25 Hot, dry environments: deserts

Two of the most popular expedition destinations, deserts and tropical forests, seem very different. Deserts appear barren, arid, and have a very limited range of highly adapted plants and animals, while in contrast the hot and humid tropical forests (Chapter 26) offer the world’s richest land ecosystem.

Both environments require humans to live and work in high temperatures, but the high humidity in forests places additional stresses on body physiology. Those living in temperate climates such as Northern Europe rarely experience heat waves, and may be physically and mentally unprepared for heat stress. Heat-related illness is a cause of death amongst travellers including previously fit young adults. Endurance athletes and the military are especially at risk as they deliberately push their bodies even when conditions are hazardous. An occasional short spell of heat in an otherwise temperate zone is especially hazardous to endurance competitors as they will have little physiological acclimatization to the conditions.

A desert is a region with little vegetation and much exposed bare soil, where average annual rainfall is <20% of the amount needed to support optimum plant growth, and where plants and animals show clear adaptations for survival during long droughts. Covering almost 20% of the Earth’s landmass, deserts are mainly found between 25° and 35° north and south of the equator, and are home to around 1 billion people.

Some deserts consist of the sand dunes of popular imagination, but others are rocky wastelands or semi-arid grasslands. Diurnal and seasonal temperature ranges can be considerable, with the landscape sculpted by freeze/thaw cycles and by powerful winds driving sand, soil or snow.¹

Wear light, loose-fitting clothes made of natural materials that allow air to circulate and sweat to evaporate. Protect the head with a hat, scarf, or keffiyeh (shemagh). Shorts and T-shirts are also convenient and usually perfectly adequate, but be conscious that such dress—particularly if worn by women—may cause offence in some countries. Where possible check the ultraviolet protection factor (UVF) offered by clothing—some materials are more effective at preventing sunburn than others. Exposed skin must be properly protected by sunblock while both sunglasses and goggles are essential.

Footwear needs to be light, comfortable, and tough; boots, shoes, and trainers all have disadvantages. Enclosed feet become sweaty, smelly, soft, and prone to fungal infections. On the other hand, bare feet or light shoes expose the feet to heat from the ground, injury by rock or thorns, and bites from snakes or scorpions. In sand and gravel deserts, walking sandals, trainers, or desert boots suffice; heavier footwear is required in stony and volcanic areas.

It is possible to travel for months in deserts without the need for formal shelter. A camp bed off the ground is protection against snakes and scorpions (but remember to shake your shoes out in the morning). A tent or impregnated mosquito net will protect against insects. Guard the area against human or large animal invasion. Beware of making camps in dry riverbeds or wadis, which can be susceptible to flash flooding from rainfall miles away.

Most desert expeditions use vehicles. These must be well maintained and have adequate tyres, tool kits, and spares to cope with the stresses of travel. Fuel consumption will be high. Ensure the party knows how to extricate a bogged vehicle, and do not lose the vehicle keys.

Travellers relying on more traditional travel methods require suitable skills in animal husbandry to keep pack animals healthy, content, and tethered at night.

Journey times are often very long. Maps may be unreliable and roads little more than parallel tracks on the ground, invisible if the wind is blowing. GPS devices are invaluable, but ensure suitable backup.

Considerable reserves of water, fuel, and food are essential to ensure safety in the event of bad weather, mechanical failure, or navigational problems.

Dust storms can develop with little warning in any arid or semi-arid environment. These take the form of an advancing wall of dust and debris that may be miles long and several thousand feet high; they can appear from any direction though generally follow the prevailing winds. Most storms pass within an hour, but some persist for several hours. High winds can destroy tents and strip campsites bare. Health risks include suffocation and silicosis from dust inhalation, and extremely low visibility both on roads and in the air leading to disorientation and the possibility of serious accident.

Snakes and scorpions (see Chapter 17) live in deserts and may enter discarded footwear or containers. Plants in arid areas can have thorns, tough spiny surfaces, or serrated leaves.

