Humans are ill-equipped to survive in the cold conditions. We have only limited physiological resources to keep warm and are therefore dependent on protection from the environment, from clothing and from external heat sources. I previously wrote about the potential risks of working (and recreating) in the cold, giving an overview of the various cold injuries. See these articles for that

• Risks of the cold: cold injuries
• Consider cold injuries with and without freezing

An extensive review has recently been published that discusses cold stress and cold injuries and discusses how humans adapt to cold and the ways in which we can live and work safely in the cold. Special attention was paid to individual differences in risk factors, in which body weight and peripheral blood flow play a role. There appears to be a large individual variability in construction, insulation and metabolism. This will be taken into account in the protective measures to be taken.

Freezing

Even military operations in a cold climate show great inter-individual diversity in construction, physiology and also psychological factors, and cold injuries are common. For example, in a joint exercise by Canadian and US troops in 2016, 215 medical problems occurred, approximately 20% of which were cases of frostbite with an average wind chill of −44°C. More accurate identification of people vulnerable to cold exposure could be a means to reduce risks and increase employability.

Cold stress

Cold stress and the associated physiological consequences are highly variable and depend on what one is exposed to (air, water), the temperature and relative humidity and the duration (minutes, hours, days). Cold stress can be thought of as any environmental exposure that increases heat output and triggers our defense mechanisms to keep core temperatures at around 37°C.

Heat loss occurs when the heat output exceeds the heat produced by the body. The transfer of heat to the environment occurs through evaporation, radiation, conduction and flow. This is strongly influenced by the environmental conditions, but also by what we are doing and the protective clothing we wear.

But in addition, individual differences in build, physiology and metabolism also play a role. This means that under the same environmental conditions, with the same protective clothing and performing the same tasks, there are still differences in cold stress due to individual factors. The article provides a comprehensive overview of all factors that play a role.

Heat production

One of the first reactions or cold is to reduce the blood flow at the surface by squeezing the blood vessels (vasoconstriction). If this is insufficient to prevent heat loss, the metabolism increases. If this is sufficient to maintain the core temperature, we call the cold exposure compensable. The capacity to compensate differs per individual and it is not yet clear what determines this.

According to the researchers, it is not related to the available amount of glycogen or fatty acids, as is the case for physical exercise. Humans have different heat production mechanisms available that complement each other. This high flexibility is probably related to the low metabolic levels required for maximum heat production: these are about 60% lower than the levels seen during exercise. However, this is at the expense of the capacity to produce sufficient heat when it gets colder. This means that we can only compensate for a limited degree of cold exposure.

Water vs. Air

If heat production falls short of compensating for heat loss, we enter the non-compensable region of cold exposure and our core temperature drops. When it drops below 35 °C, it affects our cognitive functions and organs and systems can fail. This mainly occurs in water colder than 20 °C, which conducts heat four times better than air.

Hypothermia occurs especially and quickly when immersed in water below 18 °C. In cold air this is much less the case and often the result of unforeseen circumstances or inappropriate decisions. Especially because over a longer period of time the systems to produce heat in the body become tired. People then have to take other measures to keep warm.

The cooling rate increases as the water is colder: Is this about half a degree per hour for water at 18 °C, or 7 degrees per hour for water at 2,5 °C. In the latter case, the survival time in lightly dressed drowning persons is only about an hour. When the core temperature drops to 31 °C, the body can no longer warm itself and heat is required from outside. Humans are very vulnerable to cold exposure compared to similarly sized mammals.

In the cold climate room

In an experiment by Haman et al., cold-accustomed men with a body weight of approximately 100 kg stayed in a climate chamber at 7,5 °C. Only half of them managed to complete the 24 hours there. Despite the fact that they had thick cotton overalls, shoes, mittens, hats for clothing. They also had company, were given food and were kept busy with various tasks. On average, their skin temperature dropped about 6 °C and the core temperature remained constant about 0.8 °C below normal.

The cold stress increased the body's heat production by 50% in the first six hours and this continued until the end of exposure. The dropout of these experienced men was striking because the conditions were different and difficult, but still far from the physiological limits. It shows that survival in the cold itself with experience is a problem. Source: Haman F, Mantha OL, Cheung SS, et al. Oxidative fuel selection and shivering thermogenesis during a 12- and 24-h cold-survival simulation. J Appl Physiol. 2016;120(6):640–648.

Body shape and composition

The article discusses in detail the influence of body shape and body composition on heat balance and the risk of cold injuries. The fat percentage of the body plays a role in this, but perhaps in a different way than is often thought. It is not so much about an insulating subcutaneous fat layer, but rather how the amount of fat relates to the body surface.

Comparative research between men and women immersed in cold water also shows that, despite correction for body fat, there are large variations in the degree of heat loss. This may have to do with body shape, fat distribution, but also with hitherto unknown factors. The influence of certain hormones such as estrogen and progesterone can also play a role.

It is generally accepted that people who have a larger mass/volume, a relatively small body surface and a high fat percentage are best able to withstand cooling in cold water. The extent to which this also applies to cooling in air has been less well studied. It is also important to know that a higher muscle mass not only facilitates the production of heat, but also acts as an insulating layer.

Genetic variation and behavioral adaptation

There is great genetic variation in the physiological response to ambient temperature such as constriction or dilation of blood vessels, the number of sweat glands, thickness of the skin, skin color and body shape, weight and composition, etc. It is expected that certain populations of people will adapt to their environmental temperature , such as, for example, people who live within the Arctic Circle. But while these peoples are very knowledgeable about their environment and know how to survive in extremely cold conditions, they are physiologically little better adapted to cold than people living near the equator.

They do have a slightly higher basal metabolic rate, a higher temperature in the hands and feet and somewhat more blood flow in the arms and hands. If these somewhat cold-adapted people move to a warmer climate, these adaptations disappear again. Various studies have shown that a behavioral approach to the heat balance is a very important factor in adapting to cold conditions. You should think of wearing good clothing, building shelters and using heat sources such as fire.

In recent years, several studies have been carried out into how variety influences adaptation to cold conditions. The results were inconclusive, with inter-individual differences possibly being more important than ethnic origin.

To be continued

Part two focuses on ideas about cold regulation mechanisms. 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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