Heat and Humidity and Exercise, Oh My!

Recently I’ve learned that while many runners and athletes can diligently recite what to do while exercising in hot and humid conditions (drink fluids, take it easy, cool down when done), they don’t have a firm grasp on the science behind the body’s reaction to humidity.  Worse, they don’t know how to recognize the symptoms of heat-and-humidity-related illness in their friends, leading to delay in seeking appropriate medical attention.  When you’re done reading this post, I hope you understand the science better, know how to prevent illness, and are equipped to seek help if necessary.  A hot, humid environment is the most stressful condition for exercise.

The Science of Sweat.  When you exercise, your body warms up and your body temperature increases. The body is amazing at self-regulating temperature, and when your temperature rises, your body’s cooling mechanism kicks in.  The body re-directs blood from the deeper, more central parts of the body—like the internal organs and your muscles—and into the blood vessels that are closest to the surface of the skin.  (The technical term for this is peripheral vasodilation)  Also, you sweat.  Sweat is the body’s main mechanism to cool the body.  Sweat is made up primarily of water, with some salts (which play a role in the movement of fluids between and among cells in the body).  The body produces sweat inside the body, which then exits the body via the pores in the skin. The water in the sweat evaporates from the water, which cools the underlying blood.

Even without heat and humidity, extended bouts of strenuous exercise will lead to dehydration as the body sweats to dissipate heat.  Dehydration is dangerous because it impairs performance (as fluids are not circulating the way they should within the bloodstream) and can cause muscle cramps.  It can also lead to other inconveniences and symptoms, which on the less dangerous end include headache and light-headedness, and on the more severe end include fatigue, confusion, lethargy, and an elevated body temperature.

The Science of Humidity.  If the air surrounding the body is humid, sweating becomes a less efficient way to cool the body.  The higher the humidity, the less efficient sweating is.  If you’ve ever experienced humidity, you know how your skin stays wet?  That’s sweat, sitting on your skin, unable to evaporate.  When sweat can’t evaporate, it doesn’t cool off the body. The body keeps pumping out sweat, but the body temperature does not drop.

At this point, a few things happen.  First, any exercise seems more difficult because the body is actively redirecting blood away from the muscles.  The muscles are not actually working any harder—in fact they are working less efficiently, because the supply of oxygen has been drastically reduced—it just seems that way.  (In fact, they are working the same as, or less than, they would be at a lower temperature.  This is why “hot” exercise does not produce a higher calorie burn.)  Second, the heart has to work harder to drive the circulatory system because (a) the blood is thicker and sludgier (as part of the water that usually keeps blood at the regular consistency has been lost due to sweat), and (b) it is being pumped through vessels that are further away from the heart (closer to the surface of the skin than usual).

At this point, it should make sense that humidity magnifies the effect of heat.  If it is hot and NOT humid (a so-called “dry heat”), you will sweat but the sweat will evaporate and the body will be cooled.  These conditions present their own dangers, as without a feeling of being sweaty, fluid loss might not be as apparent until you experience sudden or extreme thirst.  What we call “heat-related illnesses” are often caused not by heat, but by a combination of heat (actual temperature) and humidity (relative humidity). The combined effect of the two is known as the heat index. For example, at a relative humidity of 70% and an actual temperature of 70 degrees F, the body reacts as though it is 70 degrees (apparent temperature).  At the same humidity but an actual temperature of 80, the apparent temperature is 85 degrees, well within a safe temperature to exercise; at actual temperature 90 degrees the apparent temperature is 106, just over the threshold where heat cramps or heat exhaustion are likely and heat stroke is possible.

Houston, We Have A Problem.  Heat and humidity lead to sweating, which lead to elevated body temperature, dehydration, and electrolyte imbalance.  Sodium and potassium are two electrolytes. Electrolytes are compounds that become ions when they are dissolved into a solution, and they regulate the flow of substances into and out of cells. The regulated flow of things into and out of cells is crucial to muscle contraction, fluid balance in and out of cells, and nerve impulses. (Think of them as the traffic cops of the body.  No traffic cops, and no traffic signals, and you get a traffic jam with all of the cars bumper to bumper with nowhere to go.) 