In hot weather a major environmental risk is that of developing some form of HRI. Humans originated in tropical regions and most people can adapt well to heat, but only after a period of acclimatization. Individuals vary considerably in their tolerance to heat stress, and the underlying mechanisms are not fully understood. Many people successfully complete endurance races such as the Marathon des Sables and Amazon Marathon in hot environments without incident, yet others tragically die whilst exercising on a hot day in temperate latitudes. Ultimately, if the duration and intensity is sufficiently challenging, everyone is vulnerable. Prolonged heat adds to the physiological stress and the risk of serious HRI is greatest following several days of exposure to hot and humid conditions.

Classical heatstroke affects humans trapped in hot, unventilated environments, e.g. workers in mines, prisoners, stowaways in freight containers, or children left in cars during heat waves. Individuals with impaired thermoregulation such as infants, the elderly, people with underlying medical conditions, or those taking drugs known to interfere with thermoregulation are at greater risk (see Box 25.3). Increasing summer temperatures in normally temperate areas (exacerbated by urban environments where buildings can store heat and so raise night-time temperatures by 5°C) are producing urban heat waves and deaths amongst the frail and elderly. The 2003 European heat wave is thought to have been responsible for an extra 70,000 deaths, mainly in France. In the UK, the annual average mortality from classical heatstroke is thought to be about 40 cases per year.

The constant heat of jungles and urban environments constitutes a greater physiological stress than that found in deserts, where the nights are often cool.

Exertional heat stroke (EHS) occurs as a result of physical exercise, and cases may develop even in otherwise temperate conditions if someone becomes dehydrated or overexerts themselves, sometimes in inappropriate clothing. It typically occurs during military training or during endurance sporting events. EHS is much more likely on expeditions than classical heatstroke.

Hyperthermia is also associated with stimulant recreational drugs such as cocaine, ecstasy, and amphetamines (Box 25.3).

Heat exhaustion is the most commonly encountered form of HRI. It occurs when the cardiac output is insufficient to meet the demands of increased blood flow to the skin, working muscles, and vital organs. The effects are compounded by a decreased effective plasma volume (redistribution of blood), dehydration, and salt loss due to sweating. Heat exhaustion does not result in any organ damage and some individuals may be fit to resume normal activities after 24–48 h.

EHS is defined as a core body temperature of >40°C caused by strenuous exercise and/or environmental heat exposure which is associated with central nervous system dysfunction and multiple-system organ failure (usually cardiovascular collapse). Heat shock proteins and cytokines contribute to a systemic inflammatory response syndrome (SIRS) similar to that seen in other critical illnesses.

EHS is most often associated with pale, sweaty skin, as opposed to the hot, dry, flushed skin of classical heat stroke. As tissue temperatures rise, cell membranes and enzyme-dependent energy systems are disrupted leading to variable cell and organ dysfunction and death. The extent of injury relates both to the duration of exposure and the severity of the rise in core temperature, but the seriousness of the condition cannot be predicted from these two parameters alone, and requires laboratory confirmation.

Unfortunately the symptoms of EHS are non-specific. Anyone looking unwell, or behaving abnormally in a hot and/or humid environment, or during vigorous exercise in a temperate environment, should be managed as a victim of HRI until proven otherwise. Typically sufferers from EHS will have more than 2/3 of the symptoms (see Box 25.1), while someone who has a febrile illness instead of a HRI will show 1/3 or fewer. The box may be used as a prompt and all symptoms and signs should be sought.

EHS is not always recognized and may be diagnosed as exercise-associated collapse. The incidence is unknown, but appears to be on the rise as more people take up long-distance and endurance sporting events. In the US military, the incidence rose eightfold between 1981 and 2001 to 14.5 cases per 100,000 soldiers. In Singapore’s hot, humid climate the incidence in the military may be as high as 350 cases per 100,000 soldiers.

Inability to reduce core temperature below 39°C with evaporative cooling suggests that a febrile co-morbid condition may be present.

