Head and/or torso cooling during simulated cockpit heat stress.

Liquid-conditioned garments (LCG's) may be useful in alleviating heat stress on aircrew members. This study evaluated the effects of a liquid-cooled vest and/or cap on subjects exposed to a thermal environment which simulated the stress of low-level flight in hot weather. The chamber was set at dry bulb temperature 35 degrees C, wet bulb temperature 26 degrees C, and black globe temperature 43 degrees C. Subjects were eight men, aged 23-45 yr. Each subject was studied in four 100-min exposures: uncooled control (XX); head cooling only (HX); torso cooling only (XT); and combined head and torso cooling (HT). Cooling effects were evaluated by using rectal temperature (Tre); heart rate (HR); sweat rate (SRF and SRT for forehead and total body, respectively); and subjective comfort. Uncooled runs (XX) produced final values: Tre = 37.8 degrees C, HR = 94 beats/min, SRT = 25 g/m2 X h and SRF = 45 g/m2 X h. Combined head and torso cooling maintained the subjects at nearly their initial (baseline) state. Cap effects per se were assessed by comparing HX to XX, and HT to XT. Head cooling proved 2-3 times as efficient as torso cooling, although limited by the small surface area involved. We conclude that the specific effects of head cooling in improving comfort and diminishing forehead sweating, as well as the efficient systemic cooling observed, justify further work on cockpit applications.

Head-temperature effects on physiology, comfort, and performance during hyperthermia.

This study assessed the effects of head temperature on subjects under hyperthermic conditions. Six volunteers were trained to plateau on a manikin task that tested cognitive performance and reaction time. A subject's core temperature was driven with a full-length, liquid-conditioned garment perfused with water at 30 degrees or 43 degrees C, and head temperature was controlled by a cap with an inlet temperature of 8 degrees or 43 degrees C. Heavy insulation was worn overall; ambient temperature was 30 degrees C. Subjects were heated and cooled twice during each experiment to include all possible cap/suit temperature combinations. Each subject did one control and four stress experiments. Comfort and performance were measured as the subject's esophageal temperature (Tes) rose and fell through the range 37.5-39.0 degrees C. Cap temperature did not affect rectal temperature (Tre) but significantly altered the Tes rate of change; a cool head sometimes truncated the peak Tes value. Although the cap covered only 3-4% of the body surface, the head was a major determinant of subjective comfort. Body heating tended to shorten reaction time and diminish performance accuracy, while head cooling largely reversed these trends. Possible mechanisms include 1) change in overall heat balance; 2) countercurrent exchanges in the neck, and 3) change in sensory output. Head cooling deserves serious consideration for machinery operators where whole-body thermoneutrality is impractical.