Since many young athletes practice and play sports all year in extreme weather conditions, their ability to control their body temperature — called thermoregulation — is critical to avoiding heat illness or hypothermia. While scientists traditionally thought children were more susceptible to heat stroke and hypothermia, direct comparisons of children and adults exercising at the same exercise intensity do not confirm this. However, children have lower sweat rates, higher relative body surface area, higher skin blood flow and possibly reduced heat regulation with mild dehydration, which might affect their thermoregulatory response to exercise in extreme weather.
The most drastic difference between children and adults in terms of thermoregulation is that children sweat less than adults. A 1960 study published in the journal "Essential Problems in Climatic Physiology" showed that young boys have approximately half the sweat rate of grown men during rest, and later studies during low- and high-intensity exercise confirm this difference. Since sweat evaporation is the main cooling mechanism by the body during exercise, scientists originally hypothesized that lower sweat rates in children would reduce heat tolerance. However, since a young athlete’s sweat droplets are smaller and more diffuse on the skin surface, they actually might sweat more efficiently, according to a 2004 study published in the journal "Experimental Physiology."
Body Surface Area to Mass Ratio
Compared with adults, young athletes have a greater body surface area relative to their total mass, meaning that they have more skin surface area for body heat to escape and are less prone to heat illness. However, this difference diminishes approximately at pubertal age and also does not account for body composition or fitness levels. Additionally, research does not consistently show improved heat dissipation in young athletes with a high relative body surface area. This high body surface area to mass ratio might put children at greater risk for hypothermia, but more research on children exercising in cold environments is necessary to confirm this.
Skin Blood Flow During Exercise
Young athletes have greater skin blood flow during exercise than adults. Additionally, the blood vessels at the skin of young individuals dilate, or increase in size, more than adults. Therefore, the greater amount of blood flow to the skin during exercise implies that more heat can be removed through convection. Convection, in terms of human thermoregulation, involves exchanging body heat from the skin with air molecules passing over the skin. Faster blood flow to the skin surface leads to more rapid exchange of molecules, and therefore faster convective heat loss.
Since young athletes have a lower sweat rate than adults, you might think they are less susceptible to performance-impairing dehydration. However, a 1977 study in "Journal of Applied Physiology" did not show any difference in water weight loss, rate of water loss, and blood plasma volume between prepubescent and young adult women. However, a 1980 study in "Journal of Applied Physiology" showed that a young individual’s core body temperature rises more rapidly at a given level of dehydration than an adult’s. This suggests even minor levels of dehydration can impair a young athlete’s ability to regulate his body temperature during exercise.