28 YEARBOOK 2024 has… acquired an authority which it does not deserve… It affords no measure of the rate of cooling of the human body, and is, therefore, a very indifferent instrument for indicating atmospheric conditions which are comfortable and healthy to man.’ Regardless, outside medicine, air temperature as a proxy measurement for thermal comfort took hold and was soon accepted as a paradigm. This may be because of the fashion for enumeration: ‘comfort’ is impossible to describe with a number. To properly understand comfort, we first need to discard the common belief that humans are passive objects whose health depends on heat absorbed from the environment. We take in energy in the form of food and oxygen and our body transforms that into heat, to support our metabolic processes and keep the vital organs functioning optimally. If these organs are not to die, the temperature of our ‘core’ must stay within a very tight range (36.5 to 37.5°C). This means that even in the coldest conditions we always need to lose some of the heat we are generating into our surroundings, especially when we are active. Typical recommendations for indoor air condition assume that occupants require a ‘neutral thermal environment’ (where body heat is not lost into the surroundings), but this is simply not true. To stay happy and healthy occupants will need to lose different amounts of heat according to how active they are, what they are wearing and their own personal preferences. There is no such thing as ‘ideal’ interior conditions. Some of our heat is lost through touch depending on what we touch, how we touch it and the surface area of contact. Between two and 22 per cent is via transfer into the air, mostly by perspiration and evaporation. The rest is due to our bodies radiating heat out into our surroundings. This accounts for at least 60–65 per cent but can be as much as 85 per cent if we are lightly dressed and if our surroundings strongly absorb radiant energy. This is far more than could ever be lost into the air directly, whatever its temperature (air movement and humidity are more important). To maintain our core at the right temperature, we have evolved a highly complex thermoregulatory system which includes some familiar reactions. When we need to lose more heat, the blood is sent into the skin to increase radiation, our sweat glands are activated to increase evaporation and we may start to pant (losing heat by evaporation from the lungs). Metabolism slows, so we feel less need to eat and other behavioural changes are triggered (such as adopting a spread body position to maximise both radiative and evaporative loss). When we need to slow heat loss, the blood is drawn back from the skin and limbs; eventually, we start shivering and get goosebumps. Our body position draws in and our metabolism is sped up to match heat being lost, increasing our appetite, especially for food such as carbohydrates that can rapidly be transformed into energy. In all this, air temperature is conspicuous by its absence as a critical factor. Thermal physiology therefore explains the experiences of many during lockdown, when we were all forced to look into the complicated ways our own homes worked. The thermostat rarely turned out to be a useful guide. Cold surfaces such as bare floors or walls, or large areas of glass, persistently absorb our radiating heat, making us feel cold even when the air temperature is high. In winter, the heating might be set to 22°C, but if there is an unpleasant draught (perhaps because the wind is in a particular direction, or because a door has been left slightly ajar), you would still feel very uncomfortable. As the low winter sun enters the room through the windows and hits us directly or heats the surfaces around us, we instantly feel hot even in the coldest air temperatures. It also depends on how we are using the space: one person sitting quietly reading the newspaper may feel Ways of losing heat Thermoregulatory systems
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