Physiological responses and manual performance in humans following repeated exposure to severe cold at night

We evaluated human physiological responses and the performance of manual ta sks during exposure to severe cold (-25 degreesC) at night (0300-0500 hours ) and in the afternoon (1500-1700 hours). Thirteen male students wearing st andard cold protective clothing occupied a severely cold room (-25 degreesC ) for 20 min, and were then transferred to a cool room (10 degreesC) for 20 min. This pattern of exposure was repeated three times, for a total time o f exposure to extreme cold of 60 min. The experiments were started either a t 1500 hours or 0300 hours and measurements of rectal temperature, skin tem perature, blood pressure, performance in a counting task, hand tremor, and subjective responses were made in each condition. At the end of the experim ent at night the mean decrease in rectal temperature [0.68 (SEM 0.04)degree sC] was significantly greater than that at the end of the experiment in the afternoon [0.55 (SEM 0.08)degreesC, P < 0.01]. After the second cold expos ure at night the mean increase in diastolic blood pressure [90 (SEM 2.0) mm Hg] was significantly greater than that at the end of the second cold expos ure in the afternoon [82 (SEM 2.8) mmHg, P < 0.01]. At the end of the secon d cold exposure at night, mean finger skin temperature [11.8 (SEM 0.8)degre esC] was significantly higher than that at the comparable time in the after noon [9.0 (SEM 0.7)degreesC, P < 0.017. Similarly for the toe, mean skin te mperature at the start of the second cold exposure at night [25.6 (SEM 1.5) degreesC] was significantly higher than in the afternoon [20.1 (SEM 0.8)deg reesC, P <0.01]. The increased skin temperatures in the periphery resulted in increased heat loss. Since peripheral skin temperatures were highest at night, the subjects noted diminished sensations of thermal cold and pain at that time. Manual dexterity at the end of the first cold exposure at night [mean 83.7 (SEM 3.6) times(.)min(-1)] had decreased significantly more tha n at the end of the first cold exposure in the afternoon [mean 89.4 (SEM 3. 5) times (.) min(-1) P < 0.01]. These findings of a lowered rectal temperat ure and diminished manual dexterity suggest that there is an increased risk of both hypothermia and accidents for those who work at night.