For the human body to function properly, the temperature of our vital organs, i.e. the brain, heart, lungs and liver, has to remain at approximately 37°C – at all times. What we take for granted is actually a highly complex regulatory mechanism – but how does this actually work? To begin with, our organism must establish how hot or cold it is, using sensors in our skin, internal organs and our brain. Our skin contains minute little heat and cold centres, each of which is no more than one square millimetre in size. A me-tallic object, for example, feels cooler than a piece of polystyrene, even if both have exactly the same temperature. Metals and liquids are better thermal conductors and therefore appear to be colder than gases and plastics. This explains why we feel much colder in water of 20 degrees Celsius than in air of the same temperature. In addition to these temperature sensors in the skin, the body also needs a kind of thermostat, a regulatory device that measures and controls the temperature of the main part of the body, keeping it at 37°C. The heat in our bodies is generated by the energy we gain through the food we eat. During exercise, however, it is our muscles that produce the greatest amount of heat, because only a very small part of the energy we consume is actually converted into physical work.   SWEAT! How we react to heat If the body’s core temperature rises above the normal temperature of 37°C, for example as a result of muscular activity, the body reacts by increasing the circula-tion of blood to the skin. When the body becomes very hot, circulation can be increased eight or even twelve fold. In these cases, almost one third of the body’s blood is pumped to the surface of the body. In addition, the skeletal muscles are slackened, thus reducing the body’s own production of heat. However, the higher the exterior temperature rises, the more difficult it is for the body to dissipate heat to the external environment by means of vascular dilatation. As a result, the body initiates its most important cooling system: it starts to perspire. This is the time for the two million sweat glands to leap into action – the majority of them are in our heads, hands and feet. In order for sweat to have a genuine cooling effect, it has to be able to evaporate from the skin unhindered. Consequently, clothing that is impermeable to water vapour is just as uncomfortable as humid, muggy weather because, in both cases, the atmospheric humidity is too high for the sweat to evaporate. Sweating is the key to keeping your body cool. Garments that allow sweat to vaporize allow the body to regulate its tempe-rature: whether or not weather conditions are perceived as ‘comfortable’, does not therefore solely depend on the temperature, but also on the relative humidity of the air, the heat radiation in the vicinity and the wind speed of the air brushing past the body. So, for example, when riding a bike, the air sweeping past us is perceived as colder than when we are standing still. This is because the wind blows away the fine layer of air that normally surrounds us, thus increasing heat loss. On hot days, but also during physical activity, the two million sweat glands secrete at least 0.7 litres of fluid an hour. Once the body is completely covered in sweat, it has fully exhausted its most important means of cooling. If the sweat is not able to evaporate properly, the core temperature of the body will inevitability rise. Once it rises over 40 degrees Celsius, protein structures start to deform and cause tissue to become damaged. The blood pressure drops because the blood vessels in the skin start to dilate as much as possible. Headaches and dizziness occur. You start feeling mentally confused and lose consciousness – you are suffe-ring from heat-stroke. FREEZE! How we react to cold When undressed, people tend to feel most comfortable at a temperature between 29°C and 30°C. This remnant of our African origins, however, might also be the reason why our body’s protective mechanisms against the cold are somewhat sparsely developed. If, undressed and at rest, the temperature of the body falls below 28 degrees Celsius, it will start to lose more heat than it can produce. You feel cold. While subcutaneous fat helps reduce the conduction of heat, the musc-les start twitching rhythmically in order to increase the production of heat two-fold, or even threefold, by shivering and shaking. Because the veins and arteries are so close together, the cold blood in the veins, flowing back from the extremi-ties, which are often exposed to temperatures as low as 15 to 20 degrees Celsius, warms itself on the warmer blood in the arteries. By the time it reaches the pelvis the temperature will already have risen to about 34 degrees Celsius. At 34°C your attentiveness deteriorates and you cannot speak. At 33°C your life is in danger. At 25°C you are likely to suffer ventricular fibrillation or a cardiac death. However, some people have been known to survive core temperatures of 16°C. As only dry insulation keeps the body warm, it is essential to avoid wetness. Durable wa-terproofness thus avoids heat loss. In addition, windproof garments also minimize the body’s losing heat – less penetration with cold air prevents the windchill effect. Yet, fortunately, there are other means of regulating our temperature in addition to these purely physiological ones: Quite simply, when we feel cold, we will put on more clothes, and, when we get too hot, we will take off some layers.