The weather is starting to heat up and you should adjust your workout routine (if you exercise outside)! Just a quick note here: you will get quicker results running outside than you will on a treadmill or other machine! When you run or exercise outside, you must generate all the momentum, fight against gravity's forces and fight against the elements (wind, etc.)!
You should always stop exercising if you feel sick, have chills, headache, severe muscle burning or aching, dizziness or blurred vision. If your symptoms don't subside in a few minutes, you could be headed for a heat stroke that can kill you! During exercise or running events lasting 2-4 hours, fatigue, dehydration, muscle weakness/cramps and decreased coordination is caused by low levels of fluids, salt or glycogen (fuel from carbohydrate calories). It is important to keep your fluids, salt and glycogen levels adequate during competition because it is often too late once the symptoms show up!
Endurance events, such as 10K runs, require your heart to efficiently pump oxygen in your bloodstream from your lungs into your muscles. A study from the University of Connecticut shows that with dehydration, your heart beats with far less force, so it pumps far less blood with each beat. Therefore, an inadequate amount of oxygen reaches your muscles.
You also need to be aware of water intoxication (Hyponatremia) when exercising in the heat. Hyponatremia is basically a low concentration of sodium in the blood. Sodium is required to draw and distribute water through membranes in the body. When you sweat, you lose water and salt. If the sodium levels in your blood get too low ( hyponatremia ), you will no longer be able to move water across your body's membranes and you will become dehydrated - even if you are drinking enough water. Some of the signs and symptoms of hyponatremia include bloating, upset stomach, nausea, headaches, cramps, disorientation, slurred speech and confusion. Untreated, hyponatremia and dehydration can lead to collapse, convulsions, and sometimes even death!
Don't depend on thirst to tell you when you lack fluids. When you are going to exercise for more than a couple of hours, especially in hot weather, drink small amounts of water frequently and eat salted snacks or consume a sports drink.