Relationship between clothing ventilation and thermal insulation.

Air layers trapped within a clothing microenvironment contribute to the thermal insulation afforded by the ensemble. Any exchange of air between the external environment and these trapped air layers results in a change in the ensemble's thermal insulation and water vapor resistance characteristics. These effects are seldom taken into account when considering the effects of clothing on human heat balance, the thermal characteristics usually being restricted to intrinsic insulation and intrinsic evaporative resistance measurements on static manikins. Environmental assessments based on these measurements alone may therefore lead to under-(or over-) estimation of thermal stress of the worker. The aim of this study was to quantify the relationship between clothing ventilation and thermal insulation properties. A one-layer, air-impermeable ensemble and a three-layer, air-permeable ensemble were tested using an articulated, thermal manikin in a controlled climate chamber (ta = tr = 10 degrees C, PaH2O = 0.73 kPa). The manikin, which was designed for thermal insulation measurements, was also equipped with a system to determine clothing ventilation. Baseline measurements of clothing ventilation (VT) and thermal insulation (total clothing insulation: I(T)--measured, intrinsic insulation: Icl--calculated) were made of the clothing with the manikin standing stationary in still air conditions. Increased clothing ventilation was induced when the manikin "walked" (walking speeds of 0.37 m/sec and 0.77 m/sec) and by increasing the environmental air speed (Va = 1.0 m/sec). These increases in VT reduced Icl, this being ascribed to the increased heat transfer from the manikin skin surface to the cooler external environment due to the exchange of air between the clothing microenvironment and the external environment. Measured air exchanges were shown to have a potential heat exchange capacity of up to 17 and 161 W/m2 for the one- and three-layer ensembles, respectively, emphasizing the need to take clothing ventilation characteristics into consideration during thermal audits and thermal risk assessments.

Effects of dehydration, hypohydration, and hyperhydration on tolerance during uncompensable heat stress.

The present study examined the effects of dehydration from prior exercise on subsequent exercise tolerance time (TT) that involved wearing nuclear, biological, and chemical (NBC) protective clothing. It was hypothesised that TT would be reduced in the dehydrated state. Ten men undertook continuous treadmill walking at 4.8 km.h-1 at 35 degrees C and 50% relative humidity, wearing NBC clothing while euhydrated (EU) or dehydrated (D) by 2.3% of body weight. Hydration status had no impact on thermoregulatory or cardiovascular responses during exercise. Also rectal temperature at exhaustion did not differ between EU (38.52 +/- 0.39 degrees C) and D (38.43 +/- 0.45 degrees C). Exercise TT during this uncompensable heat stress was reduced significantly for D (47.7 +/- 15.3 min) compared with EU (59.0 +/- 13.6 min). It was concluded that prior exercise leading to levels of dehydration to 2.3% of body weight, together with subsequent fluid restriction during exposure to uncompensable heat stress, impaired TT while wearing the NBC protective clothing. The integration of these findings together with other comparable studies that have examined the influence of hypo- and hyperhydration on TT while wearing NBC protective clothing revealed that hydration status has less effect on TT as the severity of uncompensable heat stress increases.

Heat strain models applicable for protective clothing systems: comparison of core temperature response.

Core temperature (Tc) output comparisons were analyzed from thermal models applicable to persons wearing protective clothing. The two models evaluated were the United States (US) Army Research Institute of Environmental Medicine (USARIEM) heat strain experimental model and the United Kingdom (UK) Loughborough (LUT25) model. Data were derived from collaborative heat-acclimation studies conducted by three organizations and included an intermittent-work protocol (Canada) and a continuous-exercise/heat stress protocol (UK and US). Volunteers from the US and the UK were exposed to a standard exercise/heat stress protocol (ambient temperature 35 degrees C/50% relative humidity, wind speed 1 m/s, level treadmill speed 1.34 m/s). Canadian Forces volunteers did an intermittent-work protocol (15 min moderate work/15 min rest at ambient temperature of 40 degrees C/30% relative humidity, wind speed approximately 0.4 m/s). Each model reliably predicted Tc responses (within the margin of error determined by 1 root mean square deviation) during work in the heat with protective clothing. Models that are analytically similar to the classic Stolwijk-Hardy model serve as robust operational tools for prediction of physiological heat strain when modified to incorporate clothing heat-exchange factors.

Fluid and electrolyte homeostasis during prolonged exercise at altitude.

The combined effect of exercise and altitude on fluid and electrolyte homeostasis was studied over 13 days on six male subjects eating a diet with constant sodium and potassium content. During the first 4 and last 4 days subjects were semisedentary at an altitude of 900 m. In the middle 5 days subjects exercised by hill walking for about 7 h daily at altitudes between 2,678 and 3,629 m. There was a retention of sodium (mean of 202 mM by the end of the exercise-altitude period) and a small retention of water (mean of 0.49 liters). Plasma volume increased by 0.76 liters and packed cell volume fell from a mean of 44.5 to 41.8%. There was no change in plasma sodium concentration. The retention of sodium implies an expansion in the extracellular space of 1.44 liters at the expense of the intracellular space, which decreased by a calculated 1.05 liters. These changes are similar to those resulting from comparable exercise at sea level and opposite to the effect of altitude on resting subjects.

Effect of prolonged exercise at altitude on the renin-aldosterone system.