Heat-Related Illnesses.  There are three main heat-related illnesses: heat cramps, heat exhaustion, and heat stroke.  Heat cramps are muscle cramps that occur due to heat.  The cramps occur because the muscles are not receiving regular blood circulation.  This means that the muscles do not receive as much oxygenated blood, and the waste products produced by muscular activity are not removed from the body at the same rate, as would occur in cooler and less humid temperatures.  The resulting electrolyte imbalance means muscle contraction is not properly regulated. This is the least serious form of heat-related illness.  The muscle might feel hard to the touch (just like a “Charlie Horse” typo of calf cramp feels).  Heat cramps are the most minor form of heat-related illness.  Rest and rehydration (including salts such as those found in electrolyte-type sports drinks) are the appropriate treatment.

Both heat exhaustion and heat stroke are potentially life-threatening and must be treated immediately.  Heat exhaustion has multiple signs and symptoms, not all of which are present in any given case.  Elevated body temperatures may reach up to 104 degrees F (40 degrees C).

If left untreated, heat exhaustion can become heat stroke.  If you suspect that you or a friend have heat exhaustion, seek immediate treatment.  Treatment for Heat Exhaustion starts with stopping exercise and getting out of the heat/humidity; if possible, move to a cool, ventilated area.  Drink fluids—not just water, but also sports drinks (which contain electrolytes potassium and sodium).  Remove any tight clothing and unnecessary layers, and any non-breathable clothing.  Temperature monitoring is also recommended for heat exhaustion.  If you or a friend suffer from heat exhaustion during a distance running event, contact on-course medical support immediately.  Your race might be over, but your life will be safe; listen to on-course medical support and follow their advice.

More serious than heat exhaustion is Heat Stroke.  If you compare the signs of heat stroke with those of heat exhaustion, you’ll notice many of them are opposites: cold, clammy skin (heat exhaustion) v. hot, dry skin (heat stroke); paleness (heat exhaustion) v. redness (heat stroke); and strong pulse (heat exhaustion) v. weak pulse (heat stroke).  In heat stroke, the body temperature rises to or above 105 degrees F (41 degrees C).

More serious than heat exhaustion is Heat Stroke.  If you compare the signs of heat stroke with those of heat exhaustion, you’ll notice many of them are opposites: cold, clammy skin (heat exhaustion) v. hot, dry skin (heat stroke); paleness (heat exhaustion) v. redness (heat stroke); and strong pulse (heat exhaustion) v. weak pulse (heat stroke).  In heat stroke, the body temperature rises to or above 105 degrees F (41 degrees C).

Heat stroke is a serious, life-threatening condition.  It can cause organ failure, brain damage, or death.  If you suspect that you or another has heat stroke, stop exercise and seek immediate medical attention. Treatment for Heat Stroke is to stop exercising immediately, remove as much clothing as possible, cool the body in any way possible, give fluids to the person suffering, and get them to an emergency room immediately.

Heat stroke is a serious, life-threatening condition.  It can cause organ failure, brain damage, or death.  If you suspect that you or another has heat stroke, stop exercise and seek immediate medical attention. Treatment for Heat Stroke is to stop exercising immediately, remove as much clothing as possible, cool the body in any way possible, give fluids to the person suffering, and get them to an emergency room immediately.

Risk Factors for Heat-Related Illness.  Clearly, heat—or more likely heat AND humidity—are required for a heat-related illness.  Other risk factors are more personal and include:

• Dehydration
• Obesity
• Low physical fitness/poor conditioning
• Lack of acclimatization (to the heat/humidity/elevation)
• Sleep deprivation (surprised?)
• Certain medications including diuretics and antidepressants
• Sweat gland dysfunction
• Illness, including upper respiratory illnesses (cold/flu/etc.) or gastrointestinal illness (severe upset stomach, etc.)

Prevent Heat-Related Illness! 

