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Designing for IAQ in Natatoriums

©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 4, April 2012.  

By Randy C. Baxter

About the Author
Randy C. Baxter is staff engineer, Action Research Corporation in Greer, S.C.

In the last 10 to 15 years research has shown that poor air quality in indoor swimming pools has a negative impact on the health of swimmers, coaches and pool workers. Poor indoor pool air quality, caused by compounds off-gassing from pool water, has been linked to eye, nose and throat irritation, exacerbation of asthma symptoms, as well as to a predisposition to develop bronchial hyperactivity and asthma. These health concerns are in addition to the well-known damaging effects of the indoor pool atmosphere on pool buildings and equipment.

The purpose of this article is to review the literature concerning the effects of disinfectant by-products on indoor swimming pool air quality and to propose practical methods for mitigating their impact in conventional (recreational and competition) indoor pools.

Trichloramine & Other By-Products

The literature is very clear that, of all the disinfectant by-products (DBPs) present in the pool atmosphere, trichloramine vapor (nitrogen trichloride) is the primary compound responsible for indoor pool air quality problems that cause adverse physiological responses in humans. Other inorganic chloramines (mono- and dichloramine), as well as related organic DBPs are also present in chlorine-disinfected pool water and pool air. These DBPs include trihalomethanes (such as chloroform), haloacetic acids and other compounds including haloketones, trichloroaldehydes, trichloronitromethane, and cyanogen chloride. Organic DBPs have not been found to cause adverse physiological response or discomfort in the concentrations found in indoor swimming pool air although some have been identified as potential human carcinogens.

For the purpose of this article, only the inorganic chloramines will be considered. It is logical to assert that a strategy that removes inorganic chloramines will also be effective in mitigating the impact of related DBPs.

Monochloramine is the most well-known and most prevalent inorganic chloramine. It is always present in chlorine-disinfected water and is frequently used by municipal water systems as a secondary disinfectant.9Monochloramine does not irritate the eyes or respiratory system and, in normal concentrations, it is not a significant cause of air quality problems in the indoor pool environment.

Dichloramine is more volatile and more irritating to the respiratory system than monochloramine, but it does not persist as a vapor in the air and decomposes rapidly in water. Dichloramine does not contribute significantly to air quality problems but it is associated with undesirable taste and odor in water.

Trichloramine, on the other hand, is a potent respiratory irritant; its effect on the respiratory system and eyes is similar to that of chlorine gas. Because of its toxicity and the fact that it can exist as a vapor in the air, most experts agree that trichloramine is the compound most responsible for the indoor pool air quality problem. According to Black & Veatch,trichloramine is the only species of combined chlorine that causes tearing of the eyes.

Trichloramine Formation

Inorganic chloramines are a by-product of chlorine disinfection of water that contains nitrogenous organic material.Three inorganic chloramines can be formed as disinfection by-products: monochloramine, dichloramine and trichloramine. The definitive mechanism of trichloramine formation in swimming pool water is not universally agreed. One accepted mechanism of trichloramine formation is by a series of pH-dependent substitution reactions:

At normal pool pH:        
NH3 + HOCl → NH2Cl (monochloramine) + H2O
At pH <7:
NH2Cl + HOCl → NHCl2 (dichloramine) + H2O
At pH <4.4:
NHCl2 + HOCl → NCl3 (trichloramine) + H2O

The above mechanism requires acidic conditions and precludes the formation of trichloramine in swimming pool waters that are maintained within normal (7.2–7.6) pH limits. But Holtzwarth found that even at near neutral pH, a high Cl2:N ratio enables the reaction of free chlorine and dichloramine to proceed:
At high Cl2:N ratio:       
NHCl2 + HOCl → NCl3 (trichloramine) + H2O

Researchers Blatchley and Li and Schmalz, et al., also found that the reaction of organic nitrogen compounds (such as urea) with free chlorine forms trichloramine at the normal pH values found in swimming pools. There is evidence that trichloramine can be formed when a pool is operated at breakpoint conditions, especially when there is organic nitrogen in the water.

Despite the lack of consensus as to the definitive mechanism of trichloramine formation in pool water, there is no lack of evidence of trichloramine vapor in the air above indoor swimming pools. Hery found airborne trichloramine concentrations of 0.05–1.94 mg/m3 in 13 swimming pools tested. Massin measured trichloramine in 46 public swimming pools and 17 leisure pools and found average trichloramine concentrations of 0.24±0.37 (standard deviation) mg/m3 and 0.67±0.17 (standard deviation) mg/m3, respectively. Stottmeister and Voigtmeasured trichloramine concentrations as high as 18.8 mg/m3. Jacobs studied 38 indoor pools and found a mean trichloramine concentration of 0.56 mg/m3 with a maximum of 1.34 mg/m3.

Trichloramine is Irritating and Difficult to Remove

Trichloramine is extremely volatile and immiscible in water and it readily escapes the pool water, especially when disturbed. Formation, volatilization and release of trichloramine vapor into the air typically occur during times of increased occupancy because of increased surface film area and bather soiling load. Release of trichloramine vapor is exacerbated by heavy activity such as kicking and splashing.

Trichloramine vapor causes discomfort in most people exposed to air concentrations of 0.50 mg/m3 or greater.Discomfort or physiological response to trichloramine exposure ranges from sneezing, coughing, and irritated eyes to wheezing, breathing difficulty, chest tightness and chest congestion to increased risk for asthma. Positive dose effects of trichloramine exposure have been reported on serum surfactant proteins, which are markers for lung epithelial cell injury.

Citation: ASHRAE Journal, vol. 54, no. 4, April 2012

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