Core temperature is the temperature of the vital organs such as brain, heart, liver, and kidneys. Normally it should remain relatively constant regardless of environmental conditions and at rest should be between 35.5°C and 37°C, with a measurement above 37.5°C being abnormal. However, during vigorous and prolonged exercise, such as long-distance running on a hot day, core temperature rises and measurements up to 41°C have been recorded without apparent harm to the subject. Temperatures above 41°C are almost always abnormal and harmful.

Surrounding the body core is a shell of tissues at a lower temperature, the size of which depends upon the balance between heat generation and heat loss. In hot conditions, metabolic heat generated in the core has to be transferred to the skin, a process that involves substantial increases in skin blood flow.

The most critical factor in predicting the severity of injury is the duration of heat exposure following collapse. An elevated core temperature of 42–43°C can be tolerated for short periods (5–10 min) with little damage. For example, exertional heat stroke during military training usually involves short exposures and rapid treatment; there may be large numbers of casualties, but the mortality rate is relatively low. During the papal visit to Denver, Colorado in 1993, there were 18,000 symptomatic victims among the crowd but no deaths, as the organizers ensured that immediate help was available. If body temperature remains persistently elevated, body metabolism becomes deranged and enzymes denature. The destructive processes are listed in Box 25.2.

Heat transfer and hence changes in body temperature take place as a result of radiation, conduction, convection, and evaporation.

The rate of heat transfer by conduction, convection, and radiation is dependent on the difference in temperature between the body surface and the materials or radiating surfaces in the environment. If the body surface is warmer than the environment, the body will lose energy to the environment. However, very warm air or surfaces will transfer heat to the body by conduction/convection, and sunlit surfaces or sky will transfer heat to the body by radiation.

80% of metabolic energy is produced as heat in muscles—by normal metabolism, during exercise, and through shivering—and is conducted to the skin, where it is lost to the environment. The circulation of the blood augments and regulates this heat transfer by varying superficial blood flow. Clothing further modifies heat loss by acting either as a conductor or an insulator. At rest at 20°C, conduction and convection account for only about 10% of our heat loss, the majority occurring by radiation.

Once environmental temperature rises above 35°C it is impossible to lose heat through conduction, convection, or radiation. Our ability to survive and function in higher temperatures depends upon the ability to sweat.

Sweating allows the body to lose heat at any environmental temperature through evaporation, but evaporative heat loss can only occur if the air is not saturated with water vapour. So, sweating is most efficient in hot dry deserts and is less effective in hot humid rain forests. Humidity has a greater effect on the ability to lose heat than the absolute temperature.² (See Table 25.1.)

Changes in temperature are detected by both sensory nerve endings in the skin and by direct sensing of blood temperature in the hypothalamus of the brain. At rest, the skin receives around 9% of the total circulating blood flow. A rise in core temperature of as little as 0.1°C will increase skin blood flow to dissipate the heat, and under high heat stress skin blood flow can increase fourfold. Heat energy is then lost directly to the environment by a combination of radiation, conduction, convection, and by evaporation of sweat. High environmental temperatures also lead to behavioural changes such as reduced activity, seeking shade, and drinking more.

During physical work blood flow is directed to the working muscles and away from the intestines. This limits the ability of the gut to absorb water to around 1200 mL/h. If the rate of fluid lost in sweat exceeds this amount, then dehydration will occur. Anyone working in these conditions must be allowed adequate rest periods with fluid replacement. If blood flow is further distributed to the skin to allow evaporative heat loss, then the effective circulating volume is further decreased. An adequate blood volume is therefore required to ensure that thermoregulatory blood flow can occur.

(See also Chapter 3.)

Measuring body temperature accurately is difficult and medics need to be aware of the limitations of the various techniques. In hospital the best ways to measure core temperature are by the use of central venous or oesophageal sensors but neither is practical on most expeditions.

