature does, and everything else in this episode happens underneath it. Let's talk about heat. Heat changes how gases behave in water. We covered this in episode 6 with Henry's Law. Cold water holds more dissolved gas and warm water holds less. A tub at 104 degrees Fahrenheit is about as warm as water gets in a backyard, so it holds very little carbon dioxide to begin with.

Now add the jets in the air blower. Every bubble is a chance for carbon dioxide to leave the water, and when carbon dioxide leaves, pH climbs. In a tub, pH rises a bit faster than it does in a salt pool, and salt pools are famous for it. How fast depends on how you use it. A tub that gets used every day drifts up faster than one that sits under the cover most of the week. That matters for chlorine, because chlorine does its work as hypochlorous acid, and the share of your chlorine that's hyperchlorous acid drops as pH rises. So a tub drifting toward 8.0 on pH is losing disinfecting power while the free chlorine number on the test strip hasn't moved at all.

Heat also uses up chlorine. Reactions run faster when it's hot, including chlorine reacting with everything the bathers brought in. Bromine plays by different rules though. Hypobromous acid is far less sensitive to pH than hypochlorous acid, which is one reason bromine is preferred for a tub. It doesn't solve everything, but it takes one problem completely off the table. So in a hot tub, heat pushes pH up, pushes chlorine's strength down, and burns through what's left faster, all at the same time.

Now for the sanitizer. For a tub, the recommendation is bromine, and I like it somewhere between five and ten parts per million, with liquid chlorine as your second choice between three and eight parts per million. Those numbers are higher than you'd see in a pool, and the reason is the bather load we talked about earlier. A lot of people in a small volume of hot water use up sanitizer fast, so the residual has to be bigger to still be there when the next person gets in. In a pool, free chlorine is usually tied to the cyanuric acid level. A tub shouldn't have cyanuric acid, so the number stands on its own.

We'll get to that in a minute too. Bromine is the easier fit for a tub. As we just covered, it's far less sensitive to pH, so a tub that's drifting upward doesn't take its sanitizer with it. And when bromine reacts with something and turns into a bromamine, that bromamine still disinfects at about half strength, where a chloramine is a much weaker disinfectant. The bromine tabs you'll find at the pool store are a good place to start. They dissolve slowly in a floating dispenser or in a feeder, which makes the residual steadier. Then there's ozone. A lot of tubs come with an ozone generator and it earns its keep.

Ozone oxidize what the bathers leave behind, it helps break down the combined chlorine we talked about earlier, and in a bromine tub it can turn spent bromide back into active bromine so the tablets last much longer. How much it does depends on the unit and the water temperature, and a hot tub is a hard environment for it. What it doesn't do is leave a residual. Ozone is a helper, not a sanitizer, so the bromine or chlorine number still has to be there first. So what about CYA in a hot tub? Walk into a pool store and ask for spa sanitizer, and the jar you're handed is almost always sodium dichlor.

One pound or two pound size, doesn't matter, same product. It dissolves fast and it's convenient. It's also stabilized, which means every scoop of chlorine arrives with cyanuric acid attached. Dichlore is about 52% chlorine and 48% cyanuric acid by weight, so close to half and half. In an outdoor pool, cyanuric acid has a job. It shields chlorine from sunlight. We've talked about this. A tub with a cover on it most of the time, though, doesn't have a sunlight problem, so it pays the cost of cyanuric acid and gets none of the benefit. And the cost is real. Cyanuric acid binds chlorine and slows down how fast it kills in water that's already hot and already loaded with people.

The CDC's own guidance says cyanuric acid and stabilized chlorine should not be used in hot tubs because they slow disinfection. Cyanuric acid doesn't break down and it doesn't evaporate. The only thing that removes it is draining the water. So it only goes in one direction. Say a tub takes about 10 parts per million of chlorine a week from dichlore. At that ratio, that's roughly 9 parts per million of cyanuric acid every week. By week six, it's past 50, the ceiling that we never go past. By week 11, it's at 100, and at 100, the pool rule of 7.5% says you need 7.5 parts per million of free chlorine just to hold even.

