Ergodyne - EGO12543 The Chill-Its 6665 Evaporative Cooling Vest is made with an advanced polymer-embedded fabric to provide workers with immediate core-cooling relief from the heat. It is made of comfortable and durable, quilted nylon exterior with mesh side panels for ventilation and stretch. For best results, wear this lightweight, body-cooling vest over a T-shirt. Water-repellant liner keeps the interior of the vest as dry as possible when activated though you should expect to feel some dampness/wetness. This simple, effective and comfortable solution for heat stress prevention is similar to how sweat cools the body. When activated, this cooling safety vest is comfortable and refreshing for even the hottest days of the year. Medium size fits chests from 36" to 38".