The effect of climate change on thermoregulation


Vital signs, specifically body temperature, pulse rate, respiratory rate and blood pressure reflect essential body functions and as such they are closely monitored during a patient’s stay at the hospital. More recently, additional assessments of pain, level of consciousness and urine output have been proposed as critical parts of a patient’s routine assessment.1Detection of any abnormality among vital signs may indicate an acute medical problem or a change in patient’s health status that demands immediate attention.2 In such cases, repeated measurement of vital signs is advised to help physicians formulate clinical assessment and plan for their patients.2 Now, that we are living in times of climate change we need to realize that global warming will not leave human vital signs unaffected. Scientists from the Intergovernmental Panel on Climate Change claim that “global warming is likely to reach 1.5oC (34.7oF) between 2030 and 2052 if it continues to increase at the current rate”.3 As human thermoregulation is highly affected by environmental parameters we must be aware that such an increase in our planet’s average temperature would have a tremendous effect on our body homeostasis, vital signs and health.

            It is known that humans are endothermic (warm-blooded) mammals.4 This means that they are able to maintain a constant body temperature irrespective of any variations in ambient conditions, in contrast to ectotherms (cold-blooded animals) whose thermoregulation depends on behavioral and physiological adjustments, according to their surroundings.4Surprising is the fact that natural selection facilitated the rise of endotherms despite the high energetic cost derived from their evolution.5 Endotherms spend 30 times more energy per unit time than ectotherms and this is attributed in part to their aerobic metabolism and increased muscular activity, when compared to the latter.6 Increased stamina along with an increase in resting metabolism seem to be key steps in natural selection for endothermy. Pathogens have also been proposed as additional agents of natural selection in facilitating the evolution of endothermy.7 Interestingly, fever has demonstrated an adaptive and beneficiary role in the survival of the infected organisms, by mounting an adequate immune response to the host.8 Moreover, according to geologists, “endothermic species would have a better chance to survive through important and rapid climate change due to the capacity to produce their own body heat that allowed them to live in colder environments or in areas with huge seasonal temperature contrasts”.9

            Normal human body temperature ranges from 35.7 oC (96.3 oF) to 37.4 oC (99.3 oF) as determined by a thermometer placed in the mouth. However, local conditions (such as hot or cold drinks and ambient temperature) may lead to significant variations.10 Rectal temperature which spans from 36.3 oC (97.3 oF) to 37.8 oC (98.2 oF) is the most reliable way to assess body temperature and is considered as the gold standard, since variation rate is low and precision is high with this type of measurement.11 The increased metabolic activity of fecal bacteria has been implicated for the higher temperature measurements obtained through the rectum.12 In addition, measurements of body temperature in the axilla or the groin are not considered accurate and may be influenced by the body fat mass and sweating in both sexes and the menstrual cycle among women. Yet, temperature measurements in the axilla are the most commonly performed in clinical practice by medical staff. The normal axillary temperature range is approximately between 35.0°C (95.0 oF) and 36.9 °C (98.4 oF).13 Today, ear and forehead thermometers have been developed. These thermometers record body temperature through the use of infrared sensors, provide reliable measurements and are easy to use. The normal ranges of ear and forehead temperatures are 35.8 °C (96.4 oF) to 37.5 °C (99.5 oF) and 31.0 °C (87.8 oF) to 35.6 °C (96.1 oF), respectively.14,15 It is important for clinicians to be aware of the normal ranges of body temperature measured through these different types of measurements to avoid unnecessary work-up and treatment, obtaining in parallel an accurate assessment of their patients’ body temperature.16

