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Heat and dehydration in travellers

Carolyn Driver
RGN RM RHV FPCert MSc(TravelMed)
Independent Travel Health Specialist
Cheshire
E:thecompany@driverc.freeserve. co.uk

Sadly, from time to time tourists die as a result of the effects of either heat-related illness or from dehydration as a result of infection. Just last year a British tourist in Australia is thought to have died from heat exhaustion when she decided to walk a considerable distance in the desert.(1) This was not an isolated incident as a case series of deaths due to high environmental temperatures revealed at least four tourists among the casualties during an eight-year period in Australia.(2)
Dehydration occurs when the body loses more fluid than is taken in. The process of dehydration can start during the journey as aircraft air is deliberately kept at a lower relative humidity than normal in order to protect the airline's delicate instrumentation. During a long flight, fluid intake may not be as great as normal, and the tendency for alcohol to be served can further exacerbate dehydration. On arrival at a destination airport, many people will then face lengthy road or rail travel to get to their ultimate destination. Ambient conditions can be very hot if transport is not air-conditioned, and, unless travellers are well prepared with an adequate supply of fluids and suitable clothing, a degree of t will have already established itself before they arrive at their accommodation. There is less humidity in the air at high altitude, so this is an additional factor for those travellers who fly directly to high-altitude destinations such as Cuzco in Peru. The elderly and the very young are more susceptible to heat and to the effects of dehydration. Elderly travellers may also be tempted to cut down on their fluids during a journey because of anxieties about accessing toilets along the route.
Cities can be 1-6˚C hotter than surrounding rural areas, especially at night.(3) Tourists to urban areas in very hot countries need to be very aware of the risks of heat and dehydration, especially if their accommodation is not air-conditioned.

How does dehydration occur?
The body needs to be adequately hydrated to function, and it also needs to be maintained at normal body temperature. When an individual is in a hot environment, thermal receptors relay information to the hypothalamus, which then stimulates cutaneous vasodilation and an increase in heart rate. This allows blood to rapidly flow across the skin's surface and lose heat by convection and radiation. Simultaneously, millions of sweat glands contribute to the cooling process by producing sweat, which helps to cool the body by evaporation.(4) The body can lose up to three litres per hour in this way. However, in extreme conditions where the external temperature is high, the ability to lose heat by radiation and convection is lost, and if the atmosphere is humid then sweat will not evaporate;  thus the process will merely deplete the body of fluid but will not assist in cooling
Exercise generates heat, and thus exercise that takes place in hot conditions will add to the heat that needs to be controlled by the body.
If an individual becomes very hot and starts to lose fluid through sweating or because of illness, and does not replace this fluid, they will become dehydrated. Dehydration interferes with the body's thermoregulatory mechanism and can allow core temperature to rise by more than 2˚C. This will become a vicious circle if fluid is not urgently replaced, leading to heat exhaustion and possibly heat stroke, which is potentially fatal. Heat stroke is a medical emergency and is characterised by three specific symptoms:

  • A core temperature greater than 41˚C.
  • Central nervous system disturbance.
  • Cessation of sweating (this may not always be the case with classic heat stroke).

Heat stroke can occur classically, such as in the elderly, chronically ill or those in poorly ventilated housing. Symptoms will often be gradual in these cases. The more common form to affect travellers is "exertional heat stroke" where the victim is often physically fit but exerts themselves in a very hot environment. Symptoms in these cases may come on very suddenly and lead rapidly to delirium and coma. There may be some early warning signs such as nausea, dizziness, weakness and vertigo.
Suffering from a condition such as traveller's diarrhoea, which in itself may cause dehydration, will do so much more rapidly in a hot environment where the individual is also losing fluid significantly through sweating. It is vital therefore that individuals who suffer from diarrhoea while in hot climates understand the great importance of fluid replacement. A general rule is to drink a cup of fluid following each trip to the "loo" in addition to normal intake.
Individuals who have travelled in order to participate in a physical activity, whether this be trekking or more specific sporting events, and who suffer from an episode of diarrhoeal illness, need to be aware of the great importance of adequately rehydrating themselves before they continue with the physical activity.
Cardiovascular disease may impede the body's response to excessive heat and/or the extra stress caused by the heat could aggravate the underlying condition. Thermoregulation in the very young and the very old is less efficient, and their carers need to ensure that their fluid intake is sufficient for the prevailing conditions.

Prevention of dehydration and heat-associated illness
The thermoneutral zone for a human at complete rest is 28˚C - at temperatures above this or when physical exertion occurs at temperatures close to this, the body must actively lose heat in order to maintain a normal core temperature. Even a slight variation in core temperature can result in feeling unwell.(5) Thus individuals exposed to these temperatures need to do all they can to assist the body to control temperature when exposed to temperatures at or above this level.
The following advice should be given to all those planning on travelling to a hot country:

  • All travellers should drink adequate fluids and remember that excessive alcohol and caffeine can encourage dehydration.
  • Do not wait until thirsty to replace fluids - drink beyond thirst, ideally to ensure a normal output of pale urine.
  • Use rehydration salts or sports drinks if physically active or suffering from prolonged fluid loss through diarrhoea. Homemade preparations must be carefully mixed - half a teaspoon of salt to one litre of water is sufficient and below the taste threshold. Too much salt is a gastric irritant and may lead to vomiting.
  • Reduce fluid loss from diarrhoea by using antimotility agents such as loperamide (should not be used in children under eight years or if blood or mucus is present in the stools.)
  • Wear appropriate clothing for the climate and make as much use of shade as possible.
  • Avoid being physically active at the hottest part of the day.
  • In humid conditions where air conditioning is not available, fans can help to assist with evaporation of sweat.
  • Add salt to food while in environments where sweating is significant.
  • At high-altitude destinations fluid replacement is vital, as not only does the atmosphere contain less moisture but the physiological response to the reduced air pressure leads to diuresis.
  • Allow time to acclimatise if planning physical exertion in hot conditions. Full acclimatisation can take from one to three weeks to occur and is faster in hot, dry conditions than in hot, humid environments.
  • Recognise and respond to signs of dehydration - dry mucous membranes, loss of elasticity in the skin (pinch skin on the arm - it should not remain bunched after letting go), scanty dark urine.
  • Understand the emergency treatment of severe dehydration or heat illness (see Box 1).

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Conclusion
Healthcare professionals advising travellers need to do a thorough risk assessment, part of which should look at climatic conditions and planned activities at the destination. Information should be given on the importance of remaining well hydrated and avoiding heat-related illness as well as the potential for sun damage.

References

  1. UK tourist may have died of heat exposure. Sydney Morning Herald 28 October 2004. Available from URL: http://www.smh.com.au/articles/ 2004/10/28/1098667873116.html?from=storylhs&oneclick=true
  2. Green H, Gilbert J, James R, Byard RW. An analysis of factors contributing to a series of deaths caused by exposure to high environmental temperatures. Am J Forensic Med Pathol 2001;22:196-9.
  3. Sanford C. Urban medicine: threats to health of travelers to developing world cites. J Travel Med 2004;11:313-27.
  4. Weiss E. Medical risks of temperature extremes. In: DuPont H, Steffen R, editors. Textbook of travel medicine and health. 2nd ed. Ontario: BC Decker; 2001. p. 113-8.
  5. Johnson C. Effects of climatic extremes. In: Dawood R, editor. Traveller's health. 4th ed. Oxford: Oxford University Press; 2002. p. 326-49.