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Heat, our body and what we can do (2)



Exposure to extreme temperatures that exceed the body’s thermoregulatory capacity (35 degrees) can cause physiological damage to the body and the development of heat-related illness (HRI). Part of this is due to heat exposure disrupting the body temperature regulation mechanism, leading to acute fever. The mild disease of acute HRI includes heat cramps, heat edema, heat syncope, and heat tetany. The above mild diseases are the initial mild symptoms of heat injury, and if they further develop, they will become heat exhaustion, the most common type of HRI. When heat exhaustion occurs, the core body temperature of the human body will be between 38.0°C and 40.5°C, manifesting as malaise, anorexia, nausea, vomiting, and other symptoms. There are no major neurological deficits at this stage.

Heat stroke occurs when the thermal damage worsens and the core body temperature exceeds 40.6°C. Heat stroke is an acute syndrome that can be life-threatening. Studies show that the mortality rate of heat stroke patients may reach 10%. When the core temperature is too high and exceeds the heat stroke standard, various tissues and organs will fail, accompanied by neurological dysfunction. Heat stroke is defined as a condition in which body temperature rises to a level that becomes harmful, leading to dysfunction and damage to body tissues and a characteristic multi-organ clinical and pathological syndrome. Heat stroke patients often suffer permanent damage to the central nervous system or other organs even after recovery. HS can be divided into two types, experimental and classic heat stroke. Classic heat stroke occurs when individuals with low immune function, such as the elderly and young children, cannot adapt themselves and are affected by the external environment. Exertional heat stroke usually occurs in athletes and people engaged in specific jobs. Excessive exercise in hot environments causes the body temperature to rise, exceeding the body’s heat dissipation capacity. If heat stroke patients cannot receive timely treatment, or if the attack lasts for too long or is severe, it may cause permanent damage or even endanger life.


Heat exposure can not only cause certain diseases directly, but it can also make chronic illnesses worse. The most obvious effect on human health is the aggravation of cardiovascular diseases. Heat waves lead to an increase in heart diseases such as heart failure and myocardial infarction. Heat-related cardiovascular disease not only increases the rate of emergency hospital admissions, but also causes a large proportion of deaths, and the number of out-of-hospital deaths should not be underestimated. According to an analysis of cases from 50 cities in the United States, 42% of deaths during heat waves occurred outside hospitals, and 34% of deaths were caused by cardiovascular disease.

In addition to cardiovascular disease, rising temperatures are also closely linked to respiratory health. As the core temperature rises, the body helps to dissipate heat by increasing lung ventilation, including two breathing patterns, thermal tachypnea or thermal hyperpnea. Hyperventilation, when the core temperature is too high, can lead to alveolar hyperventilation, causing respiratory alkalosis, which can lead to tissue damage. As the temperature rises, the movement of particulate matter molecules in the air increases, which has a greater impact on the respiratory system. In addition, climate change and global warming have extended the pollen season, and rising temperatures have increased pollen concentrations, which will also increase the incidence of allergies and thus affect respiratory function. An analysis of death data from 11 cities in China found that respiratory deaths accounted for 12.79% of all-cause deaths during heat waves. A study from the United States also found that the risk of respiratory death increased by 5.9% during heat waves compared with non-heat-wave days [10]. Another US study found that for every 10°F increase in average daily summer temperatures, respiratory related hospitalizations increased by 4.3% in 213 counties where daily temperatures were recorded.


Another threat to human health from heat exposure is the threat to pregnant women and newborn babies. During pregnancy, physiological changes in the body reduce a pregnant woman’s ability to regulate her body temperature, making her more vulnerable to heat-related adverse health effects. Exposure to heat increases a pregnant woman’s body core temperature and heart rate, causing fetal heart rate tachycardia and uterine contractions. In severe cases, it will cause many adverse effects, such as premature birth. Studies have shown that heat exposure increases dehydration and the secretion of antidiuretic hormone and oxytocin, leading to preterm birth. Clinical research shows that a high fever during pregnancy can cause birth defects in babies. Heat exposure also has adverse effects on newborn health, and the leading cause of neonatal morbidity and death is preterm birth. Preterm birth (PTB) is defined as the birth of an infant before 37 weeks gestation and can be associated with a variety of acute morbidity or mortality, as well as long-term physiological defects. In recent years, an increasing number of studies have demonstrated a strong association between heat exposure and preterm birth. A systematic review of data from 20 study groups showed that there are clear seasonal patterns in birth outcomes, with preterm birth rates significantly higher in summer and winter than in other seasons.



Analysis of the impact of extreme heat exposure on human mental health

Climate change is not only a threat to the natural environment. It is also a mental health crisis. Some studies suggest that heat waves can increase the risk of suicide and mental health-related hospital admissions in the general population. Studies have found that temperature can increase the incidence of suicide, mania, depression, schizophrenia, and other conditions. According to a metanalysis, suicide rates increase by 1% for every 1°C increase in local ambient temperature above the threshold. Temperature changes affect mental health by influencing physiological changes in people. For example, changes in the body’s blood flow to remove heat and changes in the functioning of the central nervous system can lead to cognitive and emotional changes. As temperatures rise, the risk of dying from mental illness increases. In many cases, medications used to treat mental wellbeing can reduce the body’s ability to regulate its temperature at high temperatures, further exacerbating underlying health conditions. A review of New York State An analysis of death data from psychiatric hospitals from 1950 to 1984 showed that the mentally ill were twice as likely to die during heat waves as the general population. And the mortality rate was even higher when high doses of antipsychotic drugs were widely used in the 1970s. The study found that both having a mental illness and taking antipsychotic drugs during a heat wave were associated with an increased risk of death.


Heat exposure not only has a negative impact on people suffering from mental illness, but can also harm the mental health of the general population. When the temperature is too high, psychological discomfort and negative emotions increase in the general population. Extreme heat can affect mental health by affecting people’s sleep. Clinical evidence shows that sleep and mood are closely linked, that almost all psychiatric and neurological disorders are associated with sleep problems, and that sleep disorders can lead to new mental health problems. Some research on sleep conditions during heatwaves shows that high temperatures during heatwaves can lead to sleep disturbances such as reduced sleep and frequent sleep interruptions. Lack of sleep can cause people to be irritable, depressed and have difficulty regulating impulsivity, and research shows that sleep deprivation can also amplify negative emotions that disrupt time and cause the brain to inappropriately regulate emotional responses to negative stimuli, triggering a range of mental health problems. And sleep is often thought to have a two-way relationship with mental health. Although insomnia increases the likelihood of anxiety and depression, the occurrence of these mental disorders also increases the likelihood of new-onset insomnia.


Extreme heat has direct and indirect consequences on mental health, both now and in the future. These effects can be influenced by a variety of circumstances. This is because climate change affects people in many different ways, often not as a result of a single event. It affects mental health through social changes such as reduced economic output, increased conflict, and violence, etc. In result, the negative effects of some psychiatric medications and the decreased behavioral flexibility of individuals with mental illness can help to explain some of the consequences of heat exposure on human mental health. In addition, sleep disturbances caused by heat exposure-related changes in human central nervous system signaling may further increase the burden on human mental health.

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