by Dr Jane Canestra, MBBS, MPH, FACEM, MRSV

Jane is a retired emergency physician with expertise in public health and emergency management. This article is based on a series of talks she gave to local government, emergency services, and hospital and health professionals over a number of years as part of the Department of Health Summer Preparedness series.

Heat is a major killer. Over 70,000 excess deaths occurred in Europe in the 2003 heatwave¹. This year, more than 2,800 excess deaths of people aged 65 and over occurred in England during the long summer heatwave². In 2009, 374 excess deaths were recorded in Victoria from a briefer heatwave³. More heatwaves, more intense, and of longer duration, are expected as the planet warms.

Here is an explanation of how our bodies respond to extreme heat, why we become ill, and how we can protect ourselves.

Achieving Balance

Like a high-performance vehicle, the human body is very finely calibrated for peak efficiency. The brain regulates body temperature, keeping it within a very narrow range for optimum functioning.

A balance must be struck between heat generating activities (such as exercise, metabolism, and hot food and drink ingested), heat loss (through sharing heat through closeness or touch, breathing, sweating, and other excretions) and the environment.

When the air around us is a comfortable 21°C, 70% of body heat is lost through radiation (think of your body as a radiator), 27% through evaporation of sweat (even though we are not aware of perspiring), and a little through breathing, urine and faeces. As the air temperature increases, we are less able to shed heat to the environment through radiation, and sweating increases as a proportion of heat loss from the body.

In a hot environment, the skin is flushed as the brain redirects blood flow to maximise heat loss via radiation. Sweating increases to maximise the effectiveness of evaporation, although dripping sweat is not very effective at cooling the body as it has little chance to evaporate. We also breathe faster. Dogs don’t sweat – it’s why they pant in hot weather.

On the other side of the balance, appetite is reduced, minimising the desire for hot foods. Apathy and inertia are the brain’s signals to avoid heat generation from undue exertion in hot weather.

The change in blood flow to the skin can increase its share of the output of the heart from almost 0% to around 30%, which is better tolerated in fit, healthy bodies. Heat is conducted from the skin to the air trapped against the skin by clothing, then to the inner and finally outer aspects of the clothes to the exterior. As the rate of air exchange is affected by the thickness and texture of fabrics worn, wearing loose light clothing is an important adaptation to hot weather.

Unfortunately, fat is an excellent insulator, reducing heat loss via the skin by up to two thirds compared to other types of tissue.

When the air temperature is greater than the temperature of the skin surface, (which typically ranges from 29 to 35°C, depending on the body part) the only means by which the body can lose heat is through evaporation of sweat. At rest in lower air temperatures, around 50 mL of sweat evaporates from skin every hour. We are not aware of this insensible sweating. If we are fully adapted to hot conditions or acclimatised, it is possible to sweat as much as 1.6 L per hour – up to a maximum of 15 L per day! High humidity reduces the rate of evaporation of sweat, and therefore its effectiveness in cooling the body.

The rate of heat loss by convection, typically through movement of air away from the body, is increased by the presence of breezes or fans. A breeze shifts warmer air away from the skin, replacing it with cooler air. This is less effective when the air temperature is greater than the temperature of the skin, and may even be uncomfortable. Fortunately, water is even more effective than air in conveying heat away from the body. Hence, the relief we feel when we take a cool shower, or bathe in a pool or the sea.

Exercise can increase the body’s heat production up to 10-fold. This creates competing demands on the circulation, and particularly the heart. Blood flow to exercising muscles is several times greater than blood flow to the skin. Additionally, there is considerable loss of fluids and electrolytes through increased sweating.

Heat that is not shed continues to raise the body’s core temperature until heat losses match the heat generated from exercise and the environment. If balance cannot be restored, illness will result.

Acclimatisation

Acclimatisation increases our tolerance for hot weather by adapting our ability to maintain the balance of our body’s heat exchange. It occurs following around 2 hours of exercise per day in hot weather for 1 to 6 weeks. The body adapts by doubling the maximal rate of sweat loss, reducing the salt content in the sweat by up to 10-fold, and the kidneys work to retain sodium which helps retain fluids and minimise dehydration.

Acclimatisation occurs more rapidly if you are fit. However, the benefits of acclimatisation are undone by:

• sleep deprivation
• excess alcohol
• dehydration
• salt depletion, and
• infection

Dehydration

Exercise in a warm to hot environment often leads to dehydration of 2% to 6 %, and fluid intake is frequently inadequate to replace these losses. Thirst is not sensed by the body until we are more than 2% dehydrated. Even drinking as much as you feel like mostly does not match fluid losses. More than 2% dehydration leads to impaired thinking, increased risk of injury, and increased susceptibility to heat-related illness. More than 3% dehydration causes greater stress on bodily systems.

