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Hiking in Hot, Humid Weather and Headaches

I have put together some useful information here about exercising in heat and high humidity as I think it is useful to anyone hiking in Hong Kong.Humidity in Hong Kong varies between 70-95% with temperatures in the summer often reaching 35 degrees Centigrade. (Remember this refers to the temperature in the shade!).

Body's heat reduction mechanisms

Because the body must maintain a relatively constant body temperature around 37°C, it needs ways to eliminate the excess heat. Most of the heat is conducted in the blood stream to the skin and then the heat is released in three ways.

  • Evaporation, in which excess heat is transferred to sweat which is then evaporated. It is the evaporation that causes the heat loss not the actual sweating and it is difficult for sweat to evaporate in high humidity. So, on a hot, humid day, you can be dripping with sweat, but because the sweat is not evaporating it is not doing you any good. In addition the sweating is making you dehydrated (You lose up to 2 litres of sweat per hour)
     
  • Radiation, in which excess heat is carried by means of electromagnetic radiation to cooler objects around the body. Blood vessels near to the skin dilate to provide a greater surface area.  However, where the temperature of the surrounding air approaches body temperature (37C), this will not have much effect.
     
  • Convection, in which excess heat it transferred to air passing over the skin, particularly if it is windy.

Body Fluid Balance

Profuse sweating leads to a decrease in blood volume, ie dehydration. You should not rely on feeling thirsty. By the time you feel thirsty you are already low on fluids.  The best indicator of proper fluid levels is urine output and colour - Your urine should be "copious and clear". Dark urine means the body has concentrated the urine because it is low on fluids. You also lose salts when sweating.

Heat Illnesses

Heat Cramps - caused by insufficient salt. Replace salt and fluid and stretch the muscle

Heat Syncope (fainting) - In an effort to increase heat loss, skin blood vessels dilate to such an extent that blood flow to the brain is reduced resulting in faintness, dizziness, headache, increased pulse rate, nausea and vomiting. Dehydration contributes significantly to this.

Heat Exhaustion - where fluid losses from sweating and respiration are greater than fluid reserves. The lack of fluid causes the body to constrict blood vessels especially in the arms and legs (making your skin pale and clammy), have decreased urine output and the person will feel weak dizzy and thirsty with nausea and vomiting

Heat Stroke - Life threatening condition where core body temperature rises to above 41degreesC. There are 2 types of heat stroke:

  • Fluid Depleted Heat Stroke- fluid loss due to sweating and inadequate fluid replacement but person continues to function.
  • Fluid Intact (fast onset) Heat Stroke - heat challenge overwhelms body's active heat loss even though fluid level is sufficient (esp if people push themselves when they are already hot). Hot pale skin (may be dry or wet). severe change in mental status and motor/sensory changes.  Pupils may be dilated and unresponsive to light.

Treatment - Sit or lie the affected person in the shade, elevate the feet and give fluids (to be drunk slowly). Use half a teaspoon of salt and half a teaspoon of baking soda per litre of water to replace this. You can also buy 'Oral Rehydration Salts' from Watsons. Use cool (but not too cold), wet cloths to reduce body temperature and use a fan to increase cooling by evaporation.

Clothing

Clothing that is worn should be made of a lightweight, breathable material so that sweat can evaporate. 100% cotton is a poor choice on hot days, since cotton holds large amounts of sweat, not allowing the sweat to evaporate . The color of clothing is another consideration: white- or light-colored clothing is best. This is so because things that are white reflect all wavelengths of light (and associated heat); thus, light-colored clothing reflects heat radiated from the sun.
 

Non-Salty Sweat

Sweat contains substantial amounts of sodium (1g per litre), modest amounts of potassium and small amounts of minerals (eg Iron and Calcium)It is possible to adapt to exercising in hot environments (but note you cannot adapt to being dehydrated). One key adaptation is that your sweat becomes more dilute (there are fewer electrolytes, including sodium, dissolved in it). The 'weaker' sweat means that more sodium is being conserved inside your body. This preserved sodium actually 'pulls' more water into the blood, keeping blood volume high even in the face of fairly heavy sweating. A key benefit is that the higher blood volume helps to keep heart rate fairly low during hot-weather exertion, making exercise feel less demanding and troublesome

The extra blood volume also lessens the intense conflict which is usually set up between the muscles and skin during hot weather. The muscles selfishly want more blood because of the oxygen it contains, while the skin demands blood for cooling. The increased blood volume allows both demands to be met fairly successfully

While your sweat glands are being stingy with sodium, your kidneys also help you adapt to the heat by holding on to water more tightly (lowering your urine output), which also boosts blood volume. You become more resistant to both overheating and steep, hot-weather-related declines in performance.

Swollen Hands and Puffy Fingers

Hiking in hot weather often causes your fingers and hands to become swollen and puffy, so much so that you find it hard to bend your hands to make a fist.  If you are not taking in adequate electrolytes, the imbalance in salt levels between the blood stream, the cells, and the extracellular spaces essentially results in a trapping of the water in the tissues as the sodium is lost in sweat. The same thing can happen if you are taking in too much salt. Centrifugal force from the swinging of your arms adds to this effect.

Water and Sports Drinks

Sports drinks tend to include a lot of sugar but also contain electolytes (sodium, choride and potassium) lost in sweating and taste nice so may encourage you to drink more than you would water. I like to take a mixture of mostly water (1 litre per hour of exercise) with one or 2 sports drinks to drink at the top of hills. However, most commerical sports drinks have modest sodium content so they appeal to the general public. A person exercising in heat needs up to 2.07 grams of sodium per litre for proper recovery which would make sports drinks taste like seawater (There is about 1 gram of sodium in 2 litres of Gatorade).  You can increase your sodium by eating sodium containing foods - see below. Drinking large amounts of plain water is not ideal in itself because it shuts off the sense of thirst and produces more urine which results in further fluid loss even though you are dehydrated. Oral Rehydration salts are also useful.

Inorganic vs Organic Sodium

There is a lot of discussion of this on the internet and it was difficult to come to any conclusions. Some people say that table salt (sodium chloride) is not a good way to get your sodium. (note there is 1 gram of sodium in every 2.5 grams of table salt (sodium chloride) and about 6 grams of table salt in a teaspoon) but other sites (particularly the sports-oriented ones) say that it is necessary. Foods high in organic sodium include pears, watermelon, celery, cabbage, okra and cucumber. Foods high in organic potassium include figs, grapes, raisins, apricots, oranges, bananas, dried fruits, celery, yoghurt, oats, rye

In Conclusion

To prevent problems, you must stay hydrated and stay cool.Before Hiking: Drink half a litre of water on waking up and add some salt to your breakfast to make sure you do not start the day dehydrated or low on sodium.During hiking - wear light clothing to allow sweating. Take plenty of water (1 litre per hour) and use cold, wet cloths with you to keep cool while hiking. Drink and rest regularly. Sweat consists sodium, chloride, potassium, proteins and fatty acids. So to replace what you are losing, eat salty snacks and foods high in Sodium and Potassium.. After hiking - continue drinking regularly for the rest of the day to replace lost fluids.

This advice should not be taken lightly - people do die from heatstroke in Hong Kong - see this article from SCMP

Don't hike if it is too hot and humid!!