Air Conditioning? Not a Fan.

Jenny Lin

We have all been victims of the summer heat wave, especially in recent months, with heatstroke and hyperthermia becoming more common by the year. Over the summer of 2022, the UK saw temperatures soar above 35℃ on multiple occasions and many of us turned to indoor air conditioning as a solution to the scorching heat. These temperatures are a useful analogy for the future of our climate when we will likely rely even more heavily on technology to provide us with the ‘ideal’ living conditions. Are humans failing to adapt to the ever-changing climate, or are we just obsessed with the perfect temperature?

The human body has developed several behavioural and physiological responses to cope with variations in external temperatures, such as vasodilation and vasoconstriction (Figure 1), and a reduction in movement in the heat, which has the opposite effect to shivering. The more time we spend in the heat, the better we adapt to it. Athletes are a prime example of this; many sportspersons train vigorously in uncomfortably hot climates, leading to various physiological adaptations, including an overall decrease in core temperature, a reduction in blood lactate, and an increase in blood plasma volume. In the months leading up to the 1984 Olympics, marathon runner Alberto Salazar was subjected to a program of temperature acclimation training, which resulted in his sweat rate rocketing to three litres per hour in the 30℃ heat, compared to the usual litre per hour. His thermoregulatory and hormonal systems were measured to be entirely normal after the race, which proved his heat acclimation training program to be a success.

Figure 1: A diagram of the human body’s physiological responses to changes in external temperatures.

Figure 2: Alberto Salazar running the 1984 Los Angeles Olympic Marathon.

The same principles can be applied to the non-athlete; allowing our bodies to experience summer’s high temperatures rather than simply turning up the air conditioning might give our bodies the chance to adapt and acclimate.

In 1847, Carl Bergmann, a German biologist, observed that mammal populations of smaller individuals are more likely to be found near the equator in warmer climates, with larger mammals generally being found in colder regions: Bergmann’s rule. He concluded that the reason for this is that cell metabolism results in a by-product of heat, so the more cells an animal is comprised of, the more internal heat it will produce. Bergmann’s rule can be demonstrated by a comparison between a polar bear and a Jerboa (Figure 3 and 4).

Figure 3 and 4: Photos depicting an Artic polar bear and a Jerboa

Polar bears generally have large, compact bodies and a small surface area-to-volume ratio, which allows for better heat retention and therefore, the ability to live in colder climates such as the Arctic. On the other hand, Jerboa are mouse-like animals with long tails and large ears; their large surface area to volume ratio allows for more efficient radiative cooling, so they are found in warmer climates such as North African and Asian deserts. A 1950’s study of human populations found a ‘strong negative correlation between body mass and mean annual temperature of region’, which supports Bergmann’s rule in application to humans. Nowadays, technologies such as air conditioning and heating may offset the effects of natural selection in shaping human bodies. Instead of allowing our bodies the possibility to adapt physiologically to more extreme temperatures, we rely on indoor heating and air conditioning to provide us with the most comfortable living environment. A power outage in the summer may become a life-threatening phenomenon if we become solely reliant on air conditioning to keep us cool.

Although only 5% of Brits currently have air conditioning installed in their homes, this figure is set to soar over the next century, with it being predicted to increase the UK’s power consumption by an estimated 15%. Without even considering the damaging effects of increased air conditioning use on climate change, it is important to consider whether it is offsetting the effects of natural selection in humans. In the future, the ability to adapt to extreme temperatures could become a selective biological pressure for us, and the easiest way to respond physiologically is by reducing our volume and maximising our surface area for more efficient heat loss. Perhaps, for the time being, we should allow our bodies to carry out the cooling mechanisms and processes that they have evolved to do, rather than relying on air conditioning to get us through extreme periods of heat. The better adapted we are, the more prepared we can be for sudden changes in world climate.

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