This study examined endotoxin-mediated cytokinemia during exertional heat stress (EHS). Subjects were divided into trained [TR; n=12, peak aerobic power (VO2peak)=70+/-2 ml.kg lean body mass(-1).min(-1)] and untrained (UT; n=11, VO2peak=50+/-1 ml.kg lean body mass(-1).min(-1)) groups before walking at 4.5 km/h with 2% elevation in a climatic chamber (40 degrees C, 30% relative humidity) wearing protective clothing until exhaustion (Exh). Venous blood samples at baseline and 0.5 degrees C rectal temperature increments (38.0, 38.5, 39.0, 39.5, and 40.0 degrees C/Exh) were analyzed for endotoxin, lipopolysaccharide binding protein, circulating cytokines, and intranuclear NF-kappaB translocation. Baseline and Exh samples were also stimulated with LPS (100 ng/ml) and cultured in vitro in a 37 degrees C water bath for 30 min. Phenotypic determination of natural killer cell frequency was also determined. Enhanced blood (104+/-6 vs. 84+/-3 ml/kg) and plasma volumes (64+/-4 vs. 51+/-2 ml/kg) were observed in TR compared with UT subjects. EHS produced an increased concentration of circulating endotoxin in both TR (8+/-2 pg/ml) and UT subjects (15+/-3 pg/ml) (range: not detected to 32 pg/ml), corresponding with NF-kappaB translocation and cytokine increases in both groups. In addition, circulating levels of tumor necrosis factor-alpha and IL-6 were also elevated combined with concomitant increases in IL-1 receptor antagonist in both groups and IL-10 in TR subjects only. Findings suggest that the threshold for endotoxin leakage and inflammatory activation during EHS occurs at a lower temperature in UT compared with TR subjects and support the endotoxin translocation hypothesis of exertional heat stroke, linking endotoxin tolerance and heat tolerance.

This study examined whether active or passive cooling during intermittent work reduced the heat strain associated with wearing firefighting protective clothing (FPC) and self-contained breathing apparatus (SCBA) in the heat (35 degrees Celsius, 50% relative humidity). Fifteen male Toronto firefighters participated in the heat-stress trials. Subjects walked at 4.5 km.h(-1) with 0% elevation on an intermittent work (50 min) and rest (30 min) schedule. Work continued until rectal temperature (T(re)) reached 39.5 degrees Celsius, or heart rate (HR) reached 95% of maximum or exhaustion. One of three cooling strategies, forearm submersion (FS), mister (M), and passive cooling (PC) were employed during the rest phases. Tolerance time (TT) and total work time (WT) (min) were significantly increased during FS (178.7 +/- 13.0 and 124.7 +/- 7.94, respectively) and M (139.1 +/- 8.28 and 95.1 +/- 4.96, respectively), compared with PC (108.0 +/- 3.59 and 78.0 +/- 3.59). Furthermore, TT and WT were significantly greater in FS compared with M. Rates of T(re) increase, HR and T-(sk) were significantly lower during active compared with passive cooling. In addition, HR and T(re) values in FS were significantly lower compared with M after the first rest phase. During the first rest phase, T(re) dropped significantly during FS (approximately 0.4 degree Celsius) compared with M (approximately 0.08 degree Celsius) while PC increased (approximately 0.2 degree Celsius). By the end of the second rest period T(re) was 0.9 degree Celsius lower in FS compared with M. The current findings suggest that there is a definite advantage when utilizing forearm submersion compared with other methods of active or passive cooling while wearing FPC and SCBA in the heat.

T underestimated and U consistently perceived their physiological strain, as defined by PhSI, in accordance with the measured increases in core temperature and heart rate throughout an exposure to uncompensable exercise-heat stress.

This study examined the independent and combined importance of aerobic fitness and body fatness on physiological tolerance and exercise time during weight-bearing exercise while wearing a semipermeable protective ensemble. Twenty-four men and women were matched for aerobic fitness and body fatness in one of four groups (4 men and 2 women in each group). Aerobic fitness was expressed per kilogram of lean body mass (LBM) to eliminate the influence of body fatness on the expression of fitness. Subjects were defined as trained (T; regularly active with a peak aerobic power of 65 ml · kg LBM−1 · min−1) or untrained (UT; sedentary with a peak aerobic power of 53 ml · kg LBM−1 · min−1) with high (High; 20%) or low (Low; 11%) body fatness. Subjects exercised until exhaustion or until rectal temperature reached 39.5°C or heart rate reached 95% of maximum. Exercise times were significantly greater in TLow(116 ± 6.5 min) compared with their matched sedentary (UTLow; 70 ± 3.6 min) or fatness (THigh; 82 ± 3.9 min) counterparts, indicating an advantage for both a high aerobic fitness and low body fatness. However, similar effects were not evident between THigh and UTHigh(74 ± 4.1 min) or between the UT groups (UTLow and UTHigh). The major advantage attributed to a higher aerobic fitness was the ability to tolerate a higher core temperature at exhaustion (the difference being as great as 0.9°C), whereas both body fatness and rate of heat storage affected the exercise time as independent factors.

