Nothing screams summer like a good sweat under the hot sun, but if you’re not replacing fluid as fast as your body’s pumping it out of your pores, your outdoor workout could be cut short by sluggishness, cramped muscles, or even life-threatening heat illness. How do you know if you’re in danger?

Symptoms vary, but mild dehydration often causes fatigue and affects athletic performance. As dehydration worsens, it can cause dizziness, cramps, and mood changes. Severe dehydration decreases sweating, which can make your body overheat; lowers blood pressure, which can cause you to faint; and can, perversely, make it difficult to drink without vomiting. Humans can go a few weeks without food, but a few days without water is often enough to cause organ damage, then death.

We asked Douglas Casa, Ph.D., an exertional heat stroke expert and chief operating officer at the University of Connecticut’s Korey Stringer Institute, to pinpoint five dehydration symptoms to watch for when exercising al fresco.