Heat and Humidity
One should consider taking their workouts indoors as the outdoor temperatures rise, especially if combined with high humidity. Even with temperatures at 82 degrees with 100% humidity, there is a possibility of developing heat cramps or heat exhaustion. And when temperatures soar into the 90’s, humidity levels of just 20% can present the same level of risk. Higher temperatures, combined with humidity can result in a greater likelihood of these heat related injuries and a humidity level of 100% combined with temperatures of as low as 91 degrees may result in heat stroke. (See below for complete list of temperatures, humidity percentages and potential risks.)
When you exercise, your body cools the internal temperature by sending blood to the capillaries of the skin’s surface to release water through the sweat glands, which will cool the body through the evaporation of sweat. When the humidity is high, and you are dripping sweat, the cooling process is not working up to speed. The idea is for the sweat to evaporate, and dripping indicates that there is already a lot of moisture in the air. This can result in reduced performance as your heart continues to send blood to the skin’s surface to cool the body. And of course, keep in mind that the more you sweat the more fluids you need to take in to avoid dehydration. But don’t wait until you get thirsty… that’s a sign that you are already well on your way to dehydration, which can cause your body’s temperature to rise and blood to thicken, further increasing your risk for heat related illness.
Most local weather stations regularly report increased risks associated with the air quality based on the Pollution Standards Index (PSI) which ranges from 0 to 500. At levels of 100, people who are very unfit or have cardiovascular or pulmonary disease are at risk, and levels greater than 150 can affect normally healthy individuals.
Local officials use a simple scale to forecast and report on smog levels and other air pollution. Depending on where you live, it might be called Air Quality Index (AQI) or Pollutant Standards Index (PSI).
Current air quality is reported as a percentage of the federal health standard for a pollutant. If the current index is above 100, air pollution exceeds the level considered safe.
At ozone smog levels above 100, children, asthmatics and other sensitive groups should limit strenuous exercise. Even otherwise healthy people should consider limiting vigorous exercise when ozone levels are at or above the health standard.
If the index is above 200, corresponding to an ozone pollution level of 0.20 parts per million (ppm), the pollution level is judged unhealthy for everyone. At this level, air pollution is a serious health concern. Everyone should avoid strenuous outdoor activity, as respiratory tract irritation can occur. (Source: American Lung Association)
Outdoor Exercise Recommendations
If you do choose to exercise outdoors, make it during the cooler morning or evening hours and be sure to drink plenty of water before you venture out. Carry a water bottle with you to drink 2-4 oz of cool water every 15 minutes during your exercise session. Wear loose fitting clothing that allows air to circulate on your skin and avoid dark colors which can further absorb heat from the sun.
Pay attention to your daily weather forecasts for information on the relative humidity, heat index and the Air Quality Index or PSI. If all is indicated as safe, it is still important to allow your body to adapt or acclimate to exercising outdoors by starting with sessions that are shorter than your usual sessions done indoors.
If risk signs are present, especially if you are a new exerciser, take your exercise indoors where you can get the most out of your time. Although most people think that the more you sweat, the greater the weight loss, that’s not necessarily the case. Exercising in cooler conditions allows you to exert more energy at a higher intensity thus burning more calories – sweating simply means that you are losing water and that your personal air conditioning system is hard at work.
So be smart, drink up and stay cool!
Heat / Humidity Chart
Heat cramps, or heat exhaustion possible.
- 93 F (34 C), 20% humidity
- 87 F (31 C), 50% humidity
- 82 F (28 C), 100% humidity
Heat cramps or heat exhaustion likely.
- 105 F (41 C), 20% humidity
- 92 F (34 C), 60% humidity
- 87 F (31 C), 100% humidity
Heat Stroke imminent.
- 120 F (49 C), 20% humidity
- 108 F (43 C), 40% humidity
- 91 F (33 C), 100% humidity
Reference, 1993 American Red Cross Standard First Aid Manual