The consequences of dehydration are complex:

• reduced blood volume reduces the amount of fluid available for sweating,
• lowered blood pressure reduces the blood flow to vital organs,
• increased heart rate, which may not be tolerated by those who are older, or with pre-existing heart disease, or on certain medications,
• increased blood viscosity (thick and sticky) and likelihood of blood clots, increasing the probability of heart attacks and strokes,
• as the kidneys work to retain fluid, the urine becomes more concentrated, with increased risk of kidney stones.

Heat-related Illness

Even with maximal sweating, the body’s capacity to lose heat may be overcome, causing a rise in core temperature. Excessive heating of the brain reduces its ability to regulate body heat and causes sweating to decrease. A set of overlapping conditions are possible. In escalating severity, these are: heat cramps, heat exhaustion, and heat stroke.

Heat cramps are associated with strenuous exercise. Loss of sodium in sweat may lower the sodium level in blood, interfering with muscle relaxation. Lactic acid build-up from the exercise, can result in hyperventilation, aggravating the cramps. The cramps usually settle with rest and electrolyte replacement.

Heat exhaustion results from dehydration and electrolyte loss due to sweating, and inadequate replacement. As a result, there may be fatigue, light-headedness, nausea, vomiting, severe headache, low blood pressure and increased heart and breathing rate. Core body temperature is not raised or minimally raised, and sweating is usually profuse. Fluid replacement is an important part of treatment.

Heat stroke occurs when the core temperature is raised. This is associated with severe symptoms:

• loss of consciousness
• irritability, confusion, bizarre behaviour
• fitting, signs of stroke
• widespread organ damage

Heat stroke victims may no longer be actively sweating, and may appear paradoxically pale. Death occurs in up to 80% of affected people: this is a medical emergency. First aid measures include removing the victim from the heat, removing excess clothing, sponging and fanning, and, if available, applying ice packs to the armpits and groin.

Other heat-related illnesses include:

• impaired judgement and problem-solving, and confusion
• impaired heart function
• kidney stones
• aggravation of asthma and other respiratory disease by increased pollution from temperature inversions associated with protracted hot weather, and associated bushfire smoke
• falls
• gastroenteritis (generally due to food spoiling more quickly outside of refrigeration)

The rates of severe illness increase with each passing day of heatwaves, reflected in increased ambulance callouts, hospital attendances and deaths. Deaths are rarely due to heat stroke. In studies of large heatwave events, numerous deaths occur and mortality increases in the following disease categories:

• cardiovascular disease,
• asthma and respiratory disease,
• kidney disease,
• diabetes,
• neurological disease, and:

Predisposition to Heat-related Illness

There are numerous conditions that predispose someone to a heat-related illness:

• being very young or very old
• disregarding public health messages about avoiding strenuous exercise in extreme heat
• lack of acclimatisation
• lack of aerobic fitness
• impaired ability to sweat due to:
  • dehydration
  • skin disorders, especially sunburn and prickly heat, but any extensive dermatological condition
  • congenital conditions, including cystic fibrosis
  • neurological conditions
  • pre-existing heart disease
• certain medications
• infections
• obesity
• immobility
• heavy alcohol use.

Avoiding Heat-related Illness

Long-term strategies to minimise the risk of heat-related illness include improving home insulation, ventilation and shading. It is very important to keep direct sunlight off windows to reduce the impact of the sun’s radiant energy entering your home, through plantings or exterior coverings, even low-tech solutions like shade cloth strung outside your window.

Aspirational strategies include losing weight, getting fit and acclimatising. Remember, acclimatisation takes weeks, so start slowly in moderately hot weather. It may help with the first two recommendations! Also talk to your doctor about severe heat and your health and medications for advice specific to you.

During extreme heat, try to ensure you are fully hydrated, but minimise your alcohol intake! Salt supplements are generally unnecessary, as the average Australian diet has excessive amounts of salt. Wear loose, light clothing and stay in cool, shaded, well-ventilated spaces. Avoid unnecessary outdoor exercise, sunburn and prickly heat. And get some relief with cool sponging or bathing. Look after yourself!

References:

1. Robine, J. M., et al. (2008). Death toll exceeded 70,000 in Europe during the summer of 2003. Comptes rendus biologies, 331(2), 171–178.
2. UK Health Security Agency (UKHSA), Press Release 7/10/2022
3. DHS (2009) January 2009 Heatwave in Victoria: an Assessment of Health Impacts