The combined effect of exercise and altitude on the renin-aldosterone system was studied in six male subjects on a fixed diet. After 4 control days at rest and at low altitude, subjects ascended to 3,100 m and took about 7 h exercise daily for 5 days. There followed a 4-day recovery period at low altitude. Daily blood samples were taken for estimation of plasma renin activity (PRA), plasma aldosterone concentration (PAC), and angiotensin converting-enzyme (ACE) activity. Results showed a maximal rise in PRA and PAC with exercise at altitude maximal on the first 2 days. ACE activity fell by 23% at altitude. Compared with similar exercise at sea level, the rise in PAC was comparable but the rise in PRA was four times greater, indicating a marked decrease in PAC response to PRA. It is suggested that this loss of sensitivity of PAC to PRA is mediated by the measured reduction in ACE activity.

Sodium balance, fluid homeostasis and the renin-aldosterone system during the prolonged exercise of hill walking.

1. The effect of 5 consecutive days of hill walking on electrolyte balance, fluid homeostasis, plasma renin activity and plasma aldosterone concentration was studied in five male subjects. 2. The 5-day exercise period was preceded by a 4-day control period and followed by a 4-day recovery period. Throughout the 13-day study subjects ate a fixed diet. 3. After 5 days of exercise subjects had retained a mean of 264 mmol (SD 85) of sodium. Plasma sodium concentration remained constant at 142.0 mmol/l (SD 5.4). This indicates an expansion of the extracellular space by 1.84 litres. 4. By the end of the exercise period there was a positive water balance of about 0.9 litre. Thus there was a net movement of 0.94 litre of fluid from the intracellular to the extracellular space. 5. Packed cell volume decreased from a mean of 43.5% to 37.9% after 5 days of exercise, indicating that about 0.9 litre of the extracellular fluid entered the vascular compartment. The remaining fluid may be responsible for the significant increase in lower leg volume. 6. During the exercise period plasma aldosterone concentration and plasma renin activity rose, and there was a highly significant correlation between these values and the sodium retention. There was also a significant correlation between sodium retention and the increase in leg volume, which suggests that oedema may be the result of prolonged exercise of this type.

Effect of the exercise of seven consecutive days hill-walking on fluid homeostasis.

1. The effect of 7 consecutive days of strenuous exercise, hill-walking, on water balance and distribution was studied in five subjects. The exercise was preceded and followed by 3 control days. The diet was fixed throughout but water was allowed ad libitum. 2. Packed cell volume was measured daily. Serum electrolytes and arginine vasopressin were measured twice daily. Daily water, sodium and potassium balances were calculated. 3. During exercise there was a fall in packed cell volume, reaching a maximum of 11% by day 5 and a retention of sodium reaching a cumulative maximum of 358 mmol by day 6. During and immediately after exercise there was a retention of potassium, reaching a total of 120 mmol by day 3 after stopping exercise. 4. There was a loss of 650 ml of water on day 1 of exercise, followed by a modest retention reaching a cumulative maximum of 650 ml on day 5 of exercise. 5. Neither arginine vasopressin nor serum electrolyte concentrations were affected by exercise. 6. From the packed cell volume, sodium and water balances it was calculated that by day 5 of exercise there was an increase in plasma volume of .068 litre (22%), an increase in interstitial fluid volume of 2.0 litres (17%) and a decrease in intracellular fluid volume of 1.8 litres (8%). 7. These changes, together with the clinical observation of facial and ankle oedema during the experiemnt, suggest that continuous exercise may cause oedema and thus may be a factor in the aetiology of high-altitude oedema.

Distribution of blood flow in the hypothermic (27 degrees C) dog kidney.

1. Renal function was measured in seven normo thermic (38 degrees C) and seven hypothermic (27 degrees C) dogs. 2. The glomerular filtration rate was 60 per cent lower in the hypothermic animals, and the renal blood flow as 51 per cent lower. The intrarenal distribution of blood flow was measured by the uptake of 86Rb from the blood into different regions of the kidney. Hypothermia reduced flow by 34 per cent in the outer cortex, 72 per cent in the inner cortex, 61 per cent in the outer medulla and 69 per cent in the inner medulla. 3. Radioautography indicated a high blood flow to an area of the outer medulla of hypothermic kidneys, which may indicate medullary blood flow "shunting". 4. The results have been discussed in relation to a number of clinical and experimental observations.

Autoregulation of renal blood flow in dogs at normal body temperature and at 27 degrees C.

1. Dogs cooled to 27 degrees C were compared with control dogs maintained at 38 degrees C. The mean arterial blood pressure, renal blood flow and glomerular filtration rate were lower in the hypothermic animals. 2. The relation between mean arterial blood pressure and renal blood flow was investigated. Auto-regulation of renal blood flow occurred in the kidneys of normothermic and hypothermic animals. Thus the reduction in renal blood flow during hypothermia is not due simply to the fall in mean arterial blood pressure. 3. Similarities between recordings of renal blood flow obtained at 38 degrees C and 27 degrees C suggest that its autoregulation occurs by the same mechanism at the two temperatures. 4. Autoregulation of renal blood flow occurred in hypothermic kidneys in the presence of a cold-induced vasoconstriction. The observed responses to cold and to alterations in mean arterial blood pressure may take place in different areas of the renal vasculature.