A newborn loses heat through four physical mechanisms, and gains it from its own metabolism (plus, in preterm care, an external heater). The core temperature follows a lumped-capacitance heat balance:
m·cp · dT/dt = Q_met − Q_rad − Q_conv − Q_evap − Q_cond
Q_rad = h_r · A · (T_skin − T_wall)
Q_conv = h_c · A · (T_skin − T_air)
Q_evap = A · k_evap · (1 − RH) · skin_factor
Q_cond = k_cond · (T_skin − T_mattress)
A is body surface area, m is mass, cp ≈ 3.5 kJ/kg·°C is tissue specific heat. The killer variable in neonatology is the surface-area-to-mass ratio: a 0.6 kg extremely preterm infant has roughly triple the SA/mass of a term newborn, so the same air temperature drains heat far faster. Preterm skin is also thinner and wetter (higher skin_factor), which is why evaporative loss dominates in the first days of life before keratinisation completes.
- Air temperature — sets convective loss; incubators are tuned to each infant's "neutral thermal environment" (NTE), the air temperature at which metabolic heat production is minimised.
- Humidity — high humidity (60–90% for very preterm infants) suppresses evaporative loss through immature skin, which is why modern incubators humidify aggressively in the first week.
- Wall/radiant temperature — radiative loss depends on the temperature of surrounding surfaces, not just air; a cold single-glazed incubator wall pulls heat from the infant even in warm air. A double-walled incubator adds an inner shield close to air temperature, cutting radiative loss.
- Non-shivering thermogenesis — newborns cannot shiver effectively; instead brown adipose tissue (present mainly around the scapulae, neck and adrenals) burns fat to generate heat directly, which this model approximates as metabolic heat rising under cold stress, at the cost of higher oxygen and calorie consumption.
Clinically, this is exactly why NICU incubators servo-control air temperature to skin temperature, add humidity for very preterm infants, and use double-walled construction or radiant warmers — sustained cold stress increases oxygen consumption, depletes glucose/fat reserves, and worsens outcomes.