Rectal temperature can be measured using simple portable equipment, but there may be poor correlation between the rectal temperature and the severity of symptoms; fatalities have been reported with rectal temperatures of 39.5°C, while victims have survived core temperatures of 47°C. During active cooling, there may be rapid changes in temperature of the blood as demonstrated by the sudden onset of shivering, but there is a significant lag before the body temperature change is seen rectally.³

A systemic review of febrile critically ill patients concluded that rectal temperatures were overestimated and inaccurate, but oral and tympanic temperatures more accurately reflected core (pulmonary artery catheter) temperature.⁴

Oral temperature measurement requires a conscious, cooperative patient and accurate placement of the bulb underneath the tongue for sufficient time to ensure thermal equilibrium. The patient should not breathe through the mouth during temperature measurement.

The tympanic membrane shares its blood supply with the hypothalamus (the body’s thermostat) and changes in body temperature are measureable sooner at the tympanic membrane than at other external sites. Ear temperature probably offers the best compromise between the lag associated with rectal temperature measurement and the impracticalities and inaccuracies of oral temperature measurement, particularly in a semi-conscious patient.

However, for ease of use, most infrared ear thermometers are calibrated to read the mid-canal temperature, rather than the temperature of the tympanic membrane and this region is more susceptible to error. Some thermometers (e.g. Braun Thermoscan^® models) correct for the site of measurement, but false readings may occur for a variety of reasons. Proper technique is important—a clean probe cover should be used each time, the ear must be free of wax and water, and the ear canal should be straightened (pull the ear up and back). If in doubt, treat the clinical signs rather than be reassured by a single measurement of temperature.

Full acclimatization to heat develops at different rates in different individuals; typically 10–14 days are required for all the physiological changes to develop. The rate of acclimatization depends upon factors, including body shape, the severity of the heat stress, and pre-existing physical fitness.

Physiological acclimatization enhances evaporative heat loss while reducing cardiovascular strain. Exercise becomes easier and exercise syncope, common on day 1, rapidly declines to zero by day 5. The increased sweat rate (0.5 L/h to 2 L/h), coupled with the increased blood flow, can increase heat loss by a factor of 20, but requires a significant increase in water consumption before, during, and after activity.

Some degree of acclimatization can be obtained in temperate climates before departure. Hot baths twice a day, saunas, and exercising while wearing more clothing than normal may be effective. In a hot dry climate, rapid acclimatization requires about 2 h of exercise per day sufficient to raise heart rate to around two-thirds of maximum, which should be conducted during the cooler hours of the morning or evening. Acclimatization may be delayed if substantial portions of the day are spent in air-conditioned environments. In comparison to dry heat, acclimatizing to hot humid climates, especially if the heat is unremitting, is much harder, and initial exercise tolerance will be substantially lower.

Sweating can only remove heat if there is sufficient fluid to spare. Sweat production rates can reach 2 L an hour for short periods and can be up to 15 L a day. In low-humidity environments such as deserts where evaporation is rapid, the daily cooling capacity of the sweating mechanism is adequate to maintain body temperature even during vigorous work, but in humid environments such as tropical forest, evaporation is ineffective and slow, so exercise must be limited to avoid overheating.

Sweat is a hypotonic (dilute) solution of sodium chloride. The concentration of sodium chloride in sweat depends on the sweat rate and the degree of acclimatization. Higher sweating rates reduce the opportunity to conserve salt, and the sweat salt concentration rises, but acclimatized sweat glands conserve salt more effectively by producing more hypotonic sweat. In addition to conserving salt in sweat, humans acclimatized to heat start sweating at lower body temperatures and their kidneys conserve salt more effectively. As a consequence, an acclimatized person in a hot environment requires no more salt than an unacclimatized individual in temperate conditions, and can maintain lower body temperatures for any degree of heat stress.

Jungle acclimatization readily transfers to desert climates (hot and dry) but the reverse journey requires further acclimatization to humidity. The benefits of acclimatization are lost over 20–40 days after returning to a temperate environment.

Heat stress is the product of the interaction between:

Assessment of risk must consider each of these factors.

An individual is best able to cope with heat stress when:

Dehydration reduces both blood flow and sweating, so that a dehydrated person has reduced ability to maintain a constant body temperature in the heat. Acclimatization and physical fitness enable high temperatures to be tolerated better, but do not reduce water requirements; indeed, a fit acclimatized person will usually drink more than a new arrival to a hot environment.

