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This article was published 4/7/2011 (2210 days ago), so information in it may no longer be current.
Summertime can bring beautiful, warm weather that invites you do your physical activity outside. If you are not accustomed to a warm climate or to exercising in warm weather, you can suffer from heat illness. Of course, the safest option would be to exercise early in the day before the heat becomes intense (before 10 a.m.) or later in the day as it starts to cool down (after 4 p.m.). However, many outdoor sports or matches take place during the day, so it is important that you prepare yourself for difficulties that can occur with exercising in the heat.
Exercising in hot weather can make you feel sluggish and even exhausted. Trying to stay cool can be a challenge depending on what type of activity you are participating in. If your activity has no cool shelter, you will probably notice that you cannot exert yourself for a long period of time. It can be so uncomfortable that you can begin to feel physically and mentally unwell. Every year, there are thousands of reported cases of heat illness in North America. The true number is likely much higher because many cases are not reported.
What happens to the body when exercising in the heat?
In cool environments, your body will transfer the heat produced from exercise directly to the air. In warm weather, heat is lost from your skin by sweating and evaporation. Your body will try to keep your inner temperature from rising more than three to four degrees. If it is very warm and humid outside, it will be very difficult to keep your inner temperature from rising to dangerous levels if you are exercising.
The intensity of your exercise is also very important in the heat. If you try to perform the same workout intensity in the heat as you would in a cooler environment, your heart will work harder, with an increase in heart rate and the amount of blood pumped out per beat. Your inner temperature and skin temperature will also rise. If you are taking part in an endurance event where you are exercising non-stop for greater than 20-30 minutes, do not expect to perform at the same intensity as you would in a cooler environment. There is nothing you can do to avoid a decrease in performance when high heat and humidity are present.
What is heat illness?
Heat illness represents a category of conditions that occur when your body cannot effectively get rid of excess heat. These include heat stress, heat cramps, heat exhaustion and heatstroke. You need to consider the outdoor temperature and the humidity when you are assessing weather conditions that could lead to heat illness. As the temperature and humidity rise, it becomes more difficult to sweat as evaporation slows down. With a temperature of 35C and humidity at 90 per cent, the body loses no heat. You obviously do not want to exercise in any environment that would come close to such conditions.
What are the symptoms and signs of heat illness?
Heat stress is an early stage of heat illness where body temperature increases, heart rate and blood pressure increase, dizziness, fatigue and mental difficulties can arise. This condition usually responds to cooling and rehydrating with water.
Heat cramps can also occur at an early stage of heat illness. A lack of sodium can lead to muscle cramps (especially in the legs), weakness, marked thirst, fatigue, nausea/vomiting and elevated heart rate. If you have heat cramps, you should seek medical attention for treatment. You can initially start treatment by resting in a cool place, stretching the cramped muscles and drinking water with some salt in it (2.5 ml salt mixed in 500 ml of water, divide into 2 cups, each about 15 minutes apart). Do not take salt tablets.
Heat exhaustion is a more severe form of heat illness where the body becomes dehydrated and loses important electrolytes. Fainting may occur, a drop in blood pressure with an increase in heart rate, nausea/vomiting, profuse sweating, cool skin, lack of urine output and headache among other signs are common. You should seek a cool place and start drinking cool water. Applying cool, wet towels and removing tight clothing is also recommended. Medical assistance should be sought as soon as possible.
Heatstroke is a very serious form of heat illness and a medical emergency. The body overheats and can no longer cool down with sweating. Signs of heatstroke include hot and dry skin (lack of sweating), confusion, faintness, staggering, combativeness, bizarre behaviour, strong rapid pulse, seizures and coma. Body organs can fail and death can result. Heatstroke requires immediate emergency medical attention. While waiting for medical evacuation, the victim should be in a cool, shaded area and the body covered with wet, cool sheets. The person should be lying down. Cold packs wrapped in a cloth are also used to cool down the large blood vessels.
What other factors can make someone vulnerable to heat illness?
If you consume any beverages on the day of your activity that are caffeinated, such as coffee, tea, alcohol or soft drinks, your body will lose more fluid, leaving you dry. Certain medications such as beta blockers, diuretics, decongestants and amphetamines can make you more vulnerable to heat illness. Clothing that doesn't allow your skin to breathe and get rid of moisture can also contribute to heat illness.
Age is also a very important risk factor. Children have more surface area relative to their body mass making them more susceptible to heat loss and heat gain. The elderly are more likely to have an impaired ability to regulate fluid loss via the kidneys. Older adults also have less blood flow to the skin making sweating more difficult.
Obese individuals are a greater risk of dehydration, since more fat and less body water leads to overheating more quickly. Certain medical conditions such as heart disease can also place an individual at a greater risk for heat illness. High protein diets contribute to dehydration and should be avoided when exercising in the heat. If you spend a lot of time in a hot environment inside or outside, you can also develop signs of heat illness without physical exertion.
How much fluid do you need to drink?
Since you will sweat more in hot weather, you will lose more fluid and you need to replace it. A general guide is to drink about 2/3 cup of water every 15 minutes of exercise. However, depending on the intensity of your workout and the exact weather conditions, you may need to consume more than this amount. Water is a sufficient fluid replacement for exercise lasting an hour or less. For longer exercise periods, you can drink a six- to eight-per-cent carbohydrate drink. Sodium loss requires replacement when you exercise for three to four hours or longer.
Do not wait for your body to tell you that you are thirsty. Drink plenty of water before, during and after exercise. Wear light-coloured, breathable clothing out in the sun. Choose an exercise time earlier or later in the day, when it is cooler. Wear sunblock and use a sun hat for shade. Know the risk factors for heat illness. Stop exercising and seek medical attention if you feel unwell.
Dr. Maureen Kennedy MD, CCFP, FCFP, MSc, PhD(c) Kinesiology, Dip. Sport Med., is a sport and exercise medicine physician at Pan Am Sport Medicine in Winnipeg.
Readers can ask Dr. Kennedy questions, but due to the volume of requests, replies are not guaranteed.