The Heat Is On: To Exercise or Not To Exercise

Summer has arrived!  It’s time to separate hot weather exercise fact from fiction so you can be safe and sound throughout the dog days.

I’m a mother of two school-aged children and a fitness trainer.  These two truths intersect in this way: I am about town on the same days at the same times and along the same routes each week carting kids to and from extracurricular activities and I notice people who are exercising along these routes.  A puzzling thing happens on the hottest, most humid days.  I see a lot of individuals exercising that I don’t see on any other days, many of them during the hottest time of day.  I suspect this phenomenon is born of myth and misconception.

Exercising in hot, humid weather or overdressing (indoors or out) to induce more sweating and a higher heart rate does not equal a better workout or more calories burned.  If you sweat enough you will temporarily lose weight but all that weight is in the form of fluid loss and needs to be replenished to avoid dehydration.  Your heart rate will go up quicker and higher on very hot or humid days as compared to cool, dry days.  The reason is more blood flow is diverted away from the heart to surface tissue to aid in perspiration to keep the body from overheating.  This means the heart has to work harder to keep you going.  This has zero cardiovascular or calorie-burning benefit while putting you at higher risk for potentially serious conditions like heat cramps, dehydration, heat stroke and heat exhaustion.

Even more disturbing is that a few of the people I see exercising only in the hottest weather are also overdressed.  This is very dangerous behavior.  When the body is exercising at normal temperatures, the exercise induces an increased internal heat which triggers the body to perspire in order to keep the body’s temperature from rising more than 2-3 degrees.  The cooling comes from the evaporation of the sweat on the skin’s surface.  When a high external temperature is introduced the burden on the body is increased and the evaporation process becomes even more essential.  The evaporation process is impeded when a high amount of water vapor is in the air, such as in humid weather.  This is why humidity should be taken into consideration equally with temperature when deciding whether or not to workout.  When the skin is covered by fabric, the evaporation process is impeded even more.  There is no health or caloric expenditure benefit from taxing the body’s system in this way.  On the contrary, the greater the number of risk variables – high temperature, high humidity, excess clothing – the greater the chance that the body’s system for regulating temperature will be overwhelmed.  If the body’s temperature regulating system fails, you will become very ill or, in the case of heat stroke, organ failure and death can occur.  One should wear long sleeves and full-length pants only when exercising outdoors in cold weather.

Take a cue from the fittest people on the planet.  The Chicago and New York City marathons are run in October and November; Phoenix, Los Angeles, Boston and London do theirs in March and April; Las Vegas, Miami, Dallas and Honolulu marathons take place in the winter – December through February.  The Avon Susan G Komen 3-Day Breast Cancer walks take place throughout the United States from late September through June 1st – none in the summer months.  If  elite athletes aren’t doing the bulk of their training or racing from late June through early September, why should you?

Still, there’s no need to take four months off from outdoor exercise.  The key is knowing when it’s risky to be exercising outdoors.  Weather forecasts will post current temperature and relative humidity.  An easier indicator to use is a third number often posted:  sometimes called the “feels like”, “real feel” or the wet bulb globe temperature (WBGT).  WBGT is derived from a formula that takes into consideration temperature, humidity and radiant heat.  If the WBGT  is above 82 degrees, regular exercisers should take precautionary measures and the deconditioned or those with cardiovascular or pulmonary disease should avoid all outdoor exercise.  As the WBGT nears or exceeds 90 degrees, everyone should refrain from exercising outdoors.

Precautionary Measures to Take When WBGT Exceeds 82 Degrees (healthy, regular exercisers only)

Hydration: 2 hours prior to exercise drink 17-20 oz of water.  Sip water while exercising – approximately 7-10 oz every 10-20 minutes.  Following exercise, drink 16-24 oz for every one-hour period of your workout.  In this circumstance, it’s okay for a recreational exerciser to have a sports drink post-exercise that contains sodium and electrolytes which are also diminished while working out in heat and humidity.

Attire: Wear shorts, sleeveless or short-sleeved top, and short socks made of synthetic, light-colored fabrics that wick away moisture and apply sunscreen to all exposed skin.  Wear UV proof sunglasses and/or brimmed hat or visor.

Time of Day: Avoid the hottest time of day when sunlight is direct and shade is in short supply – 11am to 5 pm.  Opt for right after sunrise or just before sunset when the sun is low on the horizon.  Keep an eye on the sky, high heat and humidity increases the likelihood of thunderstorms.

Intensity: Avoid any high-intensity exercise.  These are the days to do low to moderate intensity, shortened workouts.  Save your best runs, power walks and steepest cycling hills for a cooler day.

If WBGT Exceeds 87 Degrees – STAY INDOORS!

Mother Nature has a way of forcing us to adapt and think outside the box.  When she heats things up, rather than take unnecessary risks, be creative and do something different.  Get a day pass for a nearby gym, find out the lap swim times for a local pool, sample a studio yoga or Pilates class, do an online cardio-strength training program in a cool indoor environment or take a day off.  Whatever you decide will be a change from what your body is anticipating.  This is an excellent thing to do occasionally from a fitness perspective.  When the heat is on, stay cool.


  1. […] ← The Heat Is On: To Exercise or Not To Exercise July 2, 2013 One Small Step Leave a comment […]

  2. […] working out in extreme heat, working out in cold temperatures can be beneficial if your goal is maximum calorie burn.  The […]

  3. Reblogged this on fitandhappier and commented:

    What does a blogger do when Wi-Fi is down & 4 people in your household are jockeying for the one Ethernet wire on premises? Answer: reblog a previous post. This post is one of my most popular ever & it happens to be a timely re-post. With the official first day of summer less than a week away, it’s a good time to review the myths, safety precautions and basic dos/don’ts of exercising in the heat of summer….

  4. […] heat and humidity can be a real and valid deterrent.  We all need to plan for that and either do less intense outdoor workouts during those times or find indoor or water-based alternatives.  Other goals are more easily […]

  5. […] natural cooling system and has nothing to do with calorie burn.  People who workout in excessive heat or overdress for their workouts in hopes that producing more sweat will mean a better workout are not only wrong, […]

  6. […] posts: Fueling & hydrating for endurance training; Exercising in High Heat/Humidity; Exercising in Extreme […]

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