It is important to realize that cold injuries, with and without frostbite, can also occur in situations where we can compensate for our heat loss and our core temperature therefore remains constant. The most vulnerable parts of the body are hands, feet and face. Also of note is that if an individual has had a previous cold injury, the risk of a new cold injury is greater. The article by Haman et al. states a schedule with factors that are important in different situations.

Part One: Individuals and cold conditions (1)

Cold injuries

Cold injuries without frostbite are caused by chronic (many hours to days) exposure to cold and are characterized by persistent sensory abnormalities, pain when warmed up and lasting consequences such as hypersensitivity to cold and sensory neuropathy. The idea is that a slow cooling of the tissue from 25°C to 10°C is responsible for the pathophysiology. The blood flow in the hands, feet, face decreases. Additional factors may include little exercise in the cold, dehydration and shoes that are too tight.

Cold injuries with frostbite caused by damage to the tissue when the skin temperature falls below the freezing point of approximately −0.55 °C. Freezing of the skin can be superficial, but also deep. While any tissue can freeze, it is also related to its location, pressure, insulation, and sensitivity to moisture. Most vulnerable are the face, fingers, hands, and the top of the nose and ears. At altitude, cold also has a greater influence on the risk of frostbite, probably because the reduced oxygen leads to thickening of the blood and thus blocks the vessels more quickly.

At the body level, the ability to maintain skin temperature is closely linked to its ability to reduce heat loss and increase heat production. When the blood vessels in the skin (periphery) constrict to keep the warm blood to the core of the body, the risk of cold injuries of the hands and feet increases.

On the other hand, research has shown that cold can also lead to higher blood pressure in an effort to keep blood circulation going. This probably occurs under the influence of noradrenaline and a sympathetic nervous system response. Often the heart rate also decreases in the cold due to a parasympathetic response of the nervous system in an attempt to maintain a good balance between metabolism and adequate heart function.

Cold and regulation mechanisms

Despite all kinds of research into predicting individual risk of cold injury, it appears that the response to cold differs greatly from person to person.

Variations between individuals in response to acute cold

Individual differences in body composition and shape are the determinants of the extent to which we lose heat, store heat and ultimately survive in the cold. Body shape and composition determines the degree of conduction, flow, radiation and evaporation that physiologically determine body temperature. Body shape and composition also determine the extent to which we can produce heat to prevent cold injuries.

Body fat would play an important role here, but further research casts doubts on this. Although subcutaneous fat is insulating, unlike the insulating blubber layer of marine mammals, there appears to be hardly any heat retention in humans. However, more body fat does increase body volume without increasing the total body surface. This changes the body shape so that there is a reduction in heat loss.

Unknown factors

But must there be other, as yet unknown, factors involved? Because if we compare the differences between men and women in exposure to cold water and correct for body fat, there are still very large differences that cannot yet be sufficiently explained. However, these sex comparisons also show that the smaller the body surface is compared to the body volume, the better the body is able to prevent heat loss in the cold. The most vulnerable groups when exposed to cold water are short people with a relatively large body surface area relative to their body volume. Whether this also applies to exposure to cold air is not entirely clear.

In humans, the ratio of body surface area to total body mass plays a decisive role in adaptation to cold. Research by Verbraecken et al. (2006) found a variation in body surface area of ​​1868 – 1,28 m in a group of 3,56 people² (factor 2,8 difference) and a variation in weight from 44 to 196 kg (factor 4,5 difference).

Verbraecken J, Van De Heyning P, De Backer W, et al. Body surface area in normal-weight, overweight, and obese adults. A comparison study. Metab. 2006;55(4):515–524. 

Cool in cold air

While a too low body temperature in cold water (and that starts below 18 °C) is almost inevitable, this does not apply to cold air. When exposed to cold air, the degree of cooling is much less and slower and can be prevented much more easily by clothing and the like. In this way, one can stay in cold air for hours to days without hypothermia. But where the situation is initially compensable, it cannot become compensable in the long term due to exhaustion of the capacity to produce heat. Measures must then be taken to limit heat loss or to increase heat production to prevent hypothermia. Therefore, individuals should monitor their own cold stress and take action as necessary.

