Pretorius T, Cahill F, Kocay S, Giesbrecht GG. Shivering heat production and core cooling during head-in and head-out immersion in 17°C water. Aviat Space Environ Med 2008; 79:495–9. Introduction: Many cold-water scenarios cause the head to be partially or fully immersed (e.g., ship wreck survival, scuba diving, cold-water adventure swim racing, cold-water drowning, etc.). However, the specific effects of head cold exposure are minimally understood. This study isolated the effect of whole-head submersion in cold water on surface heat loss and body core cooling when the protective shivering mechanism was intact. Methods: Eight healthy men were studied in 17°C water under four conditions: the body was either insulated or exposed, with the head either out of the water or completely submersed under the water within each insulated/exposed subcondition. Results: Submersion of the head (7% of the body surface area) in the body-exposed condition increased total heat loss by 11% (P < 0.05). After 45 min, head-submersion increased core cooling by 343% in the body-insulated subcondition (head-out: 0.13 ± 0.2°C, head-in: 0.47 ± 0.3°C; P < 0.05) and by 56% in the body-exposed subcondition (head-out: 0.40 ± 0.3°C and head-in: 0.73 ± 0.6°C; P < 0.05). Discussion: In both body-exposed and body-insulated subconditions, head submersion increased the rate of core cooling disproportionally more than the relative increase in total heat loss. This exaggerated core-cooling effect is consistent with a head cooling induced reduction of the thermal core, which could be stimulated by cooling of thermosensitive and/or trigeminal receptors in the scalp, neck, and face. These cooling effects of head submersion are not prevented by shivering heat production.