Although the health benefits of swimming are well documented, there are also some diseases — such as bladder cancer and asthma — that have been linked to disinfection byproducts in treated recreational water.

It is probably a good idea to define what we mean by disinfection byproduct. Generally, these are partially oxidized materials that are introduced into the water by swimmers as well as leaves and other debris.

These partially oxidized materials, such as chlorinated or brominated sweat, (skin cells, body oils, sunscreens, fecal matter, etc.) become part of the makeup of the water.

Some of these are volatile, evaporating to some extent, and depending on the air circulation may accumulate at the water/air interface.

Considering how people generally soak in a spa, faces low and close to the water surface, an obvious question to ask is, what are we breathing?

One might also ask if there is any difference in the formation of certain types of disinfection by-products as a function of the unique conditions presented by a spa as compared to a swimming pool.

The most obvious difference between the disinfection chemistry of a spa versus a swimming pool is the effect of temperature. Temperature affects disinfection chemistry in three important ways.

First, hot water increases the release of human output (or sweat). Elevated temperatures raise the rate of perspiration, which leads to an increase in the raw material that will ultimately convert into disinfection by-products.

Second, water temperature also changes the reaction rates of disinfection by-product formation. In general, these types of reactions increase at higher temperatures.

Finally, hot water also affects the volatilization rates of the disinfection by-products. In other words, the heated water leads to higher evaporation of some by-products into the air.

These effects are interrelated and were discussed in a 2017 review that appeared in the Journal of Environmental Sciences in an article entitled “Occurrence and formation of disinfection by-products in the swimming pool environment: a critical review.”

Hot water promotes the release of bather-load-derived precursors such as chemicals found in sweat, which increases with higher water temperature. The increased concentration of these precursors, combined with a generally higher usage of disinfectants, leads to an increase in disinfection by-product formation.

Many disinfection by-products have been shown to have adverse impacts on health. Several studies have found a correlation between swimming pool attendance and health issues, particularly asthma. Many studies have linked asthma to volatile disinfection by-products such as chloramines and especially trichloramine. This is further corroborated by increased negative health effects for those who frequent indoor swimming pools as compared to those outdoor. Similarly, competitive and regular swimmers have been reported to experience higher cases of asthma and other respiratory issues compared to participants in other professional sports.

Further evidence of the negative health effects of disinfection byproducts comes from a survey of lifeguards working at indoor pools, where 55% suffered from respiratory and other health issues.

Other studies have linked trihalomethane swimming exposure to bladder cancer. Collectively, these studies have associated an increased risk for bladder cancer due to dermal and inhalation exposure of certain kinds of disinfection by-products in disinfected water.

To sum it up, disinfection byproducts are not good for human health, and there are more of them in a hot tub than in a swimming pool.

Then, there is also the choice of sanitizer to consider. For spas and hot tubs, many operators prefer bromine over chlorine.

Therefore, it seems relevant to consider whether there is any difference in the harmful effects of the disinfection by-products produced by either sanitizer. This topic was explored in a 2022 journal publication entitled “ Disinfection byproducts in chlorinated or brominated swimming pools and spas: Role of brominated DBPs and association with mutagenicity.”

Researchers found that brominated disinfection by-products were significantly associated with the increased mutagenicity of treated water. Specifically, brominated pools and spas are almost twice as mutagenic as chlorinated ones. Translation: Bromine disinfection by-products are much worse than chlorine disinfection by-products.

They also found that the concentration of bromine disinfection by-products is a lot higher than chlorine disinfection by-products. Translation: Bromine-treated water produces a lot more brominated bad stuff than chlorine-treated water produces chlorinated bad stuff.

They found that increased human outputs (sweat) associated with increased use of pools and spas raises both the concentration and mutagenicity of the water. Translation: The more you use the water, the more bad stuff will form, and the worse it will be.

Finally, higher temperatures and less water exchange result in a lot more disinfection by-products, regardless of whether chlorine or bromine is used. That means that heated spas produce more disinfection by-products than a cool pool.

Reading a study like this, it seems obvious which sanitizer is best for a spa.