Chlorine gas released from hot tub water?


Platinum Supporter
Jul 1, 2014
Madison, WI
Does anyone know how much chlorine is released from a hot tub as chlorine gas? Obviously, there are a number of variables here so we'll assume perfect water chemistry and FC level. Obviously, chlorine gas is 'suboptimal' for inhalation and I've always assumed (maybe incorrectly) that the level of chorine gas one is exposed to while sitting in a hot tub is so small it's insignificant (especially considering that you're sitting in fresh air -- but some people do have hot tubs in enclosed rooms so...)

Also, does the quantity of chlorine gas related to the type of chlorine being used (dichlor, trichlor, bleach, etc) to sanitize the tub?


Active member
Jan 9, 2021
Pool Size
Liquid Chlorine
I don't think aqueous chlorine (HOCl) converts directly to Cl2 (chlorine gas) under hot tub conditions.
Chlorine gas (Cl2) hydrolyzes in water according to the following reaction:
Cl2 + H2O <-> HOCl + Cl- + H+
and then HOCl <-> ClO- + H+

You can calculate the amount of chlorine being added to the water using the dichlor, trichlor, and bleach ingredient labels. However, what you are likely smelling is Disinfection ByProducts (commonly referred to as DBPs), when HOCl (hypochlorous acid) reacts with contaminants (organic and inorganic compounds). As long as there is enough HOCl and enough contaminants, they will react and produce DBPs.

There are many types of DBPs: chloramines, trihalomethanes (THMs), haloacetic acids (HAAs), haloacids, halodiacids, iodo-THMs, haloaldehydes, halonitriles, haloketones (HKs), halonitromethanes, bromate, haloamides, haloalcohols, nitrosamines, combined available chlorine, and 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) and MX homologues, etc.

The most prevalent DBPs in swimming pool are chloramines, THMs and HAAs, (Chu and Nieuwenhuijsen, 2002; Kim et al., 2002).

In a 2018 study of drinking water treatment (Redirecting), "the major odorants responsible for chlorinous odor under acidic and non-acidic conditions were dichloramine and monochloramine, respectively." Chloramines (mono-, di-, tri-) result from reacting with aqueous ammonia (NH3 and NH4 species). Ammonia comes from sweat and urine and ammonification of urea in biofilms (Keuten et al., 2009; Schets et al., 2011; Casanovas-Massana and Blanch, 2013). "The bacteria in the biofilms utilize urea as a nutrient and release ammonia as a residual. In addition, the dead cells separated from the biofilms add organic substances into the pool water."

I think what you are trying to ask is: what are the safe levels of each type of DBPs to be breathing in, and how to minimize production of DBPs?

"As of 2014, there were over 70 peer-reviewed publications have reported various aspects of swimming pool, including types and quantities of DBPs, organic loads from bathers, factors affecting DBPs formation in swimming pool, human
exposure and their potential risks" (Redirecting)

My recommendations to minimize formation of DBPs:
(1) maintain bather cleanliness - shower and rinse off well before entering the water
(2) less sweat/pee = less DBPs
(3) allow as much fresh outdoor air into the area as possible
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