The Main Route of Heat Loss in Humans at 21°C Ambient Temperature

The human body continuously loses heat to maintain a stable core temperature around 37°C. Heat loss occurs through several mechanisms—radiation, conduction, convection, evaporation of sweat, respiration, and others. The dominant route depends largely on ambient temperature and environmental conditions.

Heat Loss Mechanisms Overview

• Radiation: Emission of infrared heat from the skin to the surrounding cooler environment.

• Conduction: Direct transfer of heat through physical contact with nearby cooler objects.

• Convection: Transfer of heat to air or water moving around the body.

• Evaporation: Loss of heat as sweat evaporates from the skin surface.

• Respiration: Heat lost during breathing exhaled air.

• Urination and Defecation: Minor heat loss through stools and urine, insignificant in body heat regulation.

At an ambient temperature of 21°C (around room temperature), the skin temperature is typically higher than the surrounding air, allowing heat loss by radiation, conduction, and convection. However, sweating and evaporative heat loss will be minimal unless there’s physical exertion or elevated body heat.

Dominant Heat Loss Route at 21°C

At this moderate ambient temperature, the maximum amount of body heat is lost through radiation and conduction combined. Radiation accounts for approximately 60% of heat loss under these conditions, while conduction and convection contribute about 15-18%. Evaporative loss through sweating is less significant at this temperature unless the individual is physically active or in a warm environment.

Respiratory heat loss and heat lost through urination and defecation are minimal contributors compared to surface heat loss through the skin.

Why Evaporation Is Not Dominant Here

Evaporation of sweat becomes the primary cooling mechanism only when:

• The ambient temperature exceeds skin temperature.

• There is significant physical activity producing excess heat.

• The relative humidity is low, allowing efficient evaporation.

At 21°C, skin temperature is above the ambient temperature, so conductive and radiant heat loss naturally cool the body without extensive sweating.

Summary Table: Heat Loss Routes at 21°C Ambient Temperature

Conclusion

At an ambient temperature of 21°C, the main route responsible for the maximum amount of body heat loss is radiation and conduction. Evaporative heat loss through vaporization of sweat, while important during heat stress or exercise, is not the dominant factor at this comfortable temperature.

Therefore, the correct answer is:
(1) Radiation and conduction.