Investigation on the relationship between thermal tolerance and heat transfer during finger contact with hot and cold surfaces

When the skin contacts hot or cold surfaces, the thermal tolerance is closely related to the amount of heat transferred. Traditional safety evaluation criteria based solely on surface temperature and contact duration are insufficient for today's synthetic materials. This study focuses on finger contact with hot and cold surfaces and investigates the relationship between cumulative heat transfer and subjective thermal tolerance. A model was developed using the Pennes bioheat equation, along with measured blood perfusion rates and temperature field data from infrared thermography. Five typical materials—steel, aluminum, stone, nylon, and wood—were selected, and multiple temperature gradients were applied. Tolerance times and skin temperature changes were recorded from healthy subjects aged 20 to 40 years during contact. Results show that, at the same surface temperature, tolerance times varied significantly across materials, with heat transfer dominated by thermal conductivity. Despite differences in material, surface temperature, and tolerance time, the cumulative heat transfer per unit volume at tolerance times showed a concentrated distribution. For hot surfaces, median values ranged from 22 to 25 J/cm 3 ; for cold surfaces, ranged from -20 to -35 J/cm 3 . Heat transfer evolved nonlinearly—rapid initially due to interfacial temperature difference, then slowing as skin temperature rose and blood perfusion regulated. Using these findings, a quantitative relationship curve was established between material temperature and tolerance time under varying thermal conductivities. These findings provide a basis for determining safety thresholds for finger contact with synthetic materials. This study lays the groundwork for a contact safety evaluation system grounded in heat transfer principles.