We, along with other mammals and birds, are endotherms. We use thermoregulation to maintain a consistent internal body temperature – between 37C and 37.5C. When the external environment changes, “A range of physiological responses is initiated, including the reduction of tissue temperature – skin, blood and muscle,” says Dr Joseph Costello, exercise and environmental physiologist at Portsmouth University’s extreme environments laboratories. “If the exposure is over a longer period of time, you may also observe a reduction in deep body temperature.” Thermoregulation is controlled by the hypothalamus, a structure deep in the brain that maintains internal balance – or homeostasis – by regulating processes such as heart rate and body temperature. If the hypothalamus senses the body growing cold, it sends signals to the skin, glands, muscles and organs, kickstarting responses that will keep the body warm and protect the vital organs. When the body cools, protecting the internal organs is the first priority. If the environmental temperature falls to 15C, blood vessels close to the skin narrow. Blood flow is redirected from the extremities – hands, feet, arms, legs and outer skin – to the core in order to keep the organs warm and protected. Cold perception begins with the skin. Some people can feel the cold more than others, in particular women, elderly people and young children. Women have more body fat than men. A thick layer of subcutaneous fat insulates the inner organs but blocks the flow of warm blood to the skin and extremities. Typically, they also have less muscle for generating heat through shivering. Having less muscle also decreases basal metabolic rate – the capacity to burn energy from food. In elderly people, the body has more difficulty pumping blood to the places that need it, as metabolic rate decreases with age. Babies, meanwhile, says neonatologist Dr Chris Dewhurst of the Liverpool Women’s NHS Foundation Trust, have poorly developed metabolic mechanisms for responding to thermal stress. “For example, babies can’t shiver,” he says. “They also have a large surface area-to-body-mass ratio, meaning they are more likely to become cold.” Babies do, however, have brown fat – the same type of fat that hibernating animals have. “Brown fat generates heat by using calories,” says Dewhurst. “Calories that would ordinarily be used to grow muscle and brain tissue.” Using calories to stay warm instead, he warns, affects growth and brain development. When body temperature drops, blood pressure spikes as more blood than usual is being pumped through a smaller space. In an attempt to reduce the volume of liquid and lower blood pressure, the kidneys respond by filtering excess fluid from the blood, so you may urinate more often. This is called cold diuresis. The lack of blood in the skin can result in a blueish tinge as the skin falls to the surrounding temperatures. When the body is exposed to cold for long periods, this protective strategy can reduce blood flow in some areas of the body to dangerously low levels. Core temperature At this point, the body has made the decision to risk sacrificing the more expendable parts – fingers, toes, ears, nose, cheeks – to preserve the core temperature. When the skin falls to -2C, body tissue begins to freeze. As the skin freezes it starts to look waxy, tingle or sting, then feels numb. Then deeper tissues freeze. This is frostbite. The skin may turn blue, grey or black. At -4C, ice crystals can form in the blood. In deep frostbite, the skin can feel hard, like wood. Muscle and bone begin to freeze. When the body can no longer maintain core temperature by constricting blood vessels, it shivers to increase heat production. Skeletal muscles contract to generate heat. This is the first symptom of hypothermia, which affects all the body’s systems – metabolic rate, mental awareness, nerve conduction and neuromuscular reaction times, as well as the cardiovascular and respiratory systems. As body temperature falls, slurred speech, lack of coordination and memory loss develop. When core body temperature falls to 32C, shivering ceases. When core temperature drops below 30C, the heart beats erratically. Kidney function falters. Liquid accumulates in the tissue and air spaces of the lungs. At 29C there may be loss of consciousness – and, at 26C, death. Perpetually cold What if you’re just moderately cold but for a long period of time – say, you can’t heat your house? “If exposed to less severe cold for prolonged periods of time, individuals may get non-freezing cold injury,” says Costello’s colleague and environmental physiologist Dr Clare Eglin. “Symptoms of non-freezing cold injury include altered sensory function, cold sensitivity and pain as a result of damage to the blood vessels and nerves in the hands or feet. If severe, these symptoms may last many months.” Are colds more likely when you are cold? Breathing in cold air for a sustained period of time can irritate the airways and lungs too – even in healthy people – and can worsen existing respiratory issues such as asthma, bronchitis or chronic obstructive pulmonary disease. When you breathe in, the nose and mouth usually warm and humidify the air before it reaches the lungs. When the air is cold, the upper airways react by narrowing, making it harder to breathe. Plus, cold air contains less moisture than warm air, so breathing it in can dry out the airways. “Respiratory infections thrive in colder temperatures,” says Erika Radford, head of health advice at Asthma + Lung UK. “Winter is already a deadly time for people with lung conditions and the last thing we want to see is more people being rushed to hospital fighting for breath, because of exposure to cold.” Keeping warm There are some scientifically proven ways to keep warm – and some old wives’ tales to avoid. Wear a hat: scientists have found that we lose more heat from our bodies than from our heads. However, it’s still worth wearing a hat because if your body is warm but your head is cold, you won’t shiver. This results in core temperature dropping fast. Wear layers: putting on lots of thin layers to trap air replicates the horripilation effect – better known as goose bumps – that many animals benefit from, where air is trapped between hairs that have been made to stand on end by the contraction of tiny muscles known as arrector pili at the base of each hair follicle. Get active: physical activity causes muscles to contract, breaking down more nutrients, which generates heat. In fact, just stepping up and down on to a step can produce an additional 200 watts of heat and raise body temperature fast. But don’t take it too far – if you overheat and start sweating, the evaporation of sweat can lead to rapid heat loss. Don’t drink alcohol: this sends blood towards the surface of the skin but away from the core, so deep body temperature drops, raising the risk of hypothermia. Alcohol has also been found to inhibit the natural shivering response as well as cold perception. Fuel poverty According to experts at University College London (UCL), this winter thousands will die and millions of children will suffer because of “epidemic levels” of fuel poverty. “We’re in uncharted territory,” says Dr Tammy Boyce, a senior research associate at the UCL and co-author of the Marmot review. “Homes that weren’t cold before will be cold for the first time ever. People who haven’t ever known fuel poverty will be fuel poor.” Disabled people, elderly people and children will be the hardest hit, she adds, and it affects everything, including school outcomes and mental health, as well as physical wellbeing. “The only good to come of this,” says Boyce, “is that people are realising the impact housing has on health.”