Since last week, the temperature in the north and south has begun to fall, but this year's "autumn tiger" still left a deep impression on everyone. In particular, the high temperature days in the south were once slaughtered after entering September, and the Central Meteorological Observatory also issued yellow warnings for high temperatures one after another. The situation abroad is similar, from May this year to this month, Phoenix, Arizona, United States, has a high temperature of more than 37.7 °C for more than 100 consecutive days, setting a new historical record.
"Dead of heat" is a mantra on the lips of people under high temperature baking, and the scientific expression of "hot death" is: how high and how long the maximum temperature a person can tolerate. In the past, there was a scientific definition of this, but now this definition has new research insights. A review article published in the journal Nature Communications recently pointed out that the maximum temperature limit that humans can tolerate is a wet-bulb temperature (WBT) of 31°C. Previous studies have suggested that this limit is 35°C.
So, how is the wet-bulb temperature different from the temperature in the weather forecast? What does it have to do with the conditions under which the human body survives at high temperatures? What implications will this change bring to the survival of human beings in the face of global climate change?
Schematic diagram of wet bulb and dry bulb thermometers
Why wet-bulb temperature is a measure of the limits of human survival
The maximum temperature that the human body can tolerate is not defined by the "Celsius" or "Fahrenheit" of the weather forecast, but by the "wet-bulb temperature".
Wet-bulb temperature gets its name from the way it is measured, that is, the sphere of the thermometer is wrapped in a damp cloth, and the moisture evaporating from the damp cloth will make the thermometer cool, and the temperature displayed on the thermometer is the wet-bulb temperature. It is different from the drybulb temperature (the temperature predicted by the weather) and cannot exceed the value of the drybulb temperature. When the humidity in the surrounding air is high, the wet-bulb temperature will be closer to the dry-bulb temperature.
Why do scientists use wet-bulb temperature to measure how well a person tolerates temperature? Because the maximum temperature that a person can tolerate is not only related to air temperature, but also to humidity. The higher the humidity, the lower the temperature that the person can tolerate, because the higher the humidity, the less heat the person dissipates through sweating and vice versa. According to a 2010 mathematical modelling study, the highest wet-bulb temperature threshold or cut-off that a healthy adult can tolerate is 35°C (95°F), and under these conditions, a person can only survive for 6 hours. If converted to a standard that can be understood by the general public, it is equivalent to a healthy adult living for only 6 hours in an environment with a temperature of 40°C (the temperature predicted for the weather) accompanied by 75% relative humidity.
Later, the World Health Organization (WHO) and the Intergovernmental Panel on Climate Change (IPCC) both considered the wet-bulb temperature of 35°C as the limit of human survival in extreme heat. However, many researchers have disputed this standard, arguing that people may not reach the wet-bulb temperature of 35°C, and people will "die of heat". In addition, objectively, although wet-bulb temperature is used to evaluate the heat stress response of people in a high-temperature environment by taking into account the effects of temperature and humidity on people, this standard also has limitations, because the human body in the model is simply regarded as a stationary object without clothing, sweating and movement, so it is difficult to conclude whether its prediction results can be completely consistent in the real environment.
What it means to be able to withstand a maximum wet-bulb temperature reduction
In 2021, a team led by Larry ·Kenny, a professor of physiology and kinesiology at Penn State University United States, concluded that the previously proposed 35°C wet-bulb temperature limit might be too high by monitoring changes in the body's core body temperature in volunteers under different combinations of temperature and humidity. Volunteers who participated in the heat-tolerant trial were young, healthy individuals who were required to ride bicycles to simulate exercise and sweating responses in high-temperature conditions, and the results showed that many experienced a runaway of body temperature at a wet-bulb temperature of around 31°C.
While Kenny's team concluded that "31°C wet-bulb temperature" is the maximum limit that humans can tolerate, Australia physiologist Oli ·Jay at the University of Sydney designed a climate chamber in 2019 that simulated current and future heat waves. The $2 million facility, which took 18 months, was completed and commissioned in 2021. Jay's team exposed participants to extreme heat under strict medical supervision, explored the limits of the human body's survival in the face of heat, and investigated effective cooling strategies.
