Conductive heat loss in simulated outdoor settings: a preliminary experimental human study

Little is known about the nature and magnitude of conductive heat loss in outdoor environments. Although manikin studies provide reproducible and consistent data, they may not fully reflect all aspects of human physiological responses. This preliminary study assessed heat loss in volunteers under simulated outdoor conditions, with a focus on heat flux toward the ground, which primarily represents heat conduction.

An experimental study was conducted with seven healthy volunteers in a thermoclimatic chamber. A two-phase rescue scenario was employed in which a person immobilized on a spineboard was placed directly on the ground (Phase 1) and then lifted upwards under windy conditions (Phase 2). Heat flux was measured via heat flow sensors placed on the volunteers' skin. The driving force for conductive heat loss was determined on the basis of the temperature difference between the skin and the spineboard. A linear regression model was used to analyse the relationship between heat flux and temperature difference.

The mean skin-spineboard temperature difference was 23,6 ± 2,9 °C and increased over time. The mean heat flux through the contact area was 467 ± 97 W/m2. A significant increase in heat flux to 560 ± 45 W/m2 was recorded in Phase 2 of the experiment. Multiplication of the heat flux per area times the contact area resulted in a mean heat loss of 159 Watts. In both phases, a strong linear relationship was found between back skin temperature and heat flux. In Phase 1, the relationship was positive (β=+31 W/m² per 1 °C decrease in skin temperature; R²=0.986; p < 0.001), whereas in Phase 2, it was negative (β=-20 W/m² per 1 °C decrease; R²=0.946; p < 0.001).

The heat flux through the skin-to-spineboard interface, which is mainly conductive loss, is one-fourth greater than the heat flux through the skin of anterior body. Heat conduction towards the ground accounts for approximately one-fifth of the total heat loss. The heat flux through the skin-to-spineboard contact surface may increase when the spineboard is lifted from the ground in windy conditions.