Children and adults cope with extreme heat differently, but are kids really at greater risk?

In Canada, Europe and around the world, temperatures are soaring and heat warnings are rampant. We are reminded to stay out of the sun and stay hydrated, and in extreme conditions, stay indoors.

We are also told that children are especially vulnerable to heat-related illness.

We are researchers with 20-35 years of experience finding out how children, adolescents and adults thermoregulate when active — that is, how they maintain their body temperature within a normal range despite the stresses of exercise and heat. We are also each parents to children who grew up as very active and competitive athletes, spending many hours training and competing outdoors, in all weather conditions. We write this article as scientists and as parents.

How adults and children regulate body heat

For many years the public has been told that children are at increased risk of heat illness, especially when being physically active in a hot environment. Being cautious in demanding situations or hazardous conditions is obviously warranted. But are children especially at risk?

In 1980, the late Oded Bar-Or, a pioneer of pediatric exercise physiology, published a seminal article describing the risks particularly relevant to children exercising in the heat. Subsequent studies consistently demonstrated that children sweat much less than adults. This was the case regardless of the environmental conditions, whether at rest or during exercise.

When we exercise, our working muscles produce heat which must be lost, and body temperature stay within our normal, healthy boundaries (36.5 to 38.5 C). Inability to lose this heat can result in heat illness symptoms (heat cramps to heat exhaustion to potentially fatal heat stroke).

Read more:
Extreme heat: Could hot baths help older adults better adapt?

Since evaporation of sweat from the skin is the most effective way to lose the heat produced by the working muscles, and since children were consistently shown to sweat less than adults, it was concluded that children are especially vulnerable to heat illness.

However, epidemiological data does not support this conclusion. In fact, studies on heat illness during sporting events or in heatwaves report that children have the lowest heat-related morbidity and mortality rates in Canada and worldwide. So, where is this contradiction coming from?

Response to hot environments

We recently conducted a systematic review of 22 studies that directly compared healthy children and adults in terms of their body temperature response when exposed to hot environments. We found that children were able to thermoregulate just as well as adults. Contrary to claims of children’s increased vulnerability, the increase in body temperature during exercise or rest in a hot environment was similar in children and adults.

The human body is able to regulate its temperature within a very narrow range, typically from 36 to 39 C. Even under unfriendly environmental conditions and prolonged activity, such as triathlon racing, body temperature will rarely increase above 40 degrees.

Heat produced by the working muscles is lost in two ways: through sweat evaporation and through sending more blood from the muscles to the skin, and from there, to the environment (conduction). The higher the sweating rate, the greater the heat lost, as long as the sweat actually evaporates rather than drips or is wiped off. Likewise, the greater the blood flow to the skin and the greater the skin area (body surface area), the greater the heat lost, as long as the skin is not covered.

Sweat vs. blood flow

In the studies reviewed, children exhibited lower sweating rates, whether resting or exercising. Their lower sweating rate was apparent regardless of whether it was measured over the whole body, over specific body segments (torso or limbs), whether expressed in absolute terms (total volume) or relative to skin surface area. In other words, sweating rate and thereby heat lost via sweat evaporation was lower in children.

On the other hand, the increase in blood flow to the skin was higher in children. Skin blood flow is difficult to measure, but in those studies in which it was determined, it was higher in children than in adults. In other words, heat lost via conduction was higher in children.

Consider also that a child’s body surface area relative to their mass is much higher than an adult’s. Heat production is dependent on body mass while heat loss is dependent on body surface area. Therefore, a greater ratio of surface area to mass can be an asset when we need to get rid of body heat. Think of how long a large block of ice takes to melt compared to if that cube were all crushed up.

A 10-year-old’s surface-area-to-mass ratio can be 35 per cent larger than a young adult’s. This provides children with an advantage. So, consider that children sweat less because they don’t need to sweat as much. They lose more heat by increasing their blood flow to the skin.

Sweat and dehydration

While sweating and sweat evaporation is an effective way to dissipate heat, it means that we lose body fluids in the process. Since children sweat less than adults, one may argue that they are actually at an advantage when it comes to hydration. However, this is not the case. Generally, despite current awareness of the importance of hydrating, we do not replace all of our fluid loss during exercise.

In other words, when we exercise, we sweat more than we drink. This is the case for children and for adults. When we dehydrate, our body temperature increases and the risk of heat illness increases.

Again, this is the case for children and for adults. However, the implications of dehydration are more serious in children. The dehydration-related rise in body temperature appears to be faster in children. For this reason, added awareness for fluid replacement during exercise (with water or sports drinks), is especially important in children.

Take-home message

The take-home message is that we should all exercise caution in hot environments — children and adults. Physiologically, children are not at a disadvantage compared with adults. They can increase their blood flow to the skin better than adults and do not need to rely on high sweating rates to keep cool. So, they manage to thermoregulate as well as adults.

We should all stay hydrated — children and adults. While awareness of the importance of hydration is growing, this awareness may not be as established in children, and they may not notice early signs of dehydration and overheating such as headache and dizziness. In those cases, added precaution and direction from parents, coaches and teachers is warranted.