Located on the top floor of the University of Sydney, the climate chamber simulates temperatures, humidity and wind speeds, as well as simulating sunlight using infrared lights. The researchers started at 5°C and increased it by 1°C per minute to a maximum of 55°C, allowing for precise control of the experimental conditions. The climate chamber also regulates humidity, which is a key variable in the body's response to heat stress.
Volunteers were allowed to eat, sleep, and exercise in the chamber, and the researchers used sensors to collect and analyze their physiological data such as heart rate, breathing, sweating, and body temperature. By measuring the volunteers' core body temperature, heart rate, and sweating at different combinations of temperature and humidity, the researchers calculated the body's maximum wetbulb temperature threshold. To be on the safe side, the trial is stopped as soon as the ambient temperature reaches the safe limit for volunteers.
After more than two years of research, in 2024, Jay's team came to the same conclusion as Kenny's team: the maximum temperature limit that humans can tolerate is a wet-bulb temperature of 31°C, which means that they can only survive for 6 hours under such conditions.
Although the Jay's team's research has not yet been measured in different age groups, there are some more specific findings from the research data. For example, the survival limit of young and old people in the shade and in direct sunlight has been calculated – the survival limit for young people is 26°C-34°C, and the survival limit for older people is 21°C-34°C.
This difference is due to the fact that the person's ability to tolerate heat and cold gradually decreases with age. First, as we age, the thermoregulatory center of the brain, the hypothalamus, becomes sluggish or difficult to activate, increasing sweating and causing blood vessels in the skin to dilate to dissipate heat when the body temperature rises. Secondly, the skin of the elderly will shrink, which will shrink the opening of the hair follicle and reduce the function of perspiration and cooling. In addition, the cardiovascular function of the elderly is reduced, the blood supply to the surrounding body and the surface of the body is insufficient, and sweating is reduced. The research team used a climate chamber model to test it in multiple scenarios, and the results showed that the human body's survival limit in direct sunlight was lower than the limit in the shade. The reason is also very simple, because when the temperature is higher in direct sunlight, the body's sweat will evaporate quickly, and the body cannot maintain a long period of self-cooling.
Jay's team's research also provides a scientific basis for protecting babies. Studies have found that dry white cloth can increase the temperature inside the stroller by more than 2.5°C, while wet white cloth can reduce the temperature inside the stroller by about 5°C. This result is clearly in line with the principle of wet-bulb temperature, where the human body can cool down by sweating, or by wrapping a damp cloth to dissipate heat to lower the temperature.
The evolution of cryophysiology in humans faces great challenges
Today, another reason why humans struggle to tolerate high temperatures is evolution. Due to the use of air conditioning and better medical conditions, the basal body temperature of the person is gradually decreasing, so it is difficult to tolerate higher temperatures.
For many years, 37°C was considered a normal body temperature. However, depending on the part of the human body, the time of the test, the season, and the individual, the body temperature reading can affect the reading, so it is necessary to measure the body temperature of different parts of the body.
What's more, people's basal body temperature is decreasing year by year. United States Stanford University's infectious disease epidemiologist Pasunnet's team has studied and analyzed more than 677,000 temperature measurements of United States people since 1860, and calculated that the average body temperature is lower than the textbook 37 °C, and it drops by a few tenths of a degree every decade, which is manifested in several time nodes. In 1851, the German physician Wendrich first determined that the normal body temperature of the human body was 37 ° C; In 1992, a team of researchers led by Maktuvik, an infectious disease physician at the University of Maryland School of Medicine United States, tested 148 people who participated in a vaccine trial and found that their average body temperature was 36.8°C, and in 2017, a study of United Kingdom 35,000 people found that the average temperature was 36.6°C.
As a result, researchers generally doubt the authenticity of 37°C as a normal body temperature in humans. But the object of their suspicion is, first of all, a thermometer. For example, Maktuvik argues that the thermometer used in Wendrich's time had too high readings, even exceeding 1°C, so the measurement error was the main reason why Windridge set 37°C as normal body temperature.