Thermoregulation can be impaired by:

People with any of these conditions should be watched closely for signs of heat distress and should avoid excessive exertion. If one member of a party develops symptoms of heat stress, then leaders and medics should assume that everyone else in that group who has been exposed to similar heat stress is a potential heat casualty.

A few individuals appear to have a genetic predisposition to developing a HRI. Previous HRI should alert one to a recurrence. However the data is variable and victims of previous exertional heat stroke may cope perfectly well with subsequent heat stress.

Four environmental characteristics influence perceived heat stress:

Environmental heat stress can vary greatly and unpredictably over short periods of time and space. On a calm, sunny day an open field may present a greater heat stress than an adjacent forest, but on a windy, cloudy day the forest may present the greater heat stress.

Three of these four factors are combined into an internationally accepted measure of heat stress, the WBGT index, developed by the US military in the 1950s.

Several manufacturers now produce relatively cheap devices capable of measuring and calculating WGBT; suppliers can be readily accessed on-line. In some states in the USA, measurement of WGBT is mandatory before sports are played in hot weather, with risk management procedures dependent upon the heat stress. Elsewhere organizers of sporting events and expedition leaders should consider planning activities around the value of WGBT. Further details of this and other temperature indices, including use of ambient temperature are available at:  http://www.bom.gov.au/info/thermal_stress

If you do not have access to equipment for measuring WBGT, consider evaluating heat stress using the workload calculations (see Table 25.2 and Workload calculations, p. 758).

The wet bulb temperature is the most important component of the WBGT index, which reflects the thermoregulatory importance of evaporation in hot (especially humid) environments, but the index does not include wind speed, another important environmental modifier, within its calculation. Air movement increases convective heat transfer and will assist evaporation; cool winds reduce heat stress, but hot winds increase it. The American College of Sports Medicine provides guidelines for exercise in hot environments and recommends cancelling sporting events if the WBGT is >28°C.

For expeditions lacking meteorological facilities, an alternative method of judging a safe workload pattern has been suggested:

Maintaining an appropriate fluid balance during the transition from a cool to a hot climate, or during endurance sporting events is difficult. Inadequate fluid intake, excessive ingestion of water, and inadequate or inappropriate use of electrolyte supplements can all lead to serious health issues. Fluid and electrolyte requirements will change as the body becomes acclimatized to heat. Each individual will have different needs at different times and this makes it hard to offer firm advice on requirements.

A reduction in total body water (TBW) of 1% affects thermoregulation and losses of 2% significantly impair physical and mental performance. Thirst is a poor stimulus to drink and TBW losses of 5–10% have been tolerated in experiments. Fluid must be consumed before, during and after physical activity to maintain normal (eu-) hydration. This is most easily assessed by measuring the specific gravity of the first urine of the day (a specific gravity of ≤1.020 can be considered as euhydrated), along with changes in daily body weight performed at the same time (and before and after activity). Weight loss is common on expeditions and caused not only by dehydration, but also by increased work-load, GI upset, and decreased appetite due to heat and unfamiliar food.

Even when a person is significantly dehydrated, urine is still produced and the volume of fluid required to return to full hydration must be at least 1.5 times that lost in sweat (assuming the individual was fully hydrated before the onset of activity). Women have a lower proportion of water in their bodies and may be at greater risk of dehydration than men.

If an individual drinks only enough to satisfy their thirst they may become chronically dehydrated, particularly if they drink substantial amounts of caffeine-containing drinks, which act as diuretics. It is essential that personnel working hard in any environment are made aware of the need to drink water despite not feeling thirsty. Expedition leaders must enforce work/rest cycles, and provide adequate shade. If toilet facilities are unpleasant or lack privacy, travellers may seek to avoid visits by drinking less. Clean and screened facilities will encourage proper drinking habits—especially if the party consists of easily embarrassed youngsters.

