Hotter water, more bacteria
The higher temperature of the spa also creates an environment that speeds the reproduction rate of bacteria called mesophiles (organisms), which grow most rapidly in temperatures ranging from 20 to 45 C (68 to 113 F). This range matches up very well with both the typical operating temperature of spas and normal human body temperature. In spa water measuring 37 C (98.6 F), some mesophilic bacteria, which are often those that cause disease in humans, can reproduce in as little as 14 minutes (see Figure 4).
Another impact of a hot tub’s higher operating temperature is the rapidity of chemical reactions. Put simply, reactions that tend to occur in water, such as oxidation and pH changes, will happen more quickly in a spa environment. This is why weekly service visits may work well for swimming pools, but can quickly cause issues for spas.
Higher temperature also causes more compounds, including carbon dioxide and various chlorinated and brominated compounds, to become volatile and leave the water. This results in more rapid depletion of sanitizer compared to a swimming pool. That said, brominated compounds are heavier than their chlorinated counterparts, and are less likely to volatilize and leave the water.
Hotter water also increases bathers’ production of sweat, a major source of contaminants that affect water quality. By mass, urea ([NH2]2CO) is the predominant source of organic-N in human sweat and urine. The organic-N compounds react with chlorine to form chloramines, thereby consuming chlorine that could have been used to oxidize a compound or kill bacteria. Chloramines are poor sanitizers compared to hypochlorous acid (HClO), so the introduction of more organic-N compounds can quickly deplete a chlorine sanitizer. Brominated spas will not be affected as adversely, because bromamines are almost as effective as sanitizers as hypobromous acid (HOBr).
Higher temperatures also affect the solubility of calcium compounds, especially calcium carbonate. This means there is more tendency for calcium scale to develop more often and more rapidly in spas, particularly within the heat exchanger.
Less volume, more bathers
Consider this comparison: A typical swimming pool with about 40,000 L (10,566 gal) of water with 40 swimmers results in the same volume of water per person as one bather in a 1,000-L (264-gal) spa. As such, typical spa use creates a much higher level of bather contaminants in the water than occurs with typical swimming pool use. Remember, aeration and higher water temperatures cause increased sweating and the deposition of skin flakes and oil into the water. Obviously, the sheer quantity of contaminants and bacteria in each litre of water cause a strain on the sanitizer.
Dealing with diseases
Unfortunately, the volatile nature of spa water can lead to several bather health issues, which spa operators should aim to prevent.
Pseudomonas folliculitis
This is the most common disease transmitted via spas. It is an infection of the hair follicle caused by bacteria called Pseudomonas aeruginosa. While this bacteria is naturally present on human skin, it is only in low quantities (i.e. not high enough to develop an infection). However, in the warm water of the spa, pores open and allow the bacteria to enter the area around hair follicles. If spa sanitizer levels are inadequate, the number of bacteria entering the follicle can overcome the body’s natural defences and cause an infection. For some individuals, the body’s natural defense mechanisms will overcome the folliculitis infection within a few days. However, those with compromised immune systems may need oral antibiotics to recover.
Dermatitis
Pseudomonas folliculitis is often confused with dermatitis, an irritation of the skin that can be triggered by an individual’s sensitivity to a particular compound present in the water. It can also be caused when the water balance is incorrect or sanitizer levels are excessively high.
Determining whether an individual has folliculitis or dermatitis is best left to a physician, but there are some clues of which spa operators should be aware. Typically, dermatitis reactions occur during spa use or within a few hours of using the spa. Also, dermatitis does not usually exhibit the presence of pus if there are raised areas on the skin. In contrast, folliculitis symptoms are typically seen 24 to 48 hours after spa use and pus is present in raised areas.
Legionnaires’ disease
This disease, the milder version of which is known as Pontiac fever, is actually transmitted through breathing of the aerosolized spa water droplets. A person can become infected with Legionella pneumophila without ever entering the spa; simply being around the spa when aeration is occurring can lead to exposure.
To prevent disease transmission, constant maintenance of adequate sanitizer residual is required. With all the factors working to deplete and reduce the effectiveness of the spa sanitizer, this is a fundamental requirement.