• Understand how heat and humidity affect your body; if possible, train in some humidity.
• Pay attention to how your body feels.  Take walking breaks if you are running, and otherwise reduce the intensity of other exercise.
• Acclimate yourself by increasing exercise gradually in the heat.  According to the American Counsel on Exercise, it takes a week to 10 days to acclimate to heat; ACE recommends exercising for short periods each day.  Author and Coach Jenny Hadfield, writing for Active.com, stated it takes two weeks.  (This is, I believe, a highly personalized factor.)
• Choose lightweight, breathable clothing.  At the gym, cotton is cooler; distance runners say “cotton is death” because the fabric’s propensity to retain moisture leads to blisters.  Do not wear “sauna suits” or other deliberately heated clothing in the heat and humidity.
• Choose light colors.  White clothing reflects the heat better than dark clothing.
• Hydrate, hydrate, hydrate.  Drink a moderate amount of extra fluids prior to an event in the heat and humidity.  During the event, consume small amounts of fluid frequently.  Even if you do not usually carry liquid, consider carrying it with you on hot and humid days.  If you carry fluid at an event, sip it between aid stations, and refill each time you are able.

Cooling the Body.  Heat-related illness treatment requires someone to cool the affected body as well as possible.  Here are some ways to cool the body—though seeking shade (getting the heck out of the sun and heat!) should be a top priority, if at all possible.

• In hot but not humid weather, pour cool water on your head and body.  This will act as a sort of sweat supplement, evaporating from the body and cooling it.
• Similarly, wear a wet towel around your neck.  There are specially designed cooling towels, as well as thin neck bands containing beads that hold water designed for this purpose.
• Ice, ice, baby!  Ice in the mouth melts to create cold water to cool the body’s core.  Ice on the wrists, back of the neck, and top of the head will help cool the blood near the surface of the body faster.

THIS IS SERIOUS.  For those who were at the Disneyland Half Marathon this past weekend, you may have seen a few unfortunate runners succumb to the heat and humidity.  I don’t know those runners personally, and don’t know whether they were prepared for the environment (and I’m not rude enough to stare so I don’t know what they were wearing or whether they were carrying fluids).  I also saw a few runners being treated for heat-related illness at the Santa Rosa half/full as well.  Just so you don’t think it’s only me, I asked for some input from other runners.

A fellow member of the Half Fanatics (let’s call him Fellow Fanatic) wrote to me about a race his friend (Fanatic Friend) drove to, one that took place in a hot and humid climate.  The drive prevented Fanatic Friend from hydrating as he would have—just like people drink less while traveling by air.  While Friend carried water, he drank it instead of using the aid stations (the better thing would have been to sip it in addition to the aid stations).  Fellow Fanatic observed Fanatic Friend as far out as mile 9, where he seemed to be doing fine; by mile 12 Fanatic Friend was talking to medical, but waived to Fellow Fanatic and told him to go on by.  Not 15 minutes later another runner saw Fanatic Friend being whisked off to the ER by ambulance, as the on-course medical staff were unable to lower his temperature.

Here’s what Fellow Fanatic concluded:

Bottom line is: the humidity will hit you fast. You cannot be a superstar runner in the humidity, so do not push for a PR. Use those water stations before depleting your own water source. Do not be alone at a race. Make sure you have a plan with your family or a friend who you’re racing with on what to do if something goes wrong. Where to meet, text, whatever. If you are out of state or the country, make sure you know what your insurance covers and how your family/friend can help.

Be safe out there!

Sources:

ACE Group Fitness Instructor Manual, Chapter 1: Exercise Physiology, discussion of environmental conditions while exercising.

AFAA textbook, Fitness: Theory and Practice.

The Essentials of Sport and Exercise Nutrition, Precision Nutrition Certification Manual, second edition.

Multiple articles available via PubMed.gov and the NCBI, including but not limited to:

Coris EE, Ramirez AM, and Van Durme DJ.  “Heat illness in athletes: the dangerous combination of heat, humidity and exercise.”  Sports Med  2004; 34(1): 9-16.