Hydration can be monitored by the colour and quantity of urine along with how often one needs to pass urine. Dark yellow urine is a sure indicator that the individual is dehydrated, as is the need to urinate less than twice a day. Medical officers should check lying and standing BP; a difference of >15 mmHg in the systolic pressure suggests dehydration.

Diabetics need to maintain good glucose control as blood glucose >10 mmol/L will result in glucose in the urine and consequent osmotic diuresis producing lighter urine. This could be mistaken as indicating adequate hydration whereas in reality it would be masking, and at the same time, worsening dehydration.

In hot environments, water losses can reach 15 L per day per person. Complete replacement requires realistic estimates of potable water requirements, adequate water logistics, and individuals who understand and act on their water requirement. Water for hygiene will be needed in addition to water for drinking.

Where water supplies are unsafe, expedition leaders must ensure that adequate provision exists to purify sufficient water for the group’s requirements. This may be flavoured to increase palatability. If chlorine or iodine is used, there should be a method of removing the taste at the point of use (see Water purification, p. 102). Bottled water supplies purchased in local markets may be contaminated, discarded bottles having been recycled by being refilled from the nearest water source. Carbonated water may be preferable as it is harder to tamper with. Carbonated water and soft drinks will fill the stomach with carbon dioxide before sufficient water has been ingested to combat dehydration, and should not be relied upon as the only source of fluid.

New arrivals in a hot climate will lose more salt in their sweat than normal and should supplement their salt intake until they become acclimatized. Salt tablets are best avoided as they contain an unknown amount of sodium and may irritate the stomach. Table salt should be readily available at meal times. Salt can be added to fluids in sensible amounts. Soups are an excellent source of both fluid and electrolytes.

The oral rehydration solution recommended by the WHO has a sodium content of 60–90 mmol/L, but the high sodium content of this significantly reduces palatability, resulting in reduced consumption. Whilst life-saving for diarrhoeal illnesses, its use cannot be recommended for fluid replacement in healthy people operating in the heat.

Sports drinks manufacturers have heavily promoted their products as the ideal way for active adults to replace the water and salt lost in sweat. Their value is controversial, with some athletes believing that they increase endurance and reduce the risk of heat cramps, whilst others doubt their value. Some sports drinks are sold as powders. Dissolving excessive amounts of powder in the hope of increasing absorbed energy produces hypertonic fluids that do not quench thirst and enhance the effects of dehydration. Always mix such powders according to instructions.

A 2012 BMJ review concluded that water remains the best replacement fluid, but that over-hydration is a bigger risk than dehydration during running events up to marathon length.⁵ However this advice may not apply when prolonged heat exposure leads to substantial additional salt losses.

In the absence of serious HRI or renal failure, dehydration by itself does not cause unconsciousness. Competitors participating in endurance races can develop symptomatic hyponatraemia if they drink excessive amounts of plain water or hypotonic fluids. Stomach bloating, weakness, and collapse may be followed by unconsciousness.

The use of desmopressin (DDAVP^®) for the treatment of nocturnal enuresis has been linked to the death of a young traveller on an expedition (see Nocturnal enuresis, p. 525).

Without laboratory facilities it will probably be impossible to distinguish between collapse from symptomatic hyponatraemia and collapse as a result of a heat related illness. Both are life-threatening conditions and require urgent medical support.

Treatment should focus on returning the victim’s body temperature to the normal range as rapidly as possible in the prevailing situation (see Table 25.3).

Remove the casualty from the source of heat and place them in the shade. Lying down maximizes heat loss, but only if the ground or mattress is no warmer than the surrounding environment. A string hammock is ideal for encouraging heat loss as it enables air to circulate over the whole body.

The most rapid cooling (with lowest morbidity and mortality) is achieved by immersing a casualty in a bath or pool of iced-water. A children’s paddling pool may be a suitable piece of first-aid equipment for expeditions or event organizers to consider. Aggressive cooling using ice-water soaked towels in combination with ice packs to the head, neck, axillae and groin also achieves reasonable cooling. Evaporative cooling using wet towels and fanning is less effective, especially in humid conditions, but is probably the mainstay of treatment on expeditions. The casualty should be continuously sprayed with cold water and fanned to encourage evaporation. A wet sheet may be wrapped around the casualty instead and kept constantly moist.

