Our internal system handles excess heat in a different way, and pretty much everybody knows how we do that–We perspire (i.e. sweat). When it gets hot–or when we create excess heat through vigorous activity–we secrete moisture through our skin pores, and it evaporates, which creates a cooling effect. This process generally works very well, but we sometimes forget a few things:
We must drink additional fluids to replenish what we are constantly losing.
In a sense, our bodies are "water cooled." We drink fluids and some of them end up being released as perspiration. Of course when we do that, we lose water–sometimes at a tremendous rate. In order to keep our body's own cooling system working properly, we must continuously replace the water we are sweating out through our skin. Simply put, the more we sweat, the more fluids we must drink–even if we don't feel thirsty. That's especially true for people exposed to hot and dry conditions.
Sometimes environmental conditions inhibit our body's ability to cool itself.
High humidity is the sworn enemy of the human body's cooling system. Why? Because our cooling system relies almost exclusively on evaporation to work, and humidity has an enormous influence on the process of evaporation.
The term humidity level is a measure of the amount of moisture in the air by percent as compared to the amount of moisture the air could hold at that temperature and barometric pressure. We say "at that temperature and barometric pressure" because the amount of moisture the air can hold varies by temperature and pressure–which can get complicated, so we'll end that discussion here (you're welcome).
What's really important about humidity level in regards to cooling the human body is that it is also a relative measure of how effective evaporation will be. The more moisture the air is holding, the less it can absorb additional moisture, so the process of evaporation becomes less effective as humidity level rises. That means our internal cooling system becomes less effective as the humidity level increases.
So here's the problem–We need evaporation to stay cool, but evaporation diminishes as the humidity level rises. The result is that when we get that 90ºF, 80% humidity day, the sweat we produce doesn't evaporate very quickly, it just sits on our skin, making us feel wet and sticky. And guess what happens when we don't get the benefit of evaporative cooling? We get hot. Sometimes very hot.
We may not notice the cooling process is being overwhelmed.
We humans can be a thick-headed bunch–especially when we're working or playing hard. In the heat of battle, it's easy to miss–or ignore–the signs that something's amiss. Having trouble concentrating? That's because your blood is thickening as your body begins to dry out. Brains don't care for thick blood. Makes them sloooow. Also makes it difficult for them to recognize other dehydration symptoms.
Have a headache? Feeling dizzy? Muscle cramps? Seeing spots or stars? These are all signs you may be dehydrated, and you need to take immediate action by drinking fluids and taking quick steps to cool your body down. Even then, you may still end up in the hospital with an IV needle plugged into your arm. Nice.
We may not realize the cooling process is failing.
If you ignore signs like headaches and dizziness, the next thing that's likely to happen is that your body will begin to shut down. You may faint, have a seizure–any number of things (all very bad, by the way), and trying to drink water at this point may not help at all–because you're too far gone. More than likely, you'll be watching the world go by through the rear window of an ambulance–if you're conscience, of course. . . .
So why hydrate? Basically, to keep your body from drying up and shutting down on you. If you must deal with hot conditions, hydrating is probably the most important thing you can do. Just drink plenty of fluids (not all sports drinks, and not all water–mix it up). Simple, right?
And consider this–If it's really hot, or you're working very hard, you may not be able to drink fluids fast enough to replace what you're sweating out. So if you know you're going to be working or playing in hot conditions, prepare a couple days in advance by drinking extra water.