Monthly Health Feature

July: Exercising in the Heat

By Angela Hammer, UI Wellness Practicum Student Summer 2007

Feature Index

Staying fit during the hot and humid months can be difficult. Understanding heat balance is important to maintain proper hydration and stay safe while outdoors. Here are a few tips, tricks, and guidelines to consider when things heat up outside.

Heat Production versus Heat Loss

Because the body has to maintain a constant core temperature (98.6° F), heat produced by the body must be matched by heat lost. Heat is produced through both involuntary ways like shivering and voluntary ways like exercise. Heat always travels to where there is less of it, and it can be lost in four different ways:

• Radiation- heat is transferred with no physical contact, such as rays from the sun to the skin
• Conduction- heat is transferred through physical contact, like when sitting down on a cold bench
• Convection- a form of conduction, heat is transferred through physical contact with moving air or water
• Evaporation- heat is transferred thorough water loss via the skin (sweating)

Sweating during exercise allows the body to get rid of heat that would otherwise cause harm. Heat production increases linearly with increasing intensity, so it is very important to closely regulate exercise dose (frequency, intensity, and time).

Factors that Affect Evaporation Rate

Because sweating is vital to losing heat during exercise, individuals should be aware of what helps and hinders their ability to sweat. Since heat always travels to where there is less of it, the body can absorb heat from the environment. The air is like a sponge. It will absorb water until it is full and then the water will no longer penetrate the surface. The same is true with sweat. If the air is holding a lot of water (high humidity), it will not absorb water off the skin and the body cannot get rid of heat. Therefore, be cautious when active on highly humid days.

Sweat rate also depends on the amount of skin that is exposed, air currents (due to convection), and air temperature. Be especially careful on hot, humid days with little wind.

Guidelines

So what do you actually need to do to prevent heat cramps, heat exhaustion and heat stroke?

• Drink plenty of fluids. With increased sweating, water loss can be a concern. Do not rely on your sensation of thirst. When you become thirsty, you have already started to become dehydrated. Drink water before, during, and after exercise. How much is enough?

• Drink 2 ½ cups two hours prior to physical activity.
• Try to drink at least 1 cup every 20 minutes during exercise.
• Check your body weight before and after a work out. Drink 2 cups of fluid for each pound lost.

• Wear light-colored and light-weight clothing. Dark clothing absorbs radiation from the sun and can impair evaporation ability. Clothing that wicks sweat is best. Fabric that absorbs water, like cotton, traps heat and therefore can slow heat loss. 
• Exercise during a cool time of the day. Early mornings and evenings are generally the best time to work out during the summer because the air temperature drops and the radiation from the sun decreases.
• On days of extremely high temperatures or humidity, workout indoors if possible. Air conditioning and/or a fan can significantly boost heat loss. If exercise inside is not feasible, some days it may be best to skip physical activity or perform light exercise only.
• Be aware of signs of overheating- diminished sweating, muscular weakness, dizziness, headache, and nausea. If symptoms become overwhelming, stop exercise immediately and seek medical attention if necessary.