Meanwhile, the strip in the tub says three or four, and it looks fine against the chart. The tub looks sanitized. It is not. The answer is simpler than the problem. Use bromine tabs, or liquid chlorine, and no cyanuric acid at all. The water stays at zero CYA, the sanitizer does its job, and nothing builds up that you'd have to drain to get rid of. And why is sanitizer so important in a hot tub? A tub has the same recipe for trouble every time, and that recipe has five ingredients. Warm water, people who bring skin, oils, and everything else they were carrying, moving air from the jets in the blower that turns water into a mist, a sanitizer that's being used up faster than it's replaced, and surfaces for germs to live on.

A pool might have two or three of those on a bad day. A tub is built with all five. It's designed to be warm, bubbly, and full of people. Two germs show up most. The first is Pseudomonas, which is what causes hot tub rash. It's common in water and soil, so it's not really exotic. The rash shows up a few days after a soak in a poorly kept tub, and it usually shows up worse on the body wherever a swimsuit held water against the skin. That's why the CDC says to wash with soap after you get out and also wash the suit. The second is Legionella, and this is the one that makes people seriously ill.

The root is different because you don't catch it by swallowing water, you catch it by breathing the mist. The CDC puts the most favorable range for Legionella at 77 to 113 degrees Fahrenheit or 25 to 45 Celsius. And a tub at 104 or 40 Celsius sits right in the middle of that. The CDC also points out that hot tubs create aerosols and speed up how fast disinfectant decays. And natural doesn't fix it. At Hot Springs National Park in Arkansas, five people came down with Legionnaire's disease between 2018 and 2019, and one of them died. Three had most likely been exposed at a bathhouse using untreated spring water.

That water comes out of the ground above 135 degrees Fahrenheit or 57 Celsius, which is too hot for Legionella, but it's cooled to somewhere around 75 to 95 degrees, or 24 to 35 degrees Celsius, for bathing, and that's right where Legionella can grow. Nothing in the water was keeping it in check, and the ozonators on the jetted tubs in one bathhouse were found to be detached. Everything else in this episode is a way of keeping a real residual in the water and keeping the surfaces clean. Heat changes the Langulaire saturation index. Temperature is one of its variables, and the warmer the water, the higher the index.

Water that's balanced at 76 degrees Fahrenheit or 24 Celsius climbs by about 0.3 on the index by the time it reaches 104 from temperature alone. That's the distance from middle of the band to its edge. Take balanced pool chemistry. The LSI is about plus 0.1. Now heat that same water 28 degrees or 16 Celsius to 104. The LSI is now about plus 0.4 past the edge of the safe range, and the water wants to deposit calcium. Scale forms where the water is hottest, and in a tub, that's the heater. The lever with the most effect is calcium. Take that same tub at 200 parts per million down to 120 parts per million of calcium, and the LSI drops by about 0.4 to roughly zero.

That's why acrylic and fiberglass tubs run calcium around 100 to 120, and the low calcium also helps keep scale off of a smooth surface. Plaster is different. It needs more calcium, typically 150 to 250 parts per million, so the balance has to come from the other levers, typically lower alkalinity and lower pH. Calcium at 200, alkalinity at 90, and pH at 7.45 puts a hot plaster tub at about 0.1. Plaster is the most sensitive surface in the lineup, so the window is tighter, and because a tub stays hot around the clock, that temperature variable never gets a break. And here's another variable. Every tub has a clock on its water, and that clock is total dissolved solids or TDS.

That's everything dissolved in the water that isn't water. Fill water brings some in on day one, and from there it only climbs. It climbs four different ways. The fill water itself, every chemical you add from sanitizer to pH adjusters to everything else, the bathers, who leave behind sweat, skin, lotion, and oils, and evaporation, because water leaves and the dissolved solids stay behind. Topping off replaces the water, but the fill water brings its own dissolved solids with it, so topping off doesn't really dilute anything much at all. That matters because only one thing removes TDS, draining. No product or chemical will take it out. As it builds, the water gets harder to manage and slower to respond to what you add.

