The clinic: it is the heat--and the humidity.

This question about summer weather's impact on a runner has interested me for some time:

A runner runs 5 miles at 6 a.m. one day with cloudy skies, an 80-degree temperature and 80 percent humidity That same runner a few days later runs 5 miles at 4 p.m. under sunny skies with a 95-degree temperature and 50 percent humidity. Which of the two external weather conditions is more taxing on the runner assuming the exact same running route and running pace? Why?

Paul Scelsi

Dallas, TX

Musclar contractions, as you probably are aware, generate heat that must be released into the environment; if not, internal core temperature will increase and eventually threaten health and possibly even life (see A Summer Scorcher [LINK]). High ambient heat and humidity make it more difficult to dissipate this heat. The primary factors that affect body core temperature are reflected in the composite temperature known as the Wet Bulb Globe Temperature (WBGT). This index integrates humidity, air temperature and the temperature of the globe into a single value. It is determined with the use of specialized instruments that are commercially available. However, the risk of a heat-related illness can be assessed by simply combining the influence of ambient temperature and relative humidity (both readily available from local weather stations).

Thus to answer your question, the relative risk of running in the a.m. with the cloud cover, an 80-degree temperature and 80 percent humidity would be moderate to high. Doing that same run on the same course at the same pace on a sunny day in the late afternoon with an ambient temperature of 95 degrees and 50 percent humidity would be very high. For more information, see the American College of Sports Medicine Guidelines for Exercise Testing and Prescription, 7th ed., Lippincott, Williams and Wilkins, Appendix E. In either case it would be critical to stay hydrated but I would advise you to run in the a.m. under these conditions or inside in a controlled environment.

For a fluid-need calculator go to www.gatorade.com/hydration/fluid loss calculator/.

Tom LaFontaine, PhD, ACSM RCEP, FACSM Columbia, MO