            Despite variations that become evident by using different methods and techniques to measure body temperature, physicians should be aware of the diurnal and age variations that may be observed in clinical practice.17 Indeed, the same person may exhibit variations in body temperature throughout the day. Lowest temperatures are noted about 4 a.m. and highest about 4 to 7 p.m. These upward inflexions in the temperature curve are highly attributed to the increased muscular activity that occurs through the course of the day.18 Exercise may raise body temperature to 38oC or 40oC.19 Under such circumstances, sweating and increased skin blood flow help to dissipate heat from the body.19 In contrast, during sleep, bodily inactivity, diminished control of the temperature-regulating mechanism and vasodilatation of the cutaneous vessels, lower body temperature.20 At this point, we should mention that human sleep, which is necessary for healthy human functioning, is highly affected by both body and ambient temperature.21, 22 Heat loss facilitated by the vasodilation of the cutaneous blood vessels, decreases the core body temperature and signals sleep onset.23 This normal biological process follows a circadian 24-hour pattern that contributes not only to falling asleep but also to maintaining sleep throughout the night. Body temperature rises again just shortly before awakening.24 It has been proposed that melatonin plays a crucial role in this mechanism.25 Scientific evidence has revealed that melatonin potentiates important heat loss at the most distant sites (i.e., hands and feet), by regulating the vascular tone in selective vascular beds, as its circulating levels rise and fall throughout the night.25 Given the fact that nighttime temperatures are increasing far more rapidly than daytime temperatures, one should assume that the ongoing global warming may significantly disrupt human sleep in the future. Actually, the first evidence that climate change may disrupt human sleep in the future comes from a study conducted in the USA, by Obradovich et al.26 The researchers reported the harmful effect of the increases in nighttime temperatures on the reported nights of insufficient sleep among 765,000 U.S. residents between 2002 to 2011.26They concluded that the self-reported nights of insufficient sleep were amplified especially during the summer period, whereas they were more commonly observed among those with a median income below $50,000 per year and the elderly.26 Moreover, adults aged ≥60 years tend to have lower body temperature than younger adults by 0.23oC, and this is attributed partly to their decreased body activity and poor circulation.16, 27 Impaired heat loss mechanisms from the body among elderly individuals make them particularly vulnerable during extremes of temperature.27    Females, children and the poor also tend to suffer during extreme weather conditions.20 Age, gender, fitness status, fat content, acclimatization and socioeconomic status are important predictors of heat tolerance.27

            To our knowledge, apart from individual factors (i.e., sweating, muscular activity and clothing), the heat balance equation that regulates body temperature is also affected by several climatic variables: 1. air temperature, 2. radiant temperature, 3. surface temperature, 4. air humidity, and 5. wind speed.28

Heat Balance Equation: Heat Production=Heat Loss29

Havenith proposed that the core body temperature remains stable when the body heat produced by muscle activity and workload equals the heat lost from the body through conduction (the loss of heat from the skin to the environment through direct contact with a cooler object), convection (the transfer of heat from the skin to the environment through rising airflow, as it occurs in the case of a fan), radiation (the electromagnetic waves in the form of infrared radiation that transfer heat from the body to the environment), evaporation (any moisture present on the skin, such as sweating, that may evaporate to release heat from the body) and respiration (with the expired air up to 10% of the total body heat production may be lost).29

            Overwhelming evidence suggests that the ongoing global warming has shifted the normal “bell-shaped distribution” temperature curve to include a statistically significant higher number of hot days and nights, that in the last ten years cover the 22.1% of the Earth’s surface compared to the 0.1% of the land they used to cover in the second half of the 20th century.30 The high levels of particulate air matter released by anthropogenic activities in densely populated urban areas in combination with the poor vegetation, the heat trapped in the cements of the buildings and sidewalks and the heat that is released by cars, air conditioners and other vehicles or engines, lead to the occurrence of higher temperatures in the cities than those observed in rural areas that make them prone to heat waves.31 In addition, as daytime and nighttime temperatures increase, the oceans evaporate more moisture into the atmosphere, perpetuating further temperature increases. Warmer air can hold a lot more water vapor. Interestingly, with each additional 1oC of temperature, the atmosphere’s capacity to hold water vapor increases by 7%.32 Actually, there is already 5% more water vapor over the oceans than there was only 30 years ago.32 Under weather extremes, body heat loss mechanisms become impaired and therefore human thermoregulation is adversely affected. For example, when the ambient temperature exceeds skin temperature the body cannot dissipate heat through conduction, convection and radiation. Instead, the human body gains heat from the environment.28 Likewise, higher moisture in the environment than on skin impairs evaporative heat loss, whereas under extremely hot and humid conditions, extreme wind speed adversely affects both convective and evaporative heat losses.28

