Is It Too Hot to Exercise? Use the Heat Index

Hot Weather Walking
Hot Weather Walking. © Dirima / Depositphotos.com

Is it too hot to exercise outdoors?

The air temperature doesn't tell the whole story when it comes to walking and exercising in hot weather. The heat index tells the apparent temperature that you feel, based on air temperature and the relative humidity. You can use it to decide whether it is too hot for outdoor exercise.

Heat Index Chart: View the heat index chart at the National Weather Service. 

The heat index is calculated for shade.

If you will be exercising in direct sunlight, without shade, the heat index may be as much as 15 degrees higher, according to the Occupational Safety & Health Administration guidelines for outdoor work.

When Cancel Outdoor Exercise Based on the Heat Index

A high heat index can warn you to restrict activities to prevent heat sickness. The American College of Sports Medicine says that the risk of exertional heat sickness is raised when the heat index is above 82F/28C for people exercising for over an hour.

  • At 82F heat index they recommend canceling competitions and continuous activity.
  • At the 86-90 range, the ASCM recommends canceling athletic activities for less-fit and non-acclimated people and limiting the duration and intensity for fit and acclimated people.
  • A value over 90 should prompt cancellation of athletic activities for everyone, and is an indicator you should plan a different workout rather than a long walk or run outdoors.

    If you are trying to decide whether to walk or run outdoors, check the heat index to see whether it will be below 90 at the time you will be doing it. If your route doesn't have shade, factor in another 15 degrees to the heat index.

    Physical Risk Factors for Heat Sickness

    The ASCM notes evidence that these factors increase risk of heat sickness.

    If they apply to you, take extra precautions when the heat index is high.

    • obesity
    • low physical fitness
    • lack of acclimatization to heat
    • dehydration
    • previous history of exertional heat sickness
    • sleep deprivation
    • sunburn
    • diarrhea, illness, certain medications

    Environmental Heat Factors

    Racewalking coach Gary Westlund (ASCM Certified Health and Fitness Instructor) lists these factors that affect how hot we feel exercising outdoors:

    • Humidity and Dew point are measures of the amount of moisture in the air. We perspire in order to get the benefits of the cooling that comes when the sweat evaporates. If the air is already saturated with moisture, your perspiration can't evaporate as fast to cool you. If the humidity is low, sweating works better to cool you.
    • Heating from the Sun: Sunlight warms you with radiant heating, directly heating your body and other surfaces without actually touching them.
    • Heating from the Wind: Air can carry heat with it from one object to another, and you will feel the heat transfer when the air temperature is at 72F or above. Below that temperature, it feels like a cool breeze, while above that temperature the wind contributes to heating.
    • Heating from the Pavement: When you touch something hot, the heat is transferred directly to your skin. Coach Westlund notes this is what happens with hot pavement or asphalt heating your feet through your shoes.
    • Temperature Gradient: The temperature difference between your body and the outdoor factors will determine how much and how fast you heat up or cool down.

    Sources:

    Armstrong, Lawrence E. Ph.D., FACSM (Chair); Casa, Douglas J. Ph.D., ATC, FACSM; Millard-Stafford, Mindy Ph.D., FACSM; Moran, Daniel S. Ph.D., FACSM; Pyne, Scott W. M.D., FASCM; Roberts, William O. M.D., FACSM. "Position Stand: Exertional Heat Illness during Training and Competition." Medicine & Science in Sports & Exercise: March 2007 - Volume 39 - Issue 3 - pp 556-572 doi: 10.1249/MSS.0b013e31802fa199

    Occupational Safety and Health Administration: Using the Heat Index: A Guide for Employers. Accessed 8/3/2015.

    Gary Westlund, ASCM H/FI: Correspondence 8/1/1999

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