Previous articles have looked at why Hong Kong’s urban environment is especially hot and how we quantify and measure such heat stress, but why is heat actually bad for us? What makes it dangerous? And how does the human body interact with hot environments? This article will help answer some of those questions.

 

Human bodies are kept alive by a complex series of chemical reactions. Collectively, the chemical reactions that serve to sustain life can be referred to as metabolism. Metabolism is responsible for a suite of incredibly important tasks, including the liberation of energy from food and conversion into energy that can be used to fuel cellular processes; the conversion of food into components of proteins, lipids, nucleic acids, and carbohydrates; and the elimination of waste products. Failure to adequately perform any of these can result in serious complications and even death.

 

The set of conditions most conducive to effective metabolism is called homeostasis. Homeostasis factors in body temperature, chemical balances, fluid levels, and countless other variables that the body constantly works to maintain within an acceptable range. There are systems in place that allow the body to withstand a wide range of external conditions while maintaining this internal balance. 

 

Problems arise, however, when these systems are pushed to their limits. The optimal temperature for the human body is 37℃. This is where the reactions that make up our metabolism occur most efficiently and everything in our body stays balanced. When temperatures rise higher, whether that be due to environmental factors or increased exertion, the body makes use of a number of thermal regulation systems. There are three ways in which the body can reduce its temperature. The first two, conduction and radiation, happen spontaneously, though some mechanisms such as laying body hairs flat against the skin or dilating blood vessels to allow more blood to the surface of the skin can help increase their cooling effect. Conduction and radiation, however, only function when the temperature of the skin is higher than that of the surroundings. If the opposite is true, they will actually serve to heat your body up further.

 

This brings us to the body’s most effective technique for thermoregulation: sweating. The body has the ability to secrete a liquid mixture of water and dissolved ions through glands near the surface of the skin. This allows the body to reduce its temperature through evaporative cooling, a detailed explanation of which can be found in the previous article about wet bulb temperature. Sweating works extremely well in hot and dry climates, where liquids can easily evaporate, but becomes much less effective in hot and humid environments, such as Hong Kong. In order to benefit from the same degree of cooling in a humid environment, a body has to secrete a much greater volume of sweat than if they were in a dry environment. This drastically increases the risk of dehydration, which poses a host of its own issues beyond the scope of this article.

But what happens when the body’s cooling mechanisms aren’t sufficient? If the body is exposed to excessive heat and humidity to the point that the heat-regulating mechanisms are overwhelmed, the condition is referred to as hyperthermia. When this raises the body’s temperature to above 40℃, it becomes known as heat stroke. In heat stroke, the body may begin to stop functioning properly, leading to inflammatory and coagulation responses that damage blood vessels, as well as disruptions to the intestinal barrier which can lead to sepsis, multi-organ failure and dysfunction of the central nervous system. Aside from heat stroke, the body may also experience other heat-related illnesses, such as heat exhaustion from lack of bodily fluids, fainting, cramps, seizures, and the break-down of muscle tissue which can clog the kidneys and cause extensive damage to the point of kidney failure.

 

In summary, the human body, despite its reliance on finely tuned chemical reactions and delicate internal parameters, has developed a host of mechanisms with which it can maintain these conditions reliably. These systems are not, however, fool proof and can be overwhelmed. When they are, the consequences are often fast onset and disastrous in their proportions. This far removed from homeostatic conditions, the body quickly shuts down without being able to rely on its internal mechanisms for help, and death is far too common of a consequence.