The extreme heat of mid-summer can make it tough to stay motivated to exercise. It can also be downright dangerous without the proper precautions
. We discussed staying safe in the heat with the father of aerobics, exercise guru Kenneth Cooper, MD.
Moderator: Welcome back to WebMD Live, Dr. Cooper. How's the weather where you are? Hot?
The weather is hot, humid, and today expected to be about 100 degrees with humidity at 50%. The humidity is an important point, because anytime you have temperature and humidity together, and the number exceeds 150, the risk of heat stress is very high. Only people who are acclimatized should be allowed to exercise out of doors with that kind of heat, and I would advise them against vigorous activity. So, short and slow type of activities, if any, ought to be conducted at this time of heat stress. It is the only thing I'd recommend.
Moderator: You mentioned high humidity adding to the potential danger of exercising in extreme heat. What about very dry conditions, such us in the Southwest?
Cooper: If you have a temperature of 100 degrees and humidity of 30% or less, that doesn't carry with it the same risk of heat stress as having a lower temperature, say 90 degrees, with humidity of 60%. So the humidity is extremely important. We have a lot of unexpected heat problems in situations where the temperature is not that high, but humidity is approaching 90%-100%.
Moderator: Tell us about different age groups and how they acclimate differently to the heat.
Two age groups the most susceptible to heat are the young and the old. One of the problems is that older people just can't handle the heat stress like they could during their youth.
The reason kids absorb more heat from their environment is because they have a larger surface area, the body mass ratio, than adults. The smaller the child, the faster the heat is absorbed. Elderly adults are at danger because there are subtle age-related differences in body fluid balance. Elderly adults must practice a more gradual acclimatization to heat, emphasizing hydration. The reason for that is that elderly adults are more likely, normally, to be dehydrated. If you add dehydration
to excessive heat in an elderly adult you are dramatically increasing the risk for heat stress.
Other groups who are very likely to suffer heat stress include the people who spend much of their day in the heat, such as landscapers, construction road crews, and traffic police. With little chance for either fluid consumption or bathroom breaks they tend to drink less fluid throughout the day because bathrooms are not accessible. So it's very important for people in that situation to consume adequate fluids, and make the bathroom breaks whenever necessary.
Finally, athletes and exercisers who spend a lot of time training and competing in the hot daylight hours typically do not drink enough to match fluid loss. Why? Exercise and activity increases the core body temperatures to a point that your body's air conditioning or sweat turns on. As sweat evaporates from your skin, it cools the body. Also as blood circulates between the skin and hot muscle it helps transfer heat out of the body, keeping your body's core temperature in the safe zone. When exercising in the heat, your core body temperature rises more rapidly, because the warmer air makes it harder to dissipate heat from the body.
Your body adapts to warmer weather, as acclimatization, by learning to sweat sooner, and to sweat more, to help keep the body from reaching dangerous heat levels. By sweating sooner and faster, you lose more fluids than before. This means that dehydration is more likely when you're acclimatized to heat.
Seems strange, but true. Dehydration can dramatically impair your performance and increase the risk of heat illnesses. Heat illnesses, such as heat exhaustion and heat stroke, are major health hazards and in extreme cases can be deadly to the extent that on the average we lose about 300 people to heat illnesses each year.
Does it place extra strain on the heart to work out in the heat?
Cooper: If you exercise in the heat, because of the acceleration of the heart rate that occurs just with heat exposure, and you add to that the acceleration of heart rate with exercise, we call that a synergistic effect, or one plus one equals three. That means you get more caloric expenditure, but also much more stress on the heart. That's the reason we say that people that have heart disease of any type should avoid vigorous exercise in the heat. Even when acclimatized I still say avoid vigorous exercise in the heat if you are suffering from heart disease.
Moderator: Dr. Cooper, before we wrap up for today, do you have any final comments for us?
Cooper: We did studies years ago when I was at Lackland Air Force base in San Antonio, Texas. I was in charge of training air force recruits. We had several deaths that occurred during basic training among young men who had come to the hot humid environment from the northern states and had no heat acclimatization. We did studies that looked at the rectal temperature before and after running two miles. It was not uncommon to have temps rise from 98.6 to 105 and 106 degrees. Those men, after the run, would either lower their temperature rapidly, or have a heat stress problem.
We developed a series of flags based upon the heat index that combines humidity, dry bulb temperature, and wet bulb temperature with ambient temperature. From that index we developed four flags: Green, yellow, red, and black. All outdoor activities were canceled in a black flag situation, and we had no further problems with heat stress.
Any football team exercising in the heat should use that same guideline that we developed in the Air Force, which essentially solved our problem. You can check with the weather bureau to get that index, and you can get the categories there.