Last year, media outlets reported that at least five people contracted Legionnaire’s Disease and two died from it after spending time in American public hot tubs.

But that’s nothing compared to how many cases the experts estimate occurred.

According to the federal government’s Occupational Safety and Health Organization (OSHA), about 6,000 people contract Legionnaire’s Disease each year. Meanwhile, according to a report from the Centers from Disease Control and Prevention, there were 354 cases of Legionnaire’s Disease acquired in treated recreational water over a recent 5-year period. That works out to about 70 cases of Legionnaire’s Disease picked up — mostly from hot tubs — per year.

Those numbers may still seem small, but the proper number should have been zero, because the legionella bacteria that cause Legionnaire’s Disease has no place in our spas.

That’s because the legionella bacteria is easy to kill. Legionella generally dies within minutes of exposure to 3-5 ppm chlorine at normal temperature and pH levels.

Legionnaire’s Disease is a severe form of pneumonia, a lung infection.

Most people who catch Legionnaire’s do so by inhaling bacteria from contaminated water. And they tend to get infected in spas for a number of reasons.

For one thing, people bathing in spas generally rest with their faces in close proximity to steaming and bubbling water. That proximity, combined with the fact that the water is aerosolized, means that they are breathing water vapor, and if that water vapor is contaminated, there’s a chance they could become infected.

For another thing, the temperature in which spas are maintained is absolutely conducive to growing the bacteria. Legionella grows best in temperatures between 77°-113°F, which is just about how we like to keep our spas.

Finally, legionella thrives in biofilm, and a large volume of bather waste combined with a small volume of water promotes the formation of biofilms in hot tub plumbing.

The problem is that while the legionella bacteria is effectively controlled by chlorination, it can hide and continue to flourish in biofilm within plumbing, protected from inactivation by chlorine.

And it must be understood that even if a hot tub is currently operating under recommended chlorine levels, that may not always have been the case. If the hot tub has a history of poor maintenance, the plumbing likely contains biofilms despite currently conforming to industry guidelines.

Furthermore, even brand-new hot tubs may contain biofilms as a result of the jet-testing that is done following manufacture. After hot tub jets are tested, residual water may remain within the plumbing, and biofilms can begin to flourish before the hot tub is even purchased.

Those biofilms become a haven that protects all sorts of diseasecausing pathogens from normal chlorination, including Legionnaire’s Disease.

So the trick is to remove the biofilm.

There are a number of spa purges and jet cleaners that are readily available for the pool and spa industry. Many formulations contain enzymes, which have been demonstrated to break down the organic components that are part of the biofilm matrix such as human skin cells, hair, oils, make-up, sunscreens, dead algae, and more.

To make a biocidal claim, however, the Environmental Protection Agency requires the product to meet specific testing criteria. Furthermore, they discriminate between the different types of claims that can be made. For example, a product may claim to remove biofilm without actually killing it. A product may claim to prevent biofilm. A product may claim to control or reduce biofilm. Finally, a product may claim to kill biofilm. There are different testing guidelines for each of these claims.

To the best of our knowledge, there are now two companies that manufacture EPA-registered biofilm removal products for hot tubs and spas. They make the only products in the spa industry that remove and kill biofilm bacteria. The companies are Unique Solutions and Sterilex.

Unique Solutions manufactures two products: Hot tub Serum (EPA Reg No. 84409-2), and Ahh-some (EPA Reg. No. 84409-1), both composed of Alkyl dimethyl benzyl ammonium chloride.

Sterilex manufactures Sterilex Ultra Disinfectant Cleaner Solution (EPA Reg. No. 63761-8), composed of both Alkyl dimethyl ethyl benzyl ammonium chloride and Alkyl dimethyl benzyl ammonium chloride.

Spa Water Chemistry: Jets, Heat, and the Role of pH in Scale Formation Why does scale seem to form more readily in a spa than in a pool? And why does the pH in a hot tub rise so much faster than in a pool? These are key questions that highlight the differences between spas and pools — differences rooted in heat and aeration — and their impact on water chemistry, particularly in relation to calcium carbonate (the primary component of scale).

Let's dive into the science behind this, as it involves some intricate chemical dynamics.

Scale Formation in Spas vs. Pools

Scale tends to form more easily in a spa for two main reasons: the higher water temperature and the aeration provided by the spa's jets. Additionally, the rapid rise in pH that occurs in a hot tub is an important factor. These elements influence the chemical equilibrium of calcium carbonate in the water, creating the conditions under which scale can form.

Understanding Chemical Equilibrium and pH

In both pools and spas, many chemical reactions — especially those involving calcium carbonate — are equilibrium reactions. This means the chemicals involved are in a constant state of flux, where reactants form products, and those products reverse to reform the original reactants. A key feature of equilibrium is that, once it is reached, the concentrations of the chemical species involved remain constant over time — unless something disturbs the balance.

One of the key equilibrium reactions at play in pool and spa water is: CO2 + H2O ↔ H2CO3 ↔ H

+ +

HCO3- ↔ H+ + CO3²

–

In words, carbon dioxide dissolves in water to form carbonic acid, which then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). Bicarbonate can further break down into more hydrogen ions (H+) and carbonate ions (CO3² -).

