Heat Effects

Human Health Effects of Heat

The sun’s heat radiation is the basis for life on this planet. Without it, none would exist. Radiant heat from the sun, either direct or reflected, warms your body. To a degree, your body generates some of its own heat – through its metabolism. Last night’s steak is still ‘warming’ your body and just reading this paragraph creates heat in your brain. The muscles create heat, even while sitting still, but exercise or hard work generates up to twenty times more. As with everything else though, you can have too much of a good thing.

When the temperature rises too much, discomfort and illness develop. Heat-related illnesses range from minor disorders, like swollen legs or heat rash, to the more serious and dangerous conditions of dehydration and heatstroke. The elderly, the very young and the sick are at greatest risk. Although it could be prevented, many thousands of people die each year as a direct or indirect result of heat-related illnesses.

What is Heat?

‘The children are full of energy today.’ That’s what you say whenever they are madly running around. Similarly, the molecules and atoms of a hot substance hop and dance or vibrate tirelessly. This activity is energy and heat is a form of energy. Not only do hot substances contain heat, even the tiny particles in an ice cube similarly jig up and down, although their activity level is very low. The cube does not contain much energy (heat) – it is said to be cold.

Heat energy doesn’t like to be confined. Like water it flows from a source with abundance to one with less – heat is transferred. This can happen in four ways: radiation, conduction, convection and evaporation.

Radiation is the process of heat transfer without going through a carrier substance. The sun transmits heat in such a way, through the emptiness of outer space. Stand in the sun and you can feel the radiation on your skin. Of course, this works the other way as well. Your body constantly radiates heat to the surroundings and thereby loses heat.
Conduction is heat transfer by contact. Touch a hot oven and an immense amount of heat energy transfers into your palm. If you don’t want to admit your foolishness just say, ‘I was part of the conduction heat transfer between an oven’s hot plate and the palm of my hand’, which sounds so much better than, ‘I damned well burnt my hand!’ But by the same principle, an ice cube in your hand feels cold because your body transfers heat energy to the cube.

When the sun heats the roofs of city buildings, the air above them also becomes hot. It is then lighter than the surrounding cooler air, so it rises. This rising air carries heat energy into the atmosphere. The process of moving heat within a gas or liquid is called convection.

Heat energy is required to evaporate water. To boil water for a cup of coffee requires a significant amount of energy. Even more energy is needed to turn some of the water into gas (vapour). This extra energy to create the vapour isn’t lost, however, it stores in the vapour, it is latent. When the vapour condenses, i.e. the water gas turns into water droplets, the energy originally needed to form the gas, is released again in the form of heat. Clouds, in particular thunderstorm clouds, contain an immense amount of heat energy. In other words, the expensive energy you bought from the power or gas company is wasted to evaporate some of the water in your kettle. The vapour will travel across your backyard fence until it reaches a suitable place where it can condense again. There it releases the same amount of energy that you put in. Not fair, is it?

What is Temperature

Temperature measures the flow of heat energy from one substance to another. For example, if you hold a thermometer in warm water, heat energy flows from the water to the cool thermometer. If we use a mercury-filled thermometer, the increasingly active mercury atoms need more room to move, and make the liquid mercury expand. Once the mercury atoms reaches the same level of activity as those of the warm water, the expansion stops. And we notice the mercury settle next to a  temperature reading that corresponds with that of the water.

Different types of temperature scales are in use. Worldwide the most popular is the Celsius scale, where 0°C is the freezing point of water and 100°C the boiling point. The Fahrenheit scale, still used in the US, shows 32°F as the freezing point and 212°F as the boiling point. Another scale worth mentioning is the Kelvin scale, also known as the Absolute scale. Zero degree Kelvin is equal to the lowest possible temperature, which is -273.15°C. At this temperature, no molecular or atomic activity takes place, so there is no heat.

To get a fair comparison between the temperatures of two different locations, meteorologists standardize where and when they take their measurements. It isn’t reliable to take one measurement at high noon, in full sun and near a brick wall and compare it with another taken in the morning shade under a tree. To be able to compare temperatures worldwide, meteorological offices all measure temperatures at the same time of the day and in the shade of a purpose-built instrument shelter.

Human Body Temperature

Humans survive in the extreme cold of the polar region and the hot and humid conditions of the equatorial tropics. Despite the climatic variety of the habitats we occupy, our core body temperature remains at approximately 36.90C.

Even someone acclimatized to the polar regions has an average core body temperature only 0.2°C lower than a person living in the tropics.

Our insides and our brain don’t like to depart much from the average, either. We can feel stressed as soon as the temperature rises or falls by more than one degree, and feel the need to add or remove clothing, seek shelter, or switch on a heater or cooler. Meanwhile, the body’s own temperature control mechanism goes into action.


A tiny gland that weighs less than 4g drives your sex life. It may seem disappointing, but it’s true. The hypothalamus, located in the centre of the brain at the top of the brain stem, also controls your behaviour, the metabolic process, emotions, the involuntary (autonomic) nervous system and the systems that regulate your body temperature.

The hypothalamus responds to any rise or drop in body temperature by releasing a particular hormone. This contains instructions for the body to sweat or shiver, contract or expand blood vessels, and increase or decrease the heartbeat and breathing rate. The hypothalamus detects thermal comfort or discomfort and tells you that it is time to escape from the heat or button up against the cold.

As with all of our other body parts, this gland is not perfect – an accident, stroke, disease or tumour can impair its function. When we are born, the hypothalamus isn’t fully developed and its functions degenerate with age, so that the very young, the elderly and people with a disease affecting the hypothalamus are particularly vulnerable to temperature extremes.

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