The COOL-U" Phase Change garment is designed to be worn beneath clothing without the physiological problems that would result from ice packs being placed in close proximity to the skin. COOLING ENERGY is created from a unique PHASE CHANGE MATERIAL (PCM) mechanically sealed in durable inserts. After freezing in ice water or a refrigerator (it freezes at 58 degrees so a refrigerator will freeze the chemical pack) for approximately twenty-five (25) minutes, COOL-U" PCM Cooling Vests deliver a constant temperature of 58F/14C for 2-3 hours. Our vests are ideal for wearing under HazMat and other Protective Suits (like a mascot costume) or as an alternative to the standard evaporative cooling vests which must be worn outside of clothing to be effective. The vest includes 1 set of 4 cooling inserts. Each vest has side and shoulder Velcro" adjustments to accommodate for various shapes and sizes. M-L is for anyone 100-175 lbs, L-XL for anyone 175-250 lbs.