When Is It Too Hot to Exercise?

The thermometer doesn't tell the whole story when it comes to deciding whether it's too hot to exercise. Based on both relative humidity and the air temperature, the heat index gauges the apparent temperature that your body feels and your risks of heat-related illness in hot weather.

If the air temperature is 100 degrees Fahrenheit with 55% humidity, for example, the heat index is 124 degrees. You feel much hotter because your sweat won't evaporate as easily. Conversely, if the heat index is lower—say 15%—that same 100 degrees actually would feel like 96.

You can view the heat index chart at the National Weather Service, as well as on many weather apps and weather reports. The index accounts for shade, so if you'll be exercising in direct sunlight, without shade, it may be up to 15 degrees higher.

When to Cancel Outdoor Exercise

The National Weather Service states these potential risks after prolonged exposure and/or physical activity:

• Caution: 80 to 90 degrees Fahrenheit, fatigue possible
• Extreme caution: 90 to 103 degrees Fahrenheit, heat stroke, heat cramps, or heat exhaustion possible
• Danger: 103 to 124 degrees Fahrenheit, heat cramps or heat exhaustion likely, and heat stroke possible
• Extreme danger: 125 degrees or higher, heatstroke highly likely

If you are trying to decide whether to walk or run outdoors, check the heat index to see whether it will be below 80 degrees during your workout. If your route doesn't have shade, factor in another 15 degrees to the heat index. And keep in mind that women are typically affected by heat illness more than men, due to their higher percentage of body fat and lower aerobic power.

Risk Factors for Heat Sickness

Certain factors can increase the risk of heat sickness. If they apply to you, take extra precautions when the heat index is high.

• Dehydration
• Diarrhea, illness, certain medications
• Lack of acclimatization to heat
• Obesity
• Poor physical fitness
• Previous history of exertional heat sickness
• Sleep deprivation
• Sunburn

You can acclimatize yourself with 10 to 14 days of exposure to hotter conditions for 60 to 90 minutes per day. By doing so, your body adapts and begins to sweat and try to cool itself sooner during an exercise session.

Environmental Heat Factors

There are several factors that affect how hot you feel when exercising outdoors. These all impact the way your body is heated and how it can get rid of excess heat.

• Heat from the pavement: When you touch something hot, the heat is transferred directly to your skin. This is known as conduction and it happens with hot pavement or asphalt heating your feet through your shoes.
• Heat from the sun: Sunlight warms you with radiation. Its electromagnetic waves directly heat your body and other surfaces without actually touching them.
• Heat from the wind: Air can carry heat with it from one object to another. This is known as convection. The stronger, more turbulent the wind, the more heat you will lose.
• 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.

Humidity and Dewpoint

The humidity and dewpoint both measure the amount of moisture in the air. You perspire in order to get the benefits of the cooling that comes when the sweat evaporates. This is the primary way your body keeps from overheating when it is hot outside or with exercise (or both).

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.

A Word From Verywell

No workout is worth risking heat sickness. A hot day is a good day to take your exercise indoors where there's shade and air conditioning. You are more at risk of heat effects with high temperatures, high humidity, no wind, and being exposed to the rays of the sun. Put in the hard outdoor workout on a better day.