Style A—Style A vests soak up water that slowly evaporates to keep you cool for up to four hours. Soak them in cold water for a few minutes, and polymer crystals in the fabric will absorb large amounts of water. As you move, airflow creates a cool, damp sensation to lower your body temperature. Lightweight and flexible, these vests are made of nylon, with a mesh bottom to help with ventilation.

Style B—Style B vests hold water in a reservoir, which slowly evaporates to cool you down. Though they don't get as cold as other options, they last much longer, keeping you cool all day. Plus, you'll only need to refill the reservoir every three days. The most durable option, they have a rigid polyurethane shell, with a nylon liner for comfort. These vests have elastic hook and loop straps, making them the most adjustable style.

Style C—Style C vests have cooling packs that stay at 64° F for up to two hours. They don’t require airflow to cool you down, so you can wear them indoors as well as outdoors. Rechill the cooling packs in a freezer, fridge, or even in front of an air conditioner. To keep cool for longer, keep replacement cooling packs on hand.