Sweat releases heat and cools down the body through evaporative cooling. When sweat escapes through the skin, it carries some of the body's energy with it in the form of heat. Evaporation releases this energy, siphoning it away from the body and cooling the skin and blood vessels. This cools the blood passing through the circulatory system, reducing the body's core temperature.
One of the reasons humid days feel particularly uncomfortable is that since the air is saturated with moisture, sweat cannot evaporate as efficiently. This reduces the amount of heat that can be lost through the process and simply causes sweat to run off the body rather than evaporate away. Lower humidity, as well as moving air, helps sweat evaporate quickly, producing a cooler, more comfortable feeling.
Since sweat contains salt and minerals as well as water, excessive sweating can lead to dehydration as well as a loss of electrolytes. Many sports drinks include added minerals in an attempt to replenish these vital nutrients after an extended workout.
Drinking large volumes of pure water to rehydrate may lead to water intoxication, as the sodium levels in the body dip to dangerously low levels. Common side effects of water intoxication include nausea and cramping, while high levels can result in weakness, confusion, paralysis and even death.