Your chemicals just don't work as well. You'll see cloudiness, foaming, and a ring at the waterline. High TDS also raises conductivity, which can speed corrosion on fixtures and equipment, and it makes water more aggressive towards the calcium in a plaster finish. The model aquatic health code guidance is to replace the water when TDS reaches about 1,500 parts per million above where it started. In a typical tub, that lands somewhere between 2,000 and 3,000, or about double that in a salt tub. A TDS reading is a guide that's worth testing once a month. Most tubs should be drained once every three to six months, depending on usage.

More usage equals more drain and refill cycles. I drain my tub about twice a year, but we don't use it as much as some people. So the water in a tub has a lifespan, and no chemistry changes that. The same chemistry behaves differently depending on what's holding the water. A natural hot spring has no equipment at all. Whatever the ground sends up is what's in the water, minerals and all, and nobody's controlling anything. A spring that's treated is just a pool with a heater supplied by geology. A therapy spa, like the ones in a clinic or rehab facility, is commercial. Its numbers come from the health department, and those are the only numbers that matter here.

An in-ground spa is built like a pool, and it's often plaster, which is why the tighter plaster rules we just went through apply. If it shares water with the pool, that's a different conversation because then it's the pool's chemistry. A portable tub is the one most people picture. A smooth acrylic or fiberglass shell with its own pump, heater, and filter, and often an ozone generator, holding a few hundred gallons, a thousand liters or so. It's a closed system, and the filter cartridge is the main place waste collects, so it needs regular rinsing and regular replacement. Inside the plumbing, on the walls of the pipes and behind the jets, there's a slimy layer called biofilm.

It's bacteria and other microbes sitting in a coating they make for themselves. It forms in the lines on the shell and in the filter. It matters for two reasons. The water can test perfectly and look clean, while the lines behind it don't. And once biofilm is established, it holds up against the sanitizer in the water. A big dose of chlorine and a few minutes of jets usually doesn't clear it either. The CDC lists sediment and biofilm among the key factors that affect Legionella growth, right alongside temperature and disinfectant residual. The signs are an odor that keeps coming back, cloudy water that won't clear, and sanitizer readings that won't hold steady.

And when the lines do get flushed, dark or slimy bits come out of the jazz. Ew. The fix happens when the tub is drained. Take the filter out and clean it separately. Then, with nobody in the tub, add a plumbing cleaner before draining. Orenda SPA 500 is the one I use in the field. They're not a sponsor of this podcast, and nobody pays me to mention them. It's just what we use. It's an enzyme product that helps flush organic buildup out of the lines so it leaves with the water when you drain it, and the label gives you the dose for your tub size. Run the jets and blowers for 5 to 10 minutes, shut everything off for 5 to 10 minutes, run it again for 5 to 10 minutes, and then drain the tub completely.

For the surfaces, the CDC's instruction is to scrub them vigorously every time the tub is drained. Rinse well and scrub as the tub drains and then refill. That's why the drain is more than just emptying and refilling. It's the reset, and the cleaning is part of the reset. In between, a steady sanitizer residual helps keep biofilm from forming in the first place.

So, a hot tub is a few hundred gallons or a thousand liters or so of hot water with people in it. A lot of what goes wrong with them traces back to treating one like a small swimming pool. It isn't one, and now you know why. You also know what to do about it. No cyanuric acid, which means no dichlore, bromine between five and ten, or liquid chlorine between three and eight, a pH between seven point four and seven point six, calcium low enough that the heat doesn't tip the water towards scale, and a drain, a scrub, and a clean set of pipes on schedule that your own bather count sets.

If the spa is commercial, the health department's numbers come first. There's no industry standard for a backyard tub, and the jar on the store shelf was built for a different problem. You built this standard from the chemistry, and you can explain every number on it. That's what a standard is. It doesn't have to come from anywhere official to be correct. It has to be built on why.

Thank you for listening today. If you have a hot tub, now you know what to do with it to keep it and you safe when you use it. I hope this episode helps you understand the chemistry of hot water a lot better. I'll see you next Tuesday.

Thank you for listening to the Pools Scientific Podcast, an independent production of John Cooper and Pools Scientific. Show notes, including full source citations for this and every episode, are at poolsscientific.com. We'll see you next Tuesday.