THE SWIMMING POOL ENVIRONMENT
Swimming-pool environment is a complex and dynamic ecosystem that can be affected by the type of swimming-pool (indoor, outdoor,…) and by other factors, including water temperature, ventilation, climate, location, purpose of use (competition, relaxation, recreational activities) and swimming habits, particularly swimmer´s hygiene4-6. In the case of an indoor swimming-pool, the environment consists of the water in the pool, the air above the pool within the natatorium, and the people in the pool (biota)2.
Both organic and inorganic compounds are continuously entering this ecosystem via filling waters (tap water, seawater, thermal water…), disinfectant addition (chlorine, bromine, ozone, UV…), pharmaceuticals and personal care products (analgesics, antibiotics, sunscreens, lotions, cosmetics, soaps…) and human body excretions (urine, sweat, saliva…). Interactions between all these compounds generates CBPs, some of which are of health concern7.
The addition of chlorine-based disinfectants (chlorine gas, sodium or calcium hypochlorite, di- or trichloroisocyanurates) to the swimming-pools water releases hypochlorous acid (HClO), which is the active biocide. HClO is a weak acid with a pKa of 7.5 at 25°C that reversely dissociates into hypochlorite (ClO) and hydrogen ion. The sum of HClO and ClO is referred to as free chlorine. HClO is a non-specific biocide that inactivates most waterborne pathogens but also reacts with organic matter to produce a wide range of CBPs8. Compared to tap water, CBPs formation in pools is much more important due to the higher input of organic matter and the constant addition of disinfectants9. In chlorinated-pools, major groups of CBPs include chloramines, trihalomethanes (THMs), haloacetics acids (HAAs), haloacetaldehydes (HALs) and haloacetonitriles (HANs) (Figure 1 ).
Chloramines are formed as a result of the reaction of HClO with urea and other nitrogenous compounds brought by swimmers. This group comprises monochloramine (chloramide, NH2Cl), dichloramine (chlorimide, NHCl2) and trichloramine (nitrogen trichloride, NCl3). Monochloramine and dichloramine (the sum of which is referred to as combined chlorine) are mainly found in water. Trichloramine, which is 400 times more volatile than its two congeners, is mainly found in the air at levels that are inversely proportional to the ventilation rate of indoor swimming-pools10,11. The odour and taste of water are mainly affected by the monochloramine/dichloramine ratio and the trichloramine concentration. This last compound is also responsible for the distinctive odour of indoor pools, wrongly attributed to chlorine9.
THMs represent between 5 to 10% of total organohalogens in swimming-pool water and air, with chloroform (CHCl3) being the dominant species8. THMs are generated from the complex reaction between active chlorine and naturally present or imported carbonaceous organic matter. Parameters influencing the formation of THMs include: organic matter concentration, chlorine concentration, contact time, water pH, temperature, and bromine ion concentrations12. THMs are generally well absorbed by inhalation, ingestion or skin contact.
Haloacetics acids (HAAs), haloacetaldehydes (HALs) and haloacetonitriles (HANs) are less frequent CBPs and therefore described in theonline supplementary material .