Respiratory heat loss was measured during cold water (5–6° C) excursions to depths of 300 m. Losses were computed with and without respiratory gas heating when wearing a diving helmet in current commercial and Service use. A magnitude of heat loss, sufficient to cause undue stress to the respiratory tract, was observed even with gas heating provided, particularly at maximum depth. If the hyperbaric gas is both heated and humidified to a maximum comfort level then respiratory gains in the order of 40 W are possible at 250 m. This technique of utilizing warm wet gas introduced into the respiratory tract was tested as a rewarming technique following immersion in cold water (4–5° C) to the limit of peripheral endurance.

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