In some countries a simple solution to heat exhaustion is to lie the victim in a tepid running stream, but beware that they could become unconscious, that the stream could be polluted, or that aggressive animals may be encountered.

Oral or IV fluids may be given, the latter being more effective in serious cases. At the Hajj pilgrimage, cold IV infusions of up to 1 L of normal saline or dextrose saline at 5°C for heatstroke and 12°C for heat exhaustion have been used successfully. Frequently casualties also suffer from hypoglycaemia, and glucose should be administered orally or IV to all casualties. No more than 2 L of IV fluids are normally required.

A heat-injured casualty who has not been cooled and yet is shivering is seriously ill. They may complain bitterly of feeling cold. They will not feel hot or thirsty. They will look pale and have cold skin. They will want to be wrapped in warm clothing, which only increases their core temperature further, as does shivering. They must have their core temperature measured to exclude HRI or a febrile illness such as malaria.

During cooling, the return to a normal temperature is often associated with shivering. It is important to continue to monitor core temperature, as the casualty’s thermoregulatory capacity has been damaged and these individuals are at continued risk of either hyperthermia or hypothermia.

Some of the effects of heatstroke, e.g. renal or hepatic failure, only develop after 24–72 h. As it is impossible to distinguish accurately between heat exhaustion and heatstroke, all casualties should be evacuated to a hospital with intensive care facilities.

Fainting on standing in the heat is thought to occur because of blood pooling in the legs and increased blood flow to the skin. When standing, the blood supply to the brain is temporarily interrupted, causing loss of consciousness. Although most cases of heat syncope are harmless, the potential for HRI should be considered, especially following physical work in the heat, or after the acclimatization period. Treat with rest in the cool and oral fluids.

Mild swelling of the limbs may be experienced during the first few days of exposure to heat, during the time when the plasma volume increases to allow for the increased blood flow to the skin. Cutaneous vasodilatation and pooling of increased interstitial fluid in dependent extremities results in swelling of the hands and feet. It is self-limiting, resolving in a few days.

Heat cramps are painful skeletal muscle spasms following prolonged exercise, often in the heat. They usually occur in the arms, legs, or abdomen from prolonged exercise and are thought to be due to dilutional hyponatremia, but also occur in cool conditions, e.g. swimming. Treatment is rest, prolonged stretches of affected muscle groups, and oral sodium replacement. If the individual is otherwise well, there is no association with HRI, but a raised core temperature should be treated promptly.

Miliaria rubra is an inflammatory skin eruption, which appears in actively sweating skin in humid conditions (or skin covered by clothing in dry environments). Each lesion represents a blocked sweat gland, which cannot function efficiently. The risk of HRI is increased in proportion to the amount of skin surface involved. Sleeplessness due to itching and secondary infection of occluded glands may further affect thermoregulation. Miliaria is treated by cooling and drying affected skin, avoiding sweating, controlling infection, and relieving itching. Sweat gland function recovers with replacement of the damaged skin, which takes 7–10 days.

Prevent if practicable by wearing loose, airy cotton clothing and taking regular cool showers. Treatment consists of frequent bathing in cool water, gently dabbing dry to prevent further damage, and application of talcum powder or calamine lotion. Air conditioning can help if available. Sedative antihistamines such as chlorphenamine (Piriton^®) may help to relieve symptoms and promote sleep at night, but sedative drugs should be avoided during the daytime as they may increase risk of accidents with machetes, etc.

NB Many general travellers who claim to have had ‘prickly heat’ may actually be describing polymorphic light eruption.

Sunburn reduces the thermoregulatory capacity of skin and also affects central thermoregulation; prevent by insisting on the use of adequate sun protection. Sunburnt individuals should be protected from significant heat stress until the burn has healed (see also Solar skin damage, p. 266).

Wound and skin infections are common in hot conditions and are covered in Chapter 9.