Heat strain reduction by ice-based and vapor compression liquid cooling systems with a toxic agent protective uniform

Abstract

Background: The purpose of this study was to compare a vapor compression microclimate cooling system (MCC) and a personal ice cooling system (PIC) for their effectiveness in reducing physiological strain when used with cooling garments worn under the impermeable self-contained toxic environment protective outfit (STEPO). A second comparison was done between the use of total body (TOTAL) and hooded shirt-only (SHIRT) cooling garments with both the MCC and PIC systems. It was hypothesized that the cooling systems would be equally effective, and total body cooling would allow 4 h of physical work in the heat while wearing STEPO.

Methods: Eight subjects (six men, two women) attempted four experiments at 38 degrees C (100 degrees F), 30% rh, 0.9 m x sec(-1) wind, while wearing the STEPO. Subjects attempted 4 h of treadmill walking (rest/exercise cycles of 10/20 min) at a time-weighted metabolic rate of 303 +/- 50 W.

Results: Exposure time was not different between MCC and PIC, but exposure time was greater with TOTAL (131 +/- 66 min) than with SHIRT (83 +/- 27 min) for both cooling systems (p < 0.05). Cooling rate was not different between MCC and PIC, but cooling rate while wearing TOTAL (362 +/- 52 W) was greater than with SHIRT (281 +/- 48 W) (p < 0.05). Average heat storage was lower with MCC (39 +/- 20 W x m(-2)) than with PIC (50 +/- 17 W x m(-2)) in both TOTAL and SHIRT (p < 0.05). Also, average heat storage while wearing TOTAL (34 +/- 19 W x m(-2)) was less than with SHIRT (55 +/- 13 W x m(-2)) for both cooling systems (p < 0.05). The Physiological Strain Index (PSI) was lower in MCC-TOTAL (2.4) than MCC-SHIRT (3.7), PIC-SHIRT (3.8), and PIC-TOTAL (3.3) after 45 min of heat exposure (p < 0.05).

Conclusions: Total body circulating liquid cooling was more effective than shirt-only cooling under the impermeable STEPO uniform, providing a greater cooling rate, allowing longer exposure time, and reducing the rate of heat storage. The MCC and PIC systems were equally effective during heat exposure, but neither system could extend exposure for the 4 h targeted time.