Swimming Pool NCl3: Not Innocuous As Once Assumed

ammoniaOnce used as a refrigerant and still made to produce fertilizer, ammonia (NH3) is also a chemistry teacher’s ally. Mix an ammonium chloride solution with base in a small test tube and as hydroxide accepts H+ from ammonium ion, the liberated ammonia gas can easily awaken a sleepy student. Kidding aside, the odor threshold for ammonia varies significantly from one individual to the next (Is there any correlation with the amount of spicy food eaten and insensitivity to NH3—great idea for a student project!)  The amount of ammonia present in sweat is also not constant among different people. Aside from attracting mosquitoes, ammonia released from skin pores gets into swimming pools where it reacts with hypochlorous acid from the chlorination process to produce various chloramines:

NH3 + HOCl → NH2Cl + H2O

NH2Cl + HOCl → NHCl2 + H2O

NHCl2 + HOCl → NCl3 + H2O

The mono and dichloramines, as shown, go on to react and produce the volatile trichloroamine or nitrogen trichloride(NCl3). An acidic pH can also accelerate the conversion to NCl3, which in small concentrations can cause red eyes and nose and throat irritations in swimmers, symptoms that become more pronounced in people working at indoor pools.Indoor-Swimming-Pool-1

Of course, ammonia isn’t the only source of NCl3. Sweat also contains urea, which is capable of reacting with OCl to eventually produce CNCl and NCl3. Another significant source of urea is urine. Any inappropriate relieving on the part of bathers will meet the same chemical fate.

Let’s look at the health evidence more closely. In a 2010 study published in the International Journal of Environmental Research and Public Health, investigators found an average concentration of 0.66 mg/m3 of NCl3 in the air above 20 indoor pools in northern Italy. Among 133 pool workers, those more in contact with the water (swim instructors and especially those who were both pool attendants and instructor) had higher odds ratios for itchy, watery eyes, infections and respiratory problems.

In a fair amount of internet writing, there is a current tendency to either cater to chemophobia or to fight it. In the latter case, writers fall into the trap of overusing the “too low a concentration to cause harm” argument. But here is a clear case of how a small concentration (less than 1 ppm on a per volume basis) of a poison like NCl3 isn’t as innocuous as previously assumed.

From a preventive viewpoint, the research reinforces the value of long standing recommendations to shower and use the bathroom before entering a pool, especially if it is an indoor one with a low ceiling and inadequate ventilation.









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