Posted By John Webster Bob The product of the reaction of free chlorine (Hypochlorous acid, HOCl) with nitrogen compounds which are introduced into pool water by bather pollution, (sweat,urine etc) is the formation of a group of compounds known as chloramines. These are unpleasant chemicals causing red eyes and mucosal irritation and and nitrogen trichloride in particular is responsible for the stale chlorine smell associated with poorly maintained public pools. They are corrosive gases and often cause damage to buildings which enclose swimming pools (see reactions below). HOCl + NH3 ---> NH2Cl + H2O (MONOCHLORAMINE) HOCl + NH2Cl ---> NHCl2 + H2O (DICHLORAMINE) HOCl + NHCl2 ---> NCl3 + H2O (TRICHLORAMINE or NITROGEN TRICHLORIDE) Chloramines must be broken down by raising the level of free chlorine. It takes a fourth unit of HOCI to finally convert the original molecule of ammonia into harmless nitrogen gas (N2), water and chloride ion (CI-) and a fifth unit of HOCI before a free available chlorine residual can be measured. In other words, the smell of chlorine can actually be an indication of UNDER dosing of chlorine!! However, overdosing of stabiliser can de-activate chlorine, as will the wrong pH. Get the pool chemistry right and there should be no trichloramine. Research indicates that pool attendants are typically exposed to trichloramine at concentrations in the range 0.1 to 2 mg/m3 and that there is no appreciable exposure to monochloramine or dichloramine. Hope this helps John

Posted By Ralph Ellington Bob, The publication "Swimming Pool Water" is a grand source of information. It is available from LRO Books Ltd. tel 01379 890721. The concern with nitrogen trichloride seems to be with structural damage to steelwork in the pool hall. It seems that a swimming pool in Switzerland collapsed in 1985 because of stress corrosion cracking. Nitrogen trichloride degrades to give hydrogen chloride. My employer is in the process of changing one of our pools from an ozone / hypochlorite system to a sodium bromide / hypochlorite system. This is said to eliminate the production of nitrogen trichloride as well as being cheaper than an ozone /hypochlorite system. There are still concerns with the bromide system as trihalomethanes (THMs) may still be formed. The brominated THMs are said to be carcinogenic and mutagenic. I also feel that there may be some possibility of the formation of bromate and this is a category 2 carcinogen. Possible formation of elemental bromine is also a concern particularly if the pH drops. If anyone has any experience of health and safety issues with operating a sodium bromide / hypochlorite system I would be very interested to hear from you. Bob, to get back to your question, "Swimming Pool Water" talks of the importance of good ventilation in removing volatile by-products of the disinfection process. There is a whole chapter on heating and air circulation. I do feel that providing good information to the public is also important. They should be encouraged to use the toilet and shower before entering the pool. Swimming clubs have an important role to play in passing on the hygiene message. This all helps to reduce the urea and other organic material from which chloramines (and bromamines) are formed. Regards Ralph Ellington