Heat waves

Until recently, heat waves were not perceived as a serious risk to human health and economic activity in the European region. However, the record high temperatures that have persisted for long periods of time in recent decades have led society to change this view.

• In 2003, Europe was hit by the hottest wave since 1540. France was particularly hardly affected. A health crisis was brewing in several countries, and the drought had led to poor harvests in parts of Southern Europe. The number of deaths due to extremely high temperatures was over 70,000.
• In the summer of 2019,  two consecutive heat waves were registered in Western Europe. The first wave was caused by hot winds from the Sahara Desert, which killed 567 people. In France, a record temperature of 45^oC was measured. A month later, Europe was hit by a second heat wave that killed 869 people. Along with humans, many animals raised on farms dies as the air conditioning system failed to compensate for the high temperatures. Many power plants were shutting down due to the inability to use river water for cooling.
•  In August 2021,  Europe was hit by a new heat wave. The temperature in Greece rose to 48^oC. Devastating forest fires were raging in Turkey, Greece, Italy and Finland.

There are different definitions of "hot wave", which depend on the location and characteristics of different regions and countries. A "hot wave" in Bulgaria is defined as a period of at least 3 consecutive days during which daytime temperatures exceed 30^oC, and if in this period nighttime temperatures are above 20^oC – then nights are defined as "tropical". Heat waves in Bulgaria were registered in 2000, 2007, 2012, 2015, and in 2021 in some places the daily temperature reaches 38-40^oC. Particularly vulnerable are the plains of southern and northern Bulgaria, as well as the valleys of the rivers Struma and Mesta.

Scientists have found that an increase in average summer temperatures of about 2-3 degrees leads to a double increase in periods of hot weather. The irreversible trend of continuing global warming in the coming decades has led to an increase in the intensity and duration of heat waves.

The combination of hot days and tropical nights, along with high humidity, is particularly stressful for the human body and can lead to serious health consequences.

Major health problems related to heat are:

• skin rash;
• heat fatigue;
• heat cramps – painful muscle spasms caused by dehydration and loss of electrolytes. They can be the result of hard physical work, hot weather and other factors. Heat cramps are the initial symptom of heat illness, including heat stroke and potentially fatal overheating of the body;
• heat syncope (fainting) – a mild form of heat sickness due to overheating, which is often the result of physical exertion in hot weather. Placing the patient in a horizontal position and in the shade quickly normalizes the condition;
• heat exhaustion – caused by low blood pressure and dehydration of the body. Too often, this condition occurs in young people who put a lot of strain on their bodies, even though they are not feeling well. If the high heat load in such cases is not removed, it can pass into
• heat stroke, in which extreme body temperature (above 40.5°C) leads to damage to cellular structures and the thermoregulatory system and is at high risk of fatal outcome.

Most heat-related illnesses (with the exception of skin rashes and heat cramps) are essentially the result of a malfunctioning thermoregulatory system. The brain centers that regulate body temperature try to keep the basic body temperature within healthy limits – at rest it is around 36-37°C, but during exercise the temperature can rise to 38-39°C without any harmful effects on health, as long as the thermoregulatory system is functioning properly.

The main predisposing factors for health problems related to heat are age, lack of acclimatization, dehydration due to reduced absorption of food and fluids, intestinal problems, use of diuretics and alcohol abuse, drugs affecting the temperature control system, poor  physical condition , overweight, fatigue, lack of sleep, prolonged exercise and wearing protective clothing.

The human body can be "discharged" from excess heat by:

• convection – in contact with air or water surrounding the body;
• conductivity – in contact with solids, such as floors, cold sheets;
• breathing – when inhaling cooler and drier air;
• heat radiation and evaporation of sweat.

Overcoming the effects of heat waves are related to the specific condition of each individual such as age, physiological acclimatization to hot weather, prevention of dehydration, overweight and appropriate physical activity:

•  Older people are more vulnerable to heat due to changes in the body's thermoregulatory system. In this way, they may not realize that high temperatures are harmful to them and may not try to limit their stay in a hot environment. Most often, poor physical condition is one of the factors that make older people susceptible to heat-related illnesses and deaths. Another factor is the relatively high percentage of people with diseases and disabilities among the elderly population. As adults have a reduced capacity for sweating, it is essential that their sweat evaporates. This does not happen if the humidity is high. This fact underscores the importance of researching not only air temperature in the analysis of heat-related morbidity and mortality, but also the humidity of the environment.
• Short-term thermal acclimatizationacclimatization
  is the process by which an individual organism adapts to changes in the environment (such as changes in altitude, temperature, humidity, light), allowing it to maintain good condition. usually takes 3-12 days. It causes sweat to appear on the surface of the skin at lower body temperatures. As long as the sweating process continues, people can withstand remarkably high temperatures. Full (long-term) acclimatization to an unknown thermal environment can take several years. Unlike short-term adaptation, long-term adaptive changes are stable and remain for a long time. They lead to less sweating with lower salt intake, a slight increase in core temperature and less acceleration of the pulse under heat load.
• Physical fitness deteriorates with age as the average level of physical activity decreases. The cardiovascular systemcardiovascular system
  is a system through which blood moves in the heart and blood vessels. becomes more stressed and thus the body's thermoregulatory capacity decreases as the heat exchange between the internal organs and the skin decreases. In addition, by avoiding exposure to heat or cold, the human body gradually loses its ability to acclimatize to temperature changes.
• Being overweight is another factor that increases the risk of heat-related health problems. Overweight, in order to dissipate heat, more blood must be directed to the subcutaneous vessels, which is the cause of increased heart rate and higher susceptibility to heat stress or heat stroke.
• Increased sweating can lead to dehydrationdehydration
  is the process of reducing the amount of water. Dehydration of the human body is accompanying by disruption of metabolic processes.. A healthy and acclimatized person can excrete up to 3 liters of sweat per hour, while this amount under normal conditions is about 1 liter per hour. The recommended daily fluid intake should not be less than 1600 ml per day to ensure adequate hydration. Adequate fluid intake during hot flashes is essential. Dehydration appears to be a critical factor contributing to thermal overheating mortality, especially in the weaker and older  people. Studies show that frail older people are becoming increasingly vulnerable to minor environmental or physiological stressors that can accelerate dehydration and subsequent acute health problems. Alcohol should not be used in hot weather, as it depresses the central nervous system and through increased {#}diuresis{& gl-07) can further stimulate dehydration. When heat stress levels are low, there is little chance of hyper hydration (excessive fluid intake). This usually happens to young, healthy people participating in sporting events of long duration.

The table shows the degree of dehydration and the corresponding symptoms associated with the deterioration of human health.

Who is most vulnerable to heat – Vulnerability to heat depends on climatic factors (such as the frequency of heat waves) and individual risk factors, which include:

•  old age, impaired consciousness (such as dementiadementia
  also called "old age disease", is a neurological disease characterized by deterioration of mental abilities, impaired concentration and others), existing diseases, use of certain medications, level of hydration, solitude, constant use of air conditioning in the home or workplace, housing in a particular type of building or on a higher floor, location of the high-rise apartment in urban heat island.
• Studies show that the risk does not differ significantly between men and women.
• Children and infants are at increased risk of heat-related death. Some heat deaths among children are caused by being left in cars on hot days.
• People with lower social status may be more vulnerable due to poorer housing quality and lack of air conditioning.
• Physical and social isolation of older people further increases their vulnerability during a heat wave.

Given the vulnerability to heat in adults, it is important to note that life expectancy and aging in Europe are increasing.