Seven young (Y, 22-28 yr) and seven middle-aged (MA, 49-60 yr) normotensive men of similar body size, fatness, and maximal oxygen uptake (VO2max) were exposed to a heat challenge in an environmental chamber (48 degrees C, 15% relative humidity). Tests were performed in two hydration states: hydrated (H, 25 ml water/kg body wt 1 h before the test, 2.5 h before exercise) and hypohydrated (Hypo, after 18-20 h of water deprivation). Each test began with a 90-min rest period during which the transiently increased plasma volume and decreased osmolality after drinking in the H condition returned to base line. This period was followed by 30 min of cycle exercise at a mean intensity of 43% VO2max and a 60-min resting recovery period with water ad libitum. Although prior drinking caused no sustained changes in plasma osmolality, Hypo increased plasma osmolality by 7-10 mosmol/kg in both groups. There were no significant age differences in water intake, urine output or osmolality, overall change in body weight, or sweating rate. In the H state, the percent change in plasma volume was less (P less than 0.01) during exercise for the Y group (-5.9 +/- 0.7%) than for the MA group (-9.4 +/- 0.6%). Esophageal temperature (Tes) was higher in the Hypo condition for both groups with no age-related differences. Throughout the 3-h period, mean skin temperature was higher in the Y group and significantly so (P less than 0.05) in the Hypo condition.(ABSTRACT TRUNCATED AT 250 WORDS)

These observations have two implications. First, one cannot assume that PV changes reflect those of the entire extracellular compartment. Second, since immersion also increases interstitial fluid pressure, fluid leaving the interstitium must have been rapidly replaced by intracellular water.

This project examined the effects of repeated, resting cold-water immersion on metabolic heat production and core temperature defence during subsequent rest and exercising immersions. Seven males undertook 15 days of cold-water adaptation, immersed to the fourth intercostal space, with cold-water stress tests (CWST) on days 1, 8 and 15 (18.1 SD 0.1°C: 60 min seated, followed by 30 min cycling (1 W·kg -1 )), and 90-min resting immersions (18.4 SD 0.4°C) on each of the intervening days. Adaptation elicited an habituated thermogenic response during the rest phase of CWST3 beyond 20 min, compared to CWST1 (P<0.05), with oxygen consumption averaging 11.15 (± 0.25) ml·kg -1 ·min -1 and 8.61 (± 0.90) ml·kg -1 ·min -1 by 50 min, for CWST1 and CWST3, respectively. During exercise, this metabolic blunting was only apparent over the first 10-min period (60-70 min). No significant differences were observed during either the rest or exercise phases of the CWSTs for oesophageal temperature (T es ). While repeated coldwater exposures produced an habituated-thermogenic response, for an equivalent drop in T es during rest, neither this response, nor an elevated thermogenesis, were apparent during subsequent cold-water exercise.

When work is performed by workers in protective clothing, sweat evaporation is limited and body temperature rises. In an attempt to quantify the limits such ensembles place on safe work, 6 acclimated men and women walked at 30% VO2max (150-200 W/m2) in 2 protocols involving environmental transients. In one, ambient water vapor pressure (Pw) was fixed at 10 torr, and after rectal temperature (Tre) plateaued, ambient dry-bulb temperature (Tdb) was raised 2 degrees C every 10 min. In the second, Tdb was constant and Pw was increased 2 torr every 10 min. Critical temperature (Tcrit) and pressure (Pcrit) were defined as the Tdb or Pw at which thermal balance could no longer be maintained and Tre rose sharply. Each test was performed in various clothing ensembles ranging from light cotton work clothes to "impermeable" suits. Lines connecting mean Tcrit and mean Pcrit define a limit for safe prolonged exposure/exercise for approximately 50% of the population in each ensemble. Similar lines, drawn to represent values 2 standard deviations below the mean, should provide critical environmental limits for 95% of the population.

When work is performed in heavy clothing, evaporation of sweat from the skin to the environment is limited by layers of wet clothing and air. The magnitude of decrement in evaporative cooling is a function of the clothing's resistance to permeation of water vapor. A physiological approach has been used to derive effective evaporative coefficients (he) which define this ability to evaporate sweat. We refined this approach by correcting the critical effective evaporative coefficient (K for sweating efficiency (Ke,eta') since only a portion of the sweat produced under such conditions is evaporated through the clothing. Six acclimated men and women walked at 30% maximal O2 consumption (150-200 W.m-2) at a constant dry bulb temperature as ambient water vapor pressure was systematically increased 1 Torr every 10 min. Critical pressure was defined as the partial pressure of water vapor (Pw) at which thermal balance could no longer be maintained and rectal temperature rose sharply. Each test was performed in various clothing ensembles ranging from cotton shirt and pants to "impermeable" suits. This approach was used to derive he by solving the general heat balance equation, M - W +/- (R + C) = w.he.(Psk - Pw), where M is metabolic heat production, W is external work, R is radiant heat exchange, C is convective heat transfer, w is skin wettedness, and Psk is water vapor pressure of fully wet skin.(ABSTRACT TRUNCATED AT 250 WORDS)