Sweat sodium concentration collected from the right and left arms and legs on the same day showed a very strong correlation confirming methodological consistency . However, a statistically significant intra-individual difference was demonstrated between sodium concentration in sweat secreted from the arms and legs, for both the summer and winter measurements (Table 2), also apparent in the mean data (Table 1). The sodium concentration in sweat samples taken from the legs was significantly less than from the arms (Tables 1&2). The difference in sweat sodium concentration between the arms and legs may be due to the difference in metabolic activity between the leg and arm muscles. The workload on the cycle ergometer required to reach 40 % VO2max is achieved by the leg muscles, producing significant metabolic heat energy, which has to be dissipated. However regardless of the causes of these regional differences the fact remains that sweat collection from one anatomical region may not be representative of whole body sodium loss.
There was a statistically significant change in sodium concentration between the first and second day in summer for the arms and to a lesser extent for the legs, suggesting that one heat exposure in summer is sufficient to trigger an acclimation effect. In winter this difference was not present. This short-term acclimation has previously been shown by Kirby and Convertino  however in their study sodium concentration was only measured on day 1 and day 10. As acclimation was being studied it is assumed the study was conducted in the cooler months. As the findings in the current study showed no variation in the first two days during winter, when subjects would be expected to be unacclimatised, it would appear that the triggering mechanism for increased sodium conservation in the unacclimatised state requires more than one heat exposure but is well established after 10 days. In contrast, in summer when subjects would be more acclimatised one exposure would appear to induce a sodium conservation response. The sweat glands may be more sensitive to aldosterone when in the acclimatised state. This was also postulated by Kirby and Convertino  who reported that decreased sweat sodium secretion was associated with significant reductions in plasma aldosterone during exercise in the heat following acclimation. The findings of the current study would reinforce increased sensitivity to aldosterone as the explanation for the seasonal differences. Further, the sensitivity is enhanced during summer when sodium retention would be important in order to prevent electrolyte disturbance due to chronic high sweat sodium loss.
The absence of any relationship between body composition, fitness or age and either sweat sodium concentration or sweat rate may come as a surprise, as exercise produces metabolic heat, which in turn induces sweating. On this basis it could be hypothesised that fitter individuals exercise more and therefore would have greater sodium conservation due to exercise-induced acclimatisation, however this appears not to be the case. Whether the metabolic heat generated is insufficient or environmental heat is a requirement remains to be fully demonstrated.
Sweat sodium concentrations in summer were less than in winter, the mean value for summer being 44.7 mmol.L-1 and winter 63.8 mmol.L-1. However the standard deviations for summer and winter were similar (24.1 in summer and 22.6 mmol.L-1 in winter). Therefore the variability in sweat sodium concentration was greater in summer than in winter (apparent in Figure 1). This supports variation reflecting inherent rather than lifestyle differences, as individuals seem to differ in their ability to acclimatise to the same environmental stress. The ethnicity of subjects was not recorded but genetics determining sweat gland density and sensitivity (receptors on the sweat duct) may have more influence than thought on the sweat response and the ability of the sweat gland to reabsorb sodium. Seasonal change to sodium loss reflects the well-known acclimatisation response. All the subjects were outdoor workers and were tested at the end of the summer months, when their acclimatisation would be expected to peak, and near the end of winter.
Future experiments should aim to clarify whether leg sweat glands have an inherently different capacity to sweat compared with the arms. Alternatively, since legs generally have a greater workload than arms, a training effect could occur to sweat glands in the lower limbs that results in a greater absorptive ability due to ductal hypertrophy or an increase in the concentration of enzymes involved in reabsorption, a possibility given some credibility by Fox et al  who showed a training effect on sweat glands. Similar findings were reported by Hofler .
One criticism of using local sweat collection methods has been that sodium concentration is usually higher than with using whole body techniques . Shirreffs and Maughan, who measured sodium loss using whole body washdown, reported sweat sodium as 50.8 mmol.L-1. However the time of year the study was conducted is not stated, so whether the subjects were acclimatised is not known. The mean sweat sodium concentration in summer (44.7 mmol.L-1) for the current study was slightly less than that described by Shirreffs and Maughan , the winter value (63.8 mmol.L-1) was higher as would be expected if unacclimatised were to be compared to acclimatised subjects. There is sound agreement between the two methods.
From a practical viewpoint, a number of findings from this study can be put to use by occupational physicians. It is common for miners and other manual workers to perform 12-hour shifts in hot environments. The sweat loss can be as high as 12 litres per day  but 8–10 litres is common . This represents a substantial fluid loss and demonstrates the importance of maintaining hydration status when working in the heat. These losses represent a substantial percentage of body weight and will rapidly lead to dehydration unless replacement fluid is consumed. In addition the sodium (Na) loss from sweating at this rate could exceed 10 g per day equivalent to 25 g of salt (NaCl). In this study the individual variation in both sweat rate and sodium concentration was substantial, however based on the mean data the sweat loss over a 10-hour shift even in a moderate environment would be 4.7 litres in summer and 4.1 litres in winter. There is currently no simple method to predict an individual's sweat composition, however on the basis of this study the average sodium concentration would be 45 mmol.L-1 in summer and 64 mmol.L-1 in winter (Table 3). The average acclimatised and unacclimatised sodium (Na) losses for a 10-hour shift in a moderate environment (35°C, 50 % RH) at 40% of VO2max would therefore be 4.8 g and 6 g, assuming the sweat rates and composition measured in this study were constant over the shift. The data predict that sodium loss would be greater in the unacclimatised individual (winter data) even with a lower sweat rate due to the higher sweat sodium concentration. Replacement of this daily electrolyte loss at regular intervals for individuals working in the heat is imperative in order to avoid possible electrolyte disturbance and impaired work performance. Given that the health message is to reduce sodium intake it becomes important that workers are educated as to the importance of eating during meal breaks and of having sodium rich foods when working in hostile conditions. Given the carbohydrate concentration in most sports drinks recommending these would not be sound; fluid replacement beverages should have far less carbohydrate and ideally more than 15 mmol.L-1 of sodium, although in the authors' experience palatability limits sodium content.