Narrative Review on Infants' Thermoregulatory Response to Heat

Abstract

Infants are at a higher risk of heat-related morbidity and mortality compared to children and adults. However, it remains unclear whether this vulnerability stems from immature thermoregulatory mechanisms or simply from their dependence on caregivers. This narrative review examines current literature on infant thermoregulation during heat exposure and explores how unique physiological characteristics may influence vulnerability. Key differences in infants compared to older individuals include their larger surface area-to-mass ratio, which (1) facilitates heat dissipation when skin temperature exceeds ambient temperature, but compromises heat loss in reversed conditions, and (2) likely enables a large portion of an infant's blood volume to shift to the skin, promoting heat loss but reducing blood volume in the central circulation. Infants also have a relatively high metabolic heat production. Additionally, their lower sweat output per gland may represent either a limitation or a different thermoregulatory strategy. Contrary to common assumptions, most components of infants' thermoregulatory system do not appear inherently immature; rather, their distinct physiological characteristics-combined with their reliance on caregivers-shape how and when heat exposure may become harmful. Nevertheless, further research is needed to better understand how these interacting factors influence infants' ability to maintain stable core temperature. Meanwhile, coordinated efforts by caregivers, health professionals, and policymakers are essential to minimize infants' heat-related health risks.

Keywords: core temperature; extreme temperatures; health risks; heat; infants; skin blood flow; skin temperature; surface area-to-mass ratio; sweating; thermoregulation.

Figure 1

Surface area-to-mass ratio by age. The solid line shows the values calculated for children using the formula for body surface area by Mosteller [24] and child growth standards for height and weight by WHO [27]. The dashed line shows the average surface area-to-mass ratio for adults, based on values reported by Verbraecken et al. [28].

Figure 2

Size-adjusted total (red) and basal (black) energy expenditure over the life course. Energy expenditures are expressed as percentages of adult-level adjusted total energy expenditure (~100%). BEE: basal energy expenditure. TEE: total energy expenditure. From Pontzer et al., Daily energy expenditure through the human life course. Science 373, 808–812 (2021). Reprinted with permission from AAAS [37].