The Glacier Tek Sports Cooling Vest is designed to keep you cool during and after strenuous physical activity. The vest features dual side elastic straps and over-the-shoulder adjustability for a secure fit on a wide range of body sizes. Each vest includes a set of eight cooling packs that maintain a cool -- but not cold -- temperature of 59° F for up to 2.5 hours in 100° F heat. This booster bundle contains an additional set of four booster packs, which can be used to supercharge your standard pack set. All of those packs are powered by PureTemp, a nontoxic, biobased phase change material known for its consistent, repeatable performance over thousands of melting-solidification cycles.