Evaluating physiological strain during cold exposure using a new cold strain index.

A cold strain index (CSI) based on core (Tcore) and mean skin temperatures (T sk) and capable of indicating cold strain in real time and analyzing existing databases has been developed. This index rates cold strain on a universal scale of 0-10 and is as follows: CSI = 6.67(Tcoret - Tcore 0) ⋅ (35 - Tcore 0)-1 + 3.33(T sk t -T sk 0) ⋅ (20 -T sk 0)-1, where Tcore 0 andT sk 0are initial measurements and Tcoret andT skt are simultaneous measurements taken at any time t; when Tcoret > Tcore 0, then Tcoret - Tcore 0 = 0. CSI was applied to three databases. The first database was obtained from nine men exposed to cold air (7°C, 40% relative humidity) for 120 min during euhydration and two hypohydration conditions achieved by exercise-heat stress-induced sweating or by ingestion of furosemide 12 h before cold exposure. The second database was from eight men exposed to cold air (10°C) immediately on completion of 61 days of strenuous outdoor military training, 48 h later, and after 109 days. The third database was from eight men repeatedly immersed in 20°C water three times in 1 day and during control immersions. CSI significantly differentiated ( P < 0.01) between the trials and individually categorized the strain of the subject for two of these three databases. This index has the potential to be widely accepted and used universally.