Written by: Stefanie Dumberger
Acclimatization is the ability of the human body to adapt its physiological and morphological functions to different climatic conditions 1, 2, 3. Basically it´s the ambient temperature which impacts us, but also humidity, wind velocity 4 and the progressive increasing lack of oxygen at high altitudes challenge our vital systems 5, 6.
Thermoregulation aims at keeping our body within a stable temperature range of 37°C+/-0.2°C, the optimum for smooth working of our metabolism 6. Therefore we perceive 21°C-26°C as a comfortable ambient temperature wearing clothes and 26°C-33°C when naked 7. Mainly we control the heat exchange with our environment through the skin by dilating and constricting our outer veins 8, 9, 10, 11. Other mechanisms involve shivering 12, 13 and non-shivering 14, 15, 16 heat production as well as insulation by our fat layer in cold conditions 17, 18, whereas in hot climates heat is lost by sweating 8, 19 and production diminished by lower core temperature and improved metabolic activity 20, 21, 22, 23. In addition, adapting our behaviour (e.g. seeking shelter, air conditioning, heating, clothing level) and habituating to specific conditions over a longer term are important instruments for human acclimatization 2.
With increasing temperatures due to climate change humans may reach their limitations of adaptation 24. Extreme events like heat and cold waves are likely to occur more often 24, 25, 26. During these, excess rates in morbidity and mortality are observed (figure 1) as the body cannot acclimatize rapidly enough 27, 28. It needs at least 4-6 days of daily exposure in combination with physical activity or 2-6 weeks without exercising to adapt properly 24, 29. Additionally acclimatization is only a transient state with the benefits decaying twice as fast as they are acquired 29. Therefore temperatures lying widely outside our comfortable zone are highly challenging for our vital systems. Humans can usually bear core temperatures of up to 41°C 30, 31 or down to 30°C 8, 32, but only with unlimited access to water and food as well as appropriate clothing. Beyond these temperatures heat and cold illnesses appear which finally lead to exhaustion of the defending mechanisms and thereby death 24.
Overall mortality in association with the perceived temperature in the state Baden-Württemberg, Germany; the x-axis shows the median over three days of perceived outdoor temperature at 12 am in °C and the y-axis the rate of mortality per 100000 inhabitants; the lowest rate of mortality appears around 20°C-25°C, the slope for heat mortality is steeper (1.1% per K) than the slope for cold mortality (0.6% per K), but higher rates of mortality occur in very cold conditions (around -10°C) (Credit).
Challenging factors in this context especially for vulnerable subgroups (e.g. the elderly, children) may be the habituation to overall warmer conditions which leads to impaired reactions to cold waves 24, 27. In addition augmentation of humidity as a result of rising ambient temperature will impede cooling through sweating 33. Since evaporation is the only possible mechanism of heat loss if ambient temperature is higher than the skin temperature 8, 19, heat waves or even hot summer months may overtax our thermoregulation.
Moreover indoor work performance diminishes with each degree after 26°C and outdoor working capacity will be limited or even getting impossible 34. Permanent air conditioning as it is now used in desert regions like the United Arab Emirates will be necessary to enable comfortable working and living resulting in a higher energy demand and thereby increasing energy consumption. Actually these technical adaptations delay the acclimatization process and enhance impaired wellbeing during outdoor activities. 24
We cannot predict if there may happen genetic adaptations to push the current temperature limitations of the body upwards. Probably adapting our behaviour and habituation processes will get more important 24. Furthermore migration towards colder areas of higher altitude or latitude are also likely, though space and resources are limited in these regions.
As it is an interesting field, more research should be done on the effects of acclimatization and the results taken into consideration when investigating the overall impact of climate change on human beings.
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