            According to the Centers for Disease Control and Prevention, heat cramps, heat exhaustion and heat stroke are among the most common health problems encountered during heat waves that increase patients’ visits to the emergency departments.33 Prolonged sun exposure combined with heavy exercise or increased workload may frequently lead to severe dehydration, water and salt loss from the body that cause heat cramps – muscle spasms usually in the abdomen, calves or arms – unless appropriate supplementations of salt and water are administered. Prolonged and extended (usually lasting for several days) exposure to extreme heat conditions may dehydrate the patient and trigger heat exhaustion that requires immediate admission and treatment. However, heat stroke (or hyperthermia) is a life-threatening and the most serious condition caused by extreme heat. When afflicted by heat stroke, patient’s body temperature may rise rapidly up to 41oC or higher, followed by shunting of blood from the visceral circulation to the skin and muscles leading to multiple organ failure that requires emergent medical attention and aggressive hydration to avoid death.33 In such cases, obtaining a careful medical history regarding the living conditions such as the availability of air conditioning at home, the duration of exposure of the patient to extreme heat, his/her access to a balanced diet and adequate hydration, in combination with the alarming signs and symptoms of very high body temperature, altered mental status, nausea, dizziness, circulatory collapse and loss of consciousness,  are crucial for a prompt and accurate diagnosis of heat stroke that warrants immediate attention and treatment.33, 34 At this point we should mention that a rectal temperature should be obtained in every patient suspected of heat stroke, since an oral or axillary temperature may underestimate delay a prompt and accurate diagnosis, delaying treatment.10, 35

The last years, from 2015 to 2019, have been the hottest years of all, since the temperature records started in 1880s.36 Few days ago, mercury hit 54.4oC (130oF) in Death Valley National Park, in California – the highest temperature ever reliably recorded on our planet. On August 22nd, the US National Weather Service issued a red flag warning for wildfires for much of the northern half of California. Heat-related illness and heat strokes are especially feared during such occasions. Some weeks earlier, Salas R. et al published a perspective article in the world’s leading medical journal, the New England Journal of Medicine, providing short-term strategies for managing climate-related extreme events (such as hurricanes, wildfires and extreme heat) during the COVID-19 pandemic highlighting that “ongoing adaptations and transformations in health care delivery, prompted by the COVID-19 pandemic, (i.e, investments in strengthening the health care infrastructure and delivery systems, such as supply chains), can also be effectively applied to climate-driven extreme events, to ensure resiliency during climate shocks”.37 In Greece, forceful winds and temperatures of above 40 degrees Celsius supercharged wildfires that havoced the Evia island in August 2019. A year earlier, 116 people had lost their lives during the wildfires in eastern Attica, according to data from Hellenic National Meteorological Service. It should be of special interest to record the hospital admissions of heat strokes in the area during this period, as the wildfires were coupled with high temperatures, up to 39oC. Anyway, it is undeniable that healthcare system preparedness, both in terms if infrastructure and scientific knowledge of medical personnel, is crucial to the battle against climate change.

The ban of mercury

According to the World Health Organization and the Minamata Convention which was signed in October 2013, the mercury thermometers are allowed to be used until this year (2020) with a few exceptions that may extend this deadline until 2030. Mercury is now recognized as a “substance that causes significant adverse neurological and other health problems, with particular concerns about its harmful effects on fetuses and newborns. The transfer of mercury to the environment on a global scale was one of the main reasons why it was decided that international action is needed to address the problem of mercury pollution.” The decision was implemented on May 11, 2017, while the international organization stressed that the health effects of mercury “are so serious that everyone should try to respect the 2020 date set in the contract.”

Clinical Observation

From February 2020 to May 2020 when the first wave of the COVID-19 pandemic took place, patients visiting our Internal Medicine Outpatient Office (in Athens, Greece) were complaining of episodes of body temperatures increases between 36.9oC to 37.5oC without any other additional symptoms, occurring daily usually in the afternoon. In fact, this variation in temperature was observed only when recommendations were made by health care professionals for a regular thermometry in the context of prevention and early detection of infection in order to decrease the spread of the virus through timely patient isolation. However, the clinician should be aware of the normal diurnal variations to avoid unnecessary work-up and treatment, that also puts an extra burden on an already fragile healthcare system. In this case, what was bringing our patients to the clinic was just the observation of a normal biological process.

  1. What is the age of our patient?
  2. If our patient is a woman, is this body temperature increase related to the menstrual cycle?
  3. Which was the site of measurement? The axilla, the mouth, etc?
  4. Was this temperature increase preceded by exercise?
  5. Was our patient exposed to the sun, at high temperatures?
  6. Was a heat wave taking place during the days that the temperature went up?

These are vital questions while obtaining an accurate clinical history from our patient.


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