This study examined the relationship between time to reach critical end points (tolerance time [TT] and metabolic rate for three different environmental temperatures (25 degrees C, 30 degrees C, and 35 degrees C, 50% relative humidity), while wearing firefighting protective clothing (FPC) and self-contained breathing apparatus (SCBA). Thirty-seven Toronto firefighters (33 male and 4 female) were divided into four work groups defined as Heavy (H, n = 9), Moderate (M, n = 9), Light (L, n = 10), and Very Light (VL, n = 9). At 25 degrees C, 30 degrees C, and 35 degrees C, TT (min) decreased from 56 to 47 to 41 for H, 92 to 65 to 54 for M, 134 to 77 to 67 for L, and 196 to 121 to 87 for VL. Significant differences in TT were observed across all group comparisons, excluding M versus L at 30 degrees C and 35 degrees C, and H versus M at 35 degrees C. Comparing 25 degrees C to 30 degrees C, M, L, and VL had significant decreases in TT, whereas only VL had a significant decrease when 30 degrees C was compared to 35 degrees C. For 25 degrees C to 30 degrees C, the relative change in TT was significantly greater for L (37%) and VL (41%) compared with H (16%) and M (26%). For 30 degrees C to 35 degrees C, the relative change among the groups was similar and approximately 17%. During passive recovery at 35 degrees C, rectal temperature (T(re)) continued to increase 0.5 degrees C above T(re final), whereas heart rate declined significantly. These findings show the differential impact of environmental conditions at various metabolic rates on TT while wearing FPC and SCBA. Furthermore, these findings reveal passive recovery may not be sufficient to reduce T(re) below pre-recovery levels when working at higher metabolic rates in hot environments.

This report provides a summary of research conducted through a grant provided by the Workplace Safety Insurance Board of Ontario. The research was divided into two phases; first, to define safe work limits for firefighters wearing their protective clothing and working in warm environments; and, the second, to examine strategies to reduce the thermal burden and extend the operational effectiveness of the firefighter. For the first phase, subjects wore their protective ensemble and carried their self-contained breathing apparatus (SCBA) and performed very light, light, moderate or heavy work at 25°C, 30°C or 35°C. Thermal and evaporative resistance coefficients were obtained from thermal manikin testing that allowed the human physiological responses to be compared with modeled data. Predicted continuous work times were then generated using a heat strain model that established limits for increases in body temperature to 38.0°C, 38.5°C and 39.0°C. Three experiments were conducted for the second phase of the project. The first study revealed that replacing the duty uniform pants that are worn under the bunker pants with shorts reduced the thermal strain for activities that lasted longer than 60 min. The second study examined the importance of fluid replacement. The data revealed that fluid replacement equivalent to at least 65% of the sweat lost increased exposure time by 15% compared with no fluid replacement. The last experiment compared active and passive cooling. Both the use of a mister or forearm and hand submersion in cool water significantly increased exposure time compared with passive cooling that involved only removing most of the protective clothing. Forearm and hand submersion proved to be most effective and produced dramatic increases in exposure time that approximated 65% compared with the passive cooling procedure. When the condition of no fluid replacement and passive cooling was compared with fluid replacement and forearm and hand submersion, exposure times were effectively doubled with the latter condition. The heat stress wheel that was generated can be used by Commanders to determine safe work limits for their firefighters during activities that involve wearing their protective clothing and carrying their SCBA.

