Many pool professionals won’t get into a public spa — at least, not before testing the water.

Pool pros have enough information about spa chemistry and high bather loads that they avoid public spas, preferring to do their soaking in their own, carefully maintained hot tubs.

It has been estimated that in terms of bather load, two people in a 400-gallon spa is equivalent to nearly 100 people in a typical 18,000-gallon pool.

We are talking about sweat, skin cells, hair, body oils, fecal matter, and urine. We’re talking about lotions, make-up, and detergents.

These ingredients create a chlorine demand that’s hard to meet. And when you dial in the higher temperatures of a spa relative to a swimming pool, you’ve got an environment that is ideal for bacteria growth.

Chlorine gets used up faster in a spa. It’s partly due to a spa’s smaller volume, but there are other reasons as well. Higher temperatures mean faster outgassing of chlorine. Higher temperatures also mean more sweating and faster chlorine chemical reactions.

Then you have the jets, which cause faster outgassing of chlorine. It also causes a higher bather load as the rushing water sloughs skin cells and body oils from our bodies.

It’s been estimated that where a pool consumes 4 grams of chlorine per bather per hour, a spa consumes 9 grams. And if nobody or nothing is monitoring the chlorine consumption on a spa with a lot of people in it, the chlorine could be gone in minutes — especially with peeing children present.

And if the chlorine goes, you’ve got bacteria.

Because in addition to everything else people put in the water, humans are host to millions of bacteria. The human skin hosts about 1012 bacteria — that’s a million times a million —and they all enter the small confines of the spa.

Taylor Technologies reports that, with the help of hot water and jet action, about a billion bacteria are shed when an adult human enters a spa.

Now you’ve got biofilm. Just picture the plumbing. Picture a dense network of hair and slime coating the inner walls of the pipes. They are full of bacteria that are multiplying rapidly, surviving on bather waste. Within this matrix, you might have legionella, E. coli, Pseudomonas aeruginosa, Cryptosporidium, Giardia, and more. These pathogens make people sick, and some of them will actually kill. And due to the thick layer of slime they surround themselves with in the plumbing, they are relatively impervious to any new sanitizer that might be added.

But despite these facts, it is possible to maintain a hot, clean, and calming spa and keep bathers returning day after day for more. It’s all part of what keeps the service industry in business. In this special issue of Service Industry News, we’ll examine some of the finer points of hot water chemistry.