At a given temperature and pH, these reactions reach a balance where the concentrations of each species remain stable. However, temperature and aeration (such as from the jets in a hot tub) can shift this equilibrium, which leads to changes in pH and the availability of carbonate ions.

How Jets and Heat Affect the Balance

In a spa, aeration from the jets increases the outgassing of carbon dioxide (CO2). This is similar to what happens when you open a soda can — stirring or shaking the soda causes it to lose its “fizz” (the CO2 gas). In a hot tub, the jets cause the CO2 to escape faster than in a pool, which shifts the equilibrium.

At the same time, the increased temperature of the spa also accelerates the loss of CO2. As CO2 leaves the water, the equilibrium is disrupted, causing the chemical system to react in order to restore balance.

The Impact of CO2 Loss on pH and Carbonate Ions

The escape of CO2 causes a chain reaction: Without enough CO2 in the water, carbonic acid (H2CO3) breaks down to release more CO2, and bicarbonate (HCO3-) dissociates further into carbonate ions (CO3² -)

and hydrogen ions (H+). This shift results in a reduction in hydrogen ions (H+), which leads to an increase in pH.

It’s important to note that while the dissociation of bicarbonate produces both carbonate (CO3² -) and hydrogen ions (H+), the overall effect of the pH rise is that the relative abundance of carbonate ions increases.

The system “prefers” to produce more carbonate ions when there is a higher pH, because fewer hydrogen ions are available to “consume” the carbonate ions.

Therefore, more carbonate ions are present in the water as the pH rises, which sets the stage for scale formation.

Temperature and Scale Formation

In addition to pH changes, the higher temperature of a spa contributes to the increased likelihood of scale formation. The precipitation of calcium carbonate (CaCO3) is an endothermic reaction, meaning it requires heat to proceed.

Unlike many substances that dissolve more readily with heat, calcium carbonate precipitates more easily as the temperature increases. This is why the higher temperatures of a spa, in combination with increased carbonate availability, create a perfect environment for scale to form on surfaces like tile lines and heaters.

So what do we do to prevent scale in spas?

The combination of heat and aeration in a spa changes the chemistry of the water, creating conditions that favor both a rise in pH and the formation of scale.

As CO2 escapes due to aeration and heating, the equilibrium shifts, producing more carbonate ions, which then combine with calcium ions to form calcium carbonate (scale).

This makes spas more prone to scale than pools, where the water temperature is lower and aeration is less pronounced. Understanding these chemical processes is key to managing spa water chemistry and preventing scale buildup.

Clearly, we can reduce scale by lowering the temperature and turning off the jets, but the whole point of a spa is hot water and jets.

So what can we do?

1. Maintain Balanced pH and Alkalinity

• Monitor pH Regularly: Because high pH contributes to the formation of more carbonate ions, it’s important to keep the pH within the recommended range (typically 7.4 to 7.6). If the pH rises too much, it will increase the likelihood of scale forming.

• Adjust Alkalinity: Total alkalinity (TA) helps buffer pH changes. If alkalinity is too high, it can cause the pH to drift upward, increasing the risk of scale formation. Keeping alkalinity in the optimal range (80–120 ppm) will help maintain pH stability and reduce the chance of unwanted scale buildup.

2. Control Calcium Hardness

• Lower Calcium Hardness: High calcium levels (above 200 ppm) can make scale formation more likely, especially in heated water. Reducing the calcium hardness to a manageable level (usually 150–200 ppm) can help prevent scale from forming on spa surfaces.

• Use a Sequestering Agent: Sequestering agents bind to calcium ions and prevent them from precipitating out of solution as scale. Regular use of a good sequestering agent can help prevent calcium carbonate from forming scale in the water, particularly in spas with higher calcium hardness.

3. Regular Cleaning and Maintenance

• Clean Filters and Jets: Scale can accumulate in the filters and jets. Regular cleaning helps prevent build-up that can later affect water chemistry. Use appropriate cleaners that target scale removal and keep water flowing smoothly through the system.

• Drain and Refill Water: If scale buildup is already noticeable, consider draining the spa partially or completely and refilling it with fresh water. This can dilute the high mineral content, which is often a contributing factor to scale formation.

4. Use of Scale Inhibitors

• Add Scale Inhibitors: Some pool and spa care products are designed specifically to inhibit scale formation by interfering with the chemical process that causes calcium carbonate to precipitate. These inhibitors can be added periodically, particularly in areas with hard water or high calcium levels.

5. Monitor Total Dissolved Solids (TDS)

• Control TDS: High levels of total dissolved solids (TDS) can contribute to scale formation over time. Periodically draining the spa and replacing the water can help reduce TDS and maintain water balance.

6. Use a Spa Cover • Minimize CO2 Loss: When not in use, covering the spa can reduce the escape of CO2, which helps maintain the chemical equilibrium and prevent pH from rising too rapidly.

By managing these key factors — pH, alkalinity, calcium hardness, and CO2 levels, spa service techs can effectively reduce the risk of scale formation. Regular maintenance and monitoring of water chemistry will ensure the spa remains in optimal condition, both for the enjoyment of bathers and the longevity of the equipment.