Selkirk GA, McLellan TM, Wright HE, Rhind SG. Expression of intracellular cytokines, HSP72, and apoptosis in monocyte subsets during exertional heat stress in trained and untrained individuals. Am J Physiol Regul Integr Comp Physiol 296: R575-R586, 2009. First published January 21, 2009 doi:10.1152/ajpregu.90683.2008.-This study examined intracellular cytokine, heat shock protein (HSP) 72, and cellular apoptosis in classic and inflammatory CD14 ϩ monocyte subsets during exertional heat stress (EHS). Subjects were divided into endurance-trained [TR; n ϭ 12, peak aerobic power (V O2peak) ϭ 70 Ϯ 2 ml⅐ kg lean body mass (LBM) Ϫ1 ⅐ min Ϫ1 ] and sedentary-untrained (UT; n ϭ 11, V O2peak ϭ 50 Ϯ 1 ml⅐ kg LBM Ϫ1 ⅐ min Ϫ1 ) groups before walking at 4.5 km/h with 2% elevation in a climatic chamber (40°C, 30% relative humidity) wearing protective clothing until exhaustion (Exh). Venous blood samples at baseline and 0.5°C rectal temperature increments (38.0, 38.5, 39.0, 39.5, and 40.0°C/Exh) were analyzed for cytokines (TNF-␣, IL-1␤, IL-6, IL-1ra, and IL-10) in CD14 ϩϩ CD16 Ϫ /CD14 ϩ CD16 ϩ and HSP72/apoptosis in CD14 Bri / CD14 Dim subsets. In addition, serum levels of extracellular (e)HSP72 were also examined. Baseline and Exh samples were separately stimulated with LPS (1 g/ml) or heat shocked (42°C) and cultured in vitro for 2 h. A greater temperature-dependent increase in CD14 ϩ CD16 ϩ cells was observed in TR compared with UT subjects as well as a greater LPS tolerance following in vitro LPS stimulation. TNF-␣ and IL-1␤ cytokine expression was elevated in CD14 ϩ CD16 ϩ but not in CD14 ϩϩ CD16 Ϫ cells. A greater induction of intracellular HSP72 and eHSP72 was observed in TR compared with UT subjects, which coincided with reduced apoptosis at Exh and following in vitro heat shock. Induced HSP in vitro was not uniform across CD14 ϩ subsets. Findings suggest that circulating CD14 ϩ CD16 ϩ , but not CD14 ϩϩ CD16 Ϫ monocytes, contribute to the proinflammatory cytokine profiles observed during EHS. In addition, the enhanced HSP72 response in endurance-trained individuals may confer improved heat tolerance through both anti-inflammatory and anti-apoptotic mechanisms. immune function; cardiovascular/thermoregulatory strain; flow cytometry; heat shock protein PERIPHERAL BLOOD MONOCYTES play an important role in protection against invading pathogens and activation of innate immunity (46). Based on differential expression of antigenic markers CD14 [part of the LPS receptor, CD14/Toll-like receptor (TLR-4)/MD-2] and CD16 (Fc␥RIII), monocytes can be divided into two phenotypically and functionally distinct subsets (82, 90). The bulk of monocytes are defined as classic monocytes and are strongly positive for surface receptor CD14 (CD14 ϩϩ CD16 Ϫ ), whereas the minor subset are referred to as inflammatory monocytes (CD14 ϩ CD16 ϩ ) because of their high capacity to express proinflammatory cytokines such as TNF-␣ (6, 62, 64, 90). Inflammatory monocytes have the ability to respond directly with antimicrobial activity, whereas the clas...

It was the purpose of this study to examine whether replacing long pants (P) with shorts (S) would reduce the heat stress of wearing firefighting protective clothing during exercise in a warm environment. Twenty-four Toronto Firefighters were allocated to one of four groups that performed heavy (H, 4.8 km x h(-1), 5% grade), moderate (M, 4.5 km x h(-1), 2.5% grade), light (L, 4.5 km x h(-1)) or very light (VL, 2.5 km x h(-1)) exercise while wearing their full protective ensemble and self-contained breathing apparatus. Participants performed a familiarization trial followed by two experimental trials at 35 degrees C and 50% relative humidity wearing either P or S under their protective overpants. Replacing P with S had no impact on the rectal temperature (Tre) or heart rate response during heavy or moderate exercise where exposure times were less than 1 h (40.8 +/- 5.8 and 53.5 +/- 9.2 min for H and M, respectively while wearing P, and 43.5 +/- 5.3 and 54.2 +/- 8.4 min, respectively while wearing S). In contrast, as exposure times were extended during lighter exercise Tre was reduced by as much as 0.4 degrees C after 80 min of exercise while wearing S. Exposure times were significantly increased from 65.8 +/- 9.6 and 83.5 +/- 11.6 min during L and VL, respectively while wearing P to 73.3 +/- 8.4 and 97.0 +/- 12.5 min, respectively while wearing S. It was concluded that replacing P with S under the firefighting protective clothing reduced the heat stress associated with wearing the protective ensemble and extended exposure times approximately 10 - 15% during light exercise. However, during heavier exercise where exposure times were less than 1 h replacing P with S was of little benefit.