Beyond the classic thermoneutral zone: Including thermal comfort

Abstract

The thermoneutral zone is defined as the range of ambient temperatures where the body can maintain its core temperature solely through regulating dry heat loss, i.e., skin blood flow. A living body can only maintain its core temperature when heat production and heat loss are balanced. That means that heat transport from body core to skin must equal heat transport from skin to the environment. This study focuses on what combinations of core and skin temperature satisfy the biophysical requirements of being in the thermoneutral zone for humans. Moreover, consequences are considered of changes in insulation and adding restrictions such as thermal comfort (i.e. driver for thermal behavior). A biophysical model was developed that calculates heat transport within a body, taking into account metabolic heat production, tissue insulation, and heat distribution by blood flow and equates that to heat loss to the environment, considering skin temperature, ambient temperature and other physical parameters. The biophysical analysis shows that the steady-state ambient temperature range associated with the thermoneutral zone does not guarantee that the body is in thermal balance at basal metabolic rate per se. Instead, depending on the combination of core temperature, mean skin temperature and ambient temperature, the body may require significant increases in heat production or heat loss to maintain stable core temperature. Therefore, the definition of the thermoneutral zone might need to be reformulated. Furthermore, after adding restrictions on skin temperature for thermal comfort, the ambient temperature range associated with thermal comfort is smaller than the thermoneutral zone. This, assuming animals seek thermal comfort, suggests that thermal behavior may be initiated already before the boundaries of the thermoneutral zone are reached.

Keywords: mathematical model; metabolism; theoretical biology; thermal behaviour; thermoregulation.

Figure 1. schematic overview of autonomic and behavioral control of thermal insulation. Solid arrows denote relation and/or control and dashed arrows denote heat flow.

Figure 2. Thermal network to describe heat balance in the thermoneutral zone. Tc: core temperature, T_s: mean skin temperature, T_a: ambient temperature, (1−α)M: metabolic rate corrected for respiratory heat loss, Q_r+c: combined radiative and convective heat loss, Ps: saturated vapor pressure at skin surface, P_a: vapor pressure of the air, Q_e: evaporative heat loss, h_c: convective heat loss coefficient, Fpcl: clothing evaporative permeability coefficient, I_body: body insulation, I_cl: clothing insulation, I_a: air insulation.

Figure 3. The classical thermoneutral zone (denoted by ‘classical TNZ’), is defined as the range of ambient temperature at which temperature regulation is achieved only by control of sensible (dry) heat loss, i.e., without regulatory changes in metabolic heat production or evaporative heat loss. Each band depicts for a given ambient temperature the range of mean skin temperature in which the equation constraints are satisfied for each category, e.g., ‘TNZ body’, ‘SWEAT’, ‘NST’, ‘SH⁺NST’. The light gray area labeled TNZ body depicts solutions that satisfy that internal body heat transport equals external heat loss and core temperature ranges between 36 °C ≤ T_c ≤ 38 °C. In other words, TNZ body indicates where the classical thermoneutral zone is supported from the perspective of the human body. NST: solutions for which steady-state heat loss is between 83 W ≤ Q ≤ 88 W. The body can achieve thermal balance by non-shivering thermogenesis. Shiver+NST: solutions for which steady-state heat loss is between 88 W ≤ Q ≤ 372 W. The body can achieve thermal balance by shivering thermogenesis. SWEAT: solutions for which steady-state heat loss is Q ≤ 83 W. The body can achieve thermal balance by increased evaporation.

Figure 4. The functional thermoneutral zone is denoted by ‘functional TNZ’ for a person dressed in a business suit and heat production associated with light office work. The definition of ‘TNZ functional’ is analogous to the definition of ‘TNZ classical’ with the difference that the metabolic heat production is greater than the basal metabolic rate. Each band depicts for a given ambient temperature the range of mean skin temperature in which the equation constraints are satisfied for each category, e.g., ‘TNZ body’, ‘SWEAT’, ‘NST’, ‘SH⁺NST’. The light gray area labeled TNZ body depicts solutions that satisfy that internal body heat transport equals external heat loss and core temperature ranges between 36 °C ≤ T_c ≤ 38 °C. In other words, TNZ body indicates where the functional TNZ is supported from the perspective of the human body. NST: solutions for which steady-state heat loss is between 107 W ≤ Q ≤ 118 W. The body can achieve thermal balance by non-shivering thermogenesis. Shiver + NST: solutions for which steady-state heat loss is between 118 W ≤ Q ≤ 372 W. The body can achieve thermal balance by shivering thermogenesis. SWEAT: solutions for which steady-state heat loss is Q ≤ 98 W. The body can achieve thermal balance by increased evaporation.

Figure 5. Steady-state ambient temperature ranges of the thermal comfort zone for a nude person. Each band depicts for a given ambient temperature the range of mean skin temperature in which the equation constraints are satisfied for each category, e.g., ‘TNZ body’, ‘SWEAT’, ‘NST’, ‘SH⁺NST’. The light gray area labeled ‘TNZ body’ depicts solutions that satisfy that internal body heat transport equals external heat loss and core temperature ranges between 36 °C ≤ T_c ≤ 38 °C. The Solid lines indicate comfortable mean skin temperature is as reported by Gagge et al. The dashed lines indicate a more conservative range of comfortable mean skin temperature range as reported by Weiwei et al.

Figure 6. Steady-state ambient temperature range of the thermal comfort zone for a person dressed in a business suit and heat production at resting metabolic rate. Each band depicts for a given ambient temperature the range of mean skin temperature in which the equation constraints are satisfied for each category, e.g., ‘TNZ body’, ‘SWEAT’, ‘NST’, ‘SH⁺NST’. The light gray area labeled TNZ body depicts solutions that satisfy that internal body heat transport equals external heat loss and core temperature ranges between 36 °C ≤ T_c ≤ 38 °C. Comfortable mean skin temperature is as reported by Gagge et al. The dashed lines indicate a more conservative range of comfortable mean skin temperature range as reported by Weiwei et al.