Special Communications: Technical Note
Chlorine is a greenish-yellow gas with a characteristic odor used in the sterilization of water supplies and in swimming pools. It is a potent irritant of the mucous membranes, eyes, and skin and its exposure causes pulmonary irritation (17). The toxic effect of inhaled chlorine gas on the respiratory tract has been known for nearly a century since the studies of Lehman in 1887 (6), but unfortunately the highly toxic potential effect was first widely recognized during World War I at Ypres in 1915 (10). Accidental exposures of humans to unmeasured but high concentrations have been reported(11,8,9), but there is insufficient evidence to conclude that there is a chronic impairment of pulmonary function after acute or chronic exposure (13). The most common exposure to chlorine gas, out of the work place, is in enclosed swimming pools. The sudden onset of reversible airway obstruction in young swimmers(12), increased sensitization to aeroallergens(21), and a high rate of bronchial responsiveness to methacholine in swimmers have been reported (4). The concentration of chlorine gas in the microclimate where the swimmer is breathing has not been previously investigated. The purpose of our study was to measure the levels of chlorine at this water level in different conditions, in the swimming pools where the swimmers were training, to evaluate the normal level and a possible increase during the training session.
Were selected the swimming pools used by the swimmers of the national team of Spain and the autonomous team of Catalonia. The characteristics of the swimming pools are indicated in Table 1. The measurement of the chlorine level in the air was made near the water (<10 cm) on four sides of the pool at the same times during the day for a total of 5 d. The total number of measurements was 323. The days were not consecutive and were chosen randomly between the different pools. The methods are described in detail elsewhere (1,7,14,18). The air for the measurement was obtained through a measurement pump (MSA model Fixt-Flo) with a flow range of 0.2-1 1·min-1. The sample air was drawn through two impingers (Vidra Foc) (Fig. 1) placed in series, each containing 10 ml of a solution of potassium iodine 1%, pH 3.5. Each sample consisted of 15-30 1 of air. The iodine liberated is measured in the spectrophotometer UV-VIS at 352 nm and compared with the standard curve for iodine. Chlorine levels are calculated from the equation (see legend inFig. 1). This method is valid for measurements between 0.15 and 3 mg·m-3 with a lower detection limit of 0.05 mg·m-3 (approximately 0.02 ppm). The chlorine concentration in the water is obtained automatically through the pump that added the chlorine to the swimming pools and also by the conventional manual colorimetric method. Temperature and humidity of the room were also obtained.
Results are expressed as means and standard deviations (SD). At-test was used to compare average chlorine levels between pools. A two-way analysis of variance with repeated measures was used to determine the hypothesis that each chlorine determination was significantly different from another for each variable considered. Significance was set at the P< 0.05 level.
Air chlorine levels in the different swimming pools are presented onTable 2. There were significant differences between the pools (P < 0.005) except for the relation between “B” and “C.” The mean chlorine level of all the pools when there were more than six swimmers was 0.42 mg·m-3. These levels were between 7% and 63% of the threshold limited value (TLV). The mean level of 0.42 mg·m-3 corresponded to 28% of the TLV concentration. The concentration of chlorine increased from the first to the last measurements of the day (P < 0.05) (Fig. 2). A low correlation was found between the levels of chlorine and the different variables taken into account, swimming pool surface area (m2), number of swimmers, the number of swimmers per m2, and the volume of the enclosure (Table 3). The chlorine level in the breathing zone was significantly higher with a large number of swimmers, compared with five or fewer swimmers. The concentrations of chlorine in the water were within the normal values (0.5-2 mg·l1 free chlorine) as was the temperature (24-30°C). The relative humidity of the installations was between 60% and 70% in all the swimming pools except for “D,” where it was above 90%. The air temperatures of the pool enclosures were approximately 2-4°C above the temperature of the water. All of these values met existing regulations (14) except for the humidity level of “D.” The ventilation of the enclosure was also studied and a low turnover of air was observed. Chlorine was added directly to all the swimming pools but was circulated in different ways, either from the bottom or from one, two, or four sides of the pool. This was not an important factor.
The presence of acute symptoms of chlorine inhalation in swimmers has not been reported, except for a high prevalence of bronchial responsiveness(4). In our study, the chlorine levels in the microambient where the swimmer is breathing were within the TLV for chlorine gas exposures (13). Although the levels were“normal” the use of the swimming pool for recreation or training purposes must be considered. The TLV levels are based on exposure to chlorine in the working place for a sedentary person or an individual with a moderate activity. If we assume that the swimmer trains at least 2 h·d-1 with a ventilation 10-12 times the resting value, these ventilations would allow the swimmer to inhale a total amount of chlorine similar to the TLV(Table 4). The amount of chlorine inhaled could be greater if the training sessions were conducted twice daily as is often conducted. Frequent exposure to TLV levels of chlorine could result in an increased prevalence of bronchial hyperresponsiveness(4).
All the swimming pools analyzed had a low air-turnover rate that would explain the increment of the concentration of chlorine through the day. The primary reason for this low air exchange is the energy cost for the heating, ventilation, and air conditioning.
In conclusion, we recommend that the levels of chlorine in the zone where the swimmer breathes should be considered in establishing the normal values for the TLV or at least for chlorine levels in swimming pool enclosures. An adequate turnover must be established to maintain chlorine concentration at an acceptable level. Ventilation systems should be designed to maximize air exchange without compromising energy cost reduction strategies. Further studies to elucidate the role of chlorine in the appearance of asthma-like symptoms and bronchial hyperresponsiveness in swimmers need to be performed. Chlorine may not only impair the respiratory system with chronic intermittent exposure to these levels in some subjects (15), but reduction of swimming performance could occur as has been reported in studies of exercise performance in the presence of other strong oxidants(5,19). Swimming is often encouraged for pediatric asthmatic patients (2,16,20,21) because the humidity and warm ambient conditions for the aerobic exercise help to minimize the incidence of exercise-induced bronchoconstriction. The possibility that high air-chlorine levels may exacerbate bronchial reactivity needs also to be considered.
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