The most obvious and fundamental difference between the disinfection chemistry of a hot tub 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 increase the rate of perspiration which leads to an increase in the types of disinfection by-product precursors, or species that will ultimately convert into disinfection by-products.
Next, water temperature changes the reaction rates of disinfection by-product formation. In general, these types of reactions increase at higher temperatures.
Finally, hot water affects the volatilization rates of the volatile disinfection by-products. In other words, the heated water leads to higher evaporation of some by-products to 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, particularly those that contain nitrogen, such as that found in sweat, which increases with rising water temperature. The higher concentration of these precursors, combined with a generally higher usage of disinfectants, leads to an increase in disinfection by-product formation.
This is important because many dis- infection by-products have also been shown to have adverse impacts on health. Several studies 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 higher negative health effects for those who frequent indoor swimming pools as compared to outdoor pools. Similarly, competitive and regular swimmers have been reported to experience higher cases of asthma and other respiratory issues than any other type of professional sports person.
Further evidence of the negative health effects of disinfection by-products 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.
To summarize, disinfection by-products are probably bad for human health, and there are more of them in a hot tub than in a swimming pool.
For example, one study reported higher concentrations of trihalomethanes and especially haloacetic acids in heated water relative to average swimming pool water temperatures. An additional laboratory study showed that the formation of haloacetic acids significantly increased at higher temperatures and that nearly doubling the water temperature led to twice as much halonitromethane. N-nitrosamine formation has been shown to be up to ten times higher at elevated water temperatures.
There is also the choice of sanitizer to consider. For spas and hot tubs, many professionals favor the use of bromine over chlorine. It seems pertinent, then, to consider whether there is any difference in the harmful effects of the disinfection by-products produced by either sanitizer.
The concentrations of either chlorine or bromine-based disinfection by-products has been studied but there is still much research needed for a variety of different byproducts. Of the research that has been conducted, there is evidence that many of the brominated disinfection by-products form in higher concentrations in spas than their chlorinated counterparts.
For example, tribromomethane tends to occur in higher concentrations in bromine- sanitized spas than does trichloromethane in chlorine-sanitized spas. That effect is likely due to trichloramine’s increased volatility. Similarly, bromoacetic acids also seem to occur in higher concentrations than chloroacetic acids in the differently sanitized spas.
In terms of their harmful effects, research suggests that bromine-based disinfection by-products may have greater adverse health effects than their chlorine analogs.
Brominated haloacetic acids have been shown to be more toxic than their chlorinated counterparts. Similarly, brominated trihalomethanes also increase the genotoxicity effect.
However, some of these conclusions must be taken with a grain of salt; further research is needed to examine the health impacts of both chlorine and bromine’s disinfection by-products.
