For two and a half centuries, the human body's ability to cool itself through sweat has been treated as a solved problem — a triumph of biology over heat. Now, engineers at Arizona State University have found a quiet flaw in that confidence: in still, dry air, the very act of sweating can create invisible air currents that work against evaporation, leaving the body hotter than our safety models ever predicted. It is a reminder that the most ordinary and universal of human experiences — perspiration — still holds secrets with life-or-death consequences for workers, soldiers, and athletes in the