But then the researchers realized that people born earlier tend to have higher body temperatures than people born later, and that people's great-grandparents and grandparents have higher body temperatures than modern people. According to Pasonette et al., the root cause is not the accuracy of thermometers, but human evolution and the technological means used to make life more comfortable. Since the 19th century, the average body temperature of adults has been declining, falling by 0.4°C in less than 200 years, from 37°C to 36.6°C. The reason for this is the increasing lack of physical activity of human beings, and the ubiquitous air conditioning makes the thermoregulatory center no longer sensitive, and the body adapts to the "low temperature" of the environment, so the body temperature drops. At the same time, the improvement of medical standards has also lowered people's body temperature, because the probability of modern people being infected with various pathogens and causing body temperature to rise has been reduced overall. For example, people with long-term infections such as tuberculosis and gum disease have an increased body temperature due to the inflammatory immune response in the body, but due to the use of antibiotics and other drugs, the chance of microbial infection is relatively reduced, and the course of the disease is shortened, which also causes the body temperature to drop.
The disadvantages of a drop in body temperature, in addition to a lower threshold for people to tolerate high temperatures, also lead to a decrease in immunity. Japan medical doctor Yumi Ishihara pointed out in his book "36.5 °C Determines Health" that for every 1 °C decrease in body temperature, immunity will drop by more than 30%; For every 1°C increase in body temperature, immunity increases by 5-6 times.
How to deal with higher and higher ambient temperatures
70% of the world's population is now at risk of extreme heat. Adopting a temperature that conforms to human physiology and a more scientific standard, i.e., the wet-bulb temperature standard, can better protect humans.
In fact, the concept of wet-bulb temperature was proposed as early as the 50s of the 20th century, when the United States military used the wet-bulb temperature standard to develop guidelines to ensure the safety of soldiers, when the wet-bulb temperature reached 29 °C, if someone did not adapt to the environment, it would be recommended to stop the activity, and anything above this temperature was listed as extremely dangerous, because the risk of heat stroke would rise sharply. According to the new study, if the maximum wet-bulb temperature threshold of 31°C is widely accepted by the scientific community, it can be calculated that a dry-bulb temperature of 35°C is dangerous to humans at 75% relative humidity. Similarly, if a wet-bulb temperature of 29°C is used as a critical value, a dry-bulb temperature of 33°C is not safe for humans.
Now, researchers believe that a wet-bulb temperature of 31°C (dry-bulb temperature of 35°C) is already the highest temperature that humans can tolerate. However, the basal body temperature of people is still declining, and combined with global warming, the gap between the two is likely to widen.
The temperature is rising, and the living environment of human beings is facing more and more severe challenges. The first solution to the problem is to reduce the use of fossil fuels and limit global warming to a certain extent. According to the conclusions of the latest report of the Intergovernmental Panel on Climate Change (IPCC), "Climate Change 2023", actions taken in the next seven years will determine the survival of human beings on the planet, and it is urgent to limit the global average temperature rise to less than 1.5°C.
Second, humans can rely on the power of technology to reduce the possibility of high temperatures in the living environment, and lowering the temperature of cities is one of the most feasible measures for people to withstand high temperatures. Taking large cities as an example, with a large population, a large number of cars, and a large use of air conditioning, coupled with the dense urban building complex, asphalt roads and cement pavements have a greater heat absorption rate and smaller specific heat capacity than the soil and vegetation in the suburbs, which makes the urban areas heat up faster and radiate to the surroundings and the atmosphere, further enhancing the heat island effect. It has been pointed out that greening the top floors or walls of highly dense urban complexes, and expanding urban vegetation, mainly trees rather than lawns, are the most effective ways to change the city's high temperatures. The temperature on the top floor of a building can reach up to 78°C, but even if the roof is only planted with simple greening (grass planting), the temperature on the top floor of the house will drop to 29°C-30°C in summer, and the surface temperature will be 20°C-30°C different from the temperature without greening.
In summary, wet-bulb temperature combines heat and humidity measurements with sunlight and wind to provide a more complete picture of the environment in which a person lives. The threshold study of wet-bulb temperature is not just to see the "liveliness" of the data and debate whether it is high or low, but importantly, it allows scientists to pay more attention to the relationship between human evolution and the earth's climate and environment, and continue to explore solutions to solve the problem, so that people can better survive in the high temperature environment brought about by global warming.
Source丨Beijing Daily