Deep in our brain the hypothalamus houses our body's thermostat and monitors our core temperature. When we are exposed to heat and our core temperature begins to rise above 98.6 degrees our cooling mechanism is triggered. Our heart is triggered to pump more blood from our body core to our skin, our sweat glands are triggered to bring moisture to the skin's surface, and as our sweat evaporates into the air it pulls heat from the skin and blood. This cooler blood circulates back to cool our core. For this cooling mechanism to be effective there has to be air moving across our skin into which the sweat can evaporate. The more air moving across our skin the easier it is for our sweat to evaporate. Likewise, the dryer the air the easier it is for our sweat to evaporate. This is why a breeze is cool and a dry day feels cooler than a humid day. In summary our body's ability to cool itself is affected by three things: 1) temperature, 2) humidity and 3) air movement. These three things also determine how hot it feels to us. Our body is able to sense even small differences in temperature changes but is extremely efficient at maintaining our core temperature. As Texas Tech physiology professor Thomas Pressley, PhD points out, "Despite the extremes of temperature that you might expose yourself to, your body temperature never varies more than a couple of degrees. Going from your air-conditioned office to a hot parking lot to your air-conditioned car, your body temperature probably won't vary enough to measure." Even though the impact on our body of going from one temperature to the next has minimal impact on our core temperature we are very sensitive to the change.
The temperature we feel is a function of heat, humidity and air movement. The function commonly used to measure felt temperature is heat index. The heat index formula assumes a steady air movement of 5.8 mph and then combines variable heat and humidity to calculate the temperature our body feels. The Sunstone Fire Series class targets a temperature around 98.6 degrees and a humidity of around 60%. This is a heat index or felt temperature of 124.6 degrees. Because our body is able to sense even small difference in temperature changes, large changes can be startling. When we first walk into a Fire Series class there is little doubt that it is hot. Our bodies immediately begin to cool themselves and we adjust to the temperature. Conversely, when we leave a Fire class 90 minutes later the lobby feels substantially colder than before class. The lobby has not gotten colder but it feels that way because we have adjusted to a higher temperature and also because we have sweat on our skin. Even during class small changes in the heat and humidity can make the room feel hotter or colder. These small changes will have no impact on our core temperature and very little impact beyond our skin surface, but they are likely to be felt. For example if the temperature rises 1 degree to 99.6 and the humidity rises 1 degree to 61% this represents a heat index of 129.3 which is a 4.7 degree felt temperature changei.
Until our body is acclimated heat exposure creates stressii. Our body's ability to function in the heat is not a measure of our physical condition rather of our heat conditioning. Being in good condition will help but only exposure to heat will allow our body to adjust. The following elements improve with exposure:
- Core temperature
- Sweat rate
- Blood lactate
- Blood and urine osmolality
- Rating of perceived exertion
- Skin temperature
- Heart rate
- Plasma volume changes
- Hydration status
- Index of thermal strain
Full acclimatization takes as long as 14 days but most of us will feel more comfortable after only a few consistent classes. This chart shows the number of days acclimation takes for certain elementsiii
The three main ways the body adapts with exposure are:
- Our blood volume pumped to the skin increases
- We start sweating at a lower body temperature
- Our hormones adjust so we sweat more water and less salt
Because of salt (electrolyte) loss prior to acclimatization (number 3 above) it is essential that we take electrolyte replacements. This is also true when we increase exposure by taking more classes.
It is absolutely critical that our bodies be given enough water to remain hydrated. Hydration must occur prior to heat exposure and/or exercise. Our body can absorb one-quarter liter per 10 – 15 minutes. For most of us water loss due to sweating is greater than one-quarter liter per 10 – 15 minutes. A dehydration of only 2% body weight has been shown to reduce performance by as much as 10%iv. We should try to avoid dehydration by consuming 2/3 ounce of water per pound of body weight each day. This is any addition to any water we sweat out during class which will be as high as 3 liters per hourv. During class we should avoid wiping sweat from our body. The removal of sweat does not help remove excess heat but instead stimulates additional sweating and therefore additional water loss.