Over the last several weeks, temperatures in Kentucky have soared into the triple digits, with heat indices hovering around 110-degrees Fahrenheit. When it gets this hot, I start to wonder if it’s just too hot to run outside.
When Is It Too Hot To Run Outside?
Because of back problems, the guy that first got me started with running has given it up in favor of cycling and swimming. We still squeeze in workouts together though, and he was just telling me that the only time he really misses running is on days that are really hot. Like the scorchers we’ve had over the last few weeks. When I first started running, I was all for the idea of running in the extreme heat, but as I’ve started sneaking up on 40, my desire to be outside starts fading fast when the mercury heads north of 90-degrees F.
According to the CDC, over 8000 people in the U.S. died of excessive heat between 1979 and 2003, which is more than the combined deaths from hurricanes, tornadoes, lightning, floods, and earthquakes combined. In fact, heat is considered the number one weather related cause of death in the U.S., claiming an average of 162 lives each year. So we need to take it seriously.
The human body cools itself by an evaporative mechanism. Exertion (exercise) causes your heart rate to rise, which brings blood near the skin’s surface through the capillaries, which allows heat loss. As internal temperature continues to increase, you begin to sweat, which has a cooling effect as the water evaporates from the skin’s surface. Since normal body temperature hovers around 98.6°F (37.0°C), as air temperature approaches skin temperature, sweat doesn’t evaporate as quickly, which means your body doesn’t cool itself as efficiently. When air temperature exceeds skin temperature, things start getting really ugly.
Now let’s factor in the effect of humidity, which is the amount of water vapor in the air. As humidity rises, it becomes more difficult for sweat to evaporate from your skin’s surface. This means your body doesn’t cool itself as efficiently, which is where the whole concept of heat index comes from. This explains why heat index is sometimes called the apparent temperature.
Most of the suggestions I’ve found for heat and running suggest that as air temperature starts to approach 80°F (27°C), running performance will start to suffer. As air temperatures approach 85-90°F (29-32°C), runners should expect to take substantial hits to average running pace. All of this correlates with the expected heat index values from the above table.
There is, of course, the effect of acclimating to hot weather. The CDC defines this as an average increase in sweating and decreased energy expenditure for an average workload. They also indicate that it takes a full 1-2 hours each day for at least 10 consecutive days to occur. Ceasing outdoor activity can cause the effect of acclimatization to decay in as little as a few days. Even when you’re acclimated to outdoor heat, they recommend not engaging in strenuous activity during the hottest parts of the day.
As many of you know, I generally isolate my workouts to lunch breaks, which are of course the hottest part of the day. As heat indices have exceeded 110°F over the last few weeks, I’ve moved more of my workouts indoors. As much as it pains me, I’ve done a little treadmill running, and hit the pool and weights more. As much as I enjoy running outdoors, I have no interest in becoming a victim of heat stroke.
What are your guidelines when it comes to outdoor running? Or for that matter, any outdoor activity.