Variants in cold adaptation due to genetic variations

Countless genetic variations affect your phenotype and physiological response to changes in temperature (blood flow rate, for example), skin characteristics (sweat glands, color, thickness), and body shape (shape, height, weight, body composition, etc.). Depending on the ambient temperature, people are more or less adapted. This mainly concerns the mental state regarding the ambient temperature and developing knowledge and skills to deal with extreme temperatures. Because physiologically and in terms of metabolism, there are remarkably few differences between people living above the Arctic Circle and people around the equator.

However, it has been shown a number of times that people in arctic conditions have a higher basal metabolic rate, higher temperatures and more blood flow in the hands and feet. But comparison is difficult because body shape and composition also have so much influence. Moreover, the adjustments mentioned seem temporary and the differences disappear when people of Arctic origin move to more temperate regions. This indicates that adaptation to cold is more important than predisposition based on race.

Temperature regulation

There are several models of thermoregulation in the body in which core temperature and skin temperature play an important role. The combination(s) of these temperatures can lead to the activation of defense mechanisms such as constricting blood vessels, burning brown fat or shivering. These mechanisms can operate independently depending on the circumstances. There are differences in this in the sense that the insulation is increased in one individual to prevent heat loss (blood vessel constriction, for example) and in the other individual the heat production is increased (higher metabolism). In the first case, the risk of cold injury to the hands and feet is greater.

Temperature receptors

Exposure to cold activates temperature-sensitive receptors in the skin. The information collected in this way is processed in the thalamus, cortex and preoptic area in the hypothalamus, which can control the defense mechanisms. The first task is to maintain the core temperature. A drop of two degrees in the skin temperature (~33 °C) already constricts the blood vessels on the surface and increases heat production (including shivering). The precise neurological control of the various defense mechanisms is not entirely clear. The extra heat production from shivering etc. can amount to 5 times the basal metabolic rate, but this is still 4-5 times lower than the heat production from physical exertion.

Also in the body there are cold and heat sensitive receptors. A heat sensitive receptor is present in 60% of the spinal nerves to the gastrointestinal tract and bladder, compared to 30% of the nerves to skin and muscles. Differences have been found between lean people and overweight people. In people with more body fat, fewer receptors are present in the nucleus. Perhaps because they are less needed here due to higher protection against cooling.

Sex, age, fatigue and energy balance

The study of differences between men and women with regard to their adaptation to cold does not yield clear results, other than those mentioned above (different ratio of body volume to body surface area). However, research does seem to indicate that a higher age leads to a lower tolerance for cold (particularly above 60). This could be related to a reduced capacity for peripheral vasoconstriction, which reduces the ability to prevent heat loss. The decrease in muscle mass and thus a reduction in the capacity for heat production could also play a role. In principle, this makes the elderly more vulnerable to cold injuries. When the elderly die due to cold, this often has to do with high blood pressure caused by cold and thus a burden on the heart. This can lead to a stroke or infarction.

It is important to know that the response to cold is also influenced by factors such as fatigue, diet, negative energy balance and moisture balance. Anyone who is chronically sleep deprived, is tired due to physical exertion and has a negative energy balance due to high exercise and little food is more susceptible to hypothermia. Yet you see these effects mainly in persistent and chronic situations and less in short-term lack of rest and nutrition.

Adaptation to the cold through acclimatization

Repeated exposure to cold can cause the body to adapt to cold exposure in different ways. However, it takes relatively prolonged (weeks) and repeated exposure to achieve a substantial reduction in, for example, shivering. More important seems to be the finding that after acclimatization to cold air the skin temperature remains better at the same level. This can help prevent cold injuries.

To be continued

Part three discusses the measures to prevent cold injuries.

Source

Haman, F., Souza, SC, Castellani, JW, Dupuis, MP, Friedl, KE, Sullivan-Kwantes, W., & Kingma, BR (2022). Human vulnerability and variability in the cold: Establishing individual risks for cold weather injuries. Temperature, 1-38.

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