UnderWarmer Unisex-Compression Fitted Heated Shirt with Air-Activated Heat Packs Enjoy your winter activities like never before!. The UnderWarmer heated shirt is convenient, super comfortable and best of all - it works really well. The UnderWarmer was specifically designed to be affordable and convenient for the everyday guy or gal spending a long day in the cold. That’s why UnderWarmer has some very specific features, like instant air activation (just take it out of the package) and compression fit (so you can feel every bit of heat pressed against your upper body,). For just a few bucks, you have a portable, wearable radiator, ready to go the moment you are. Unlike hand and foot warmers, which only help extremities that have already gotten cold, the UnderWarmer PREVENTS those extremities from ever getting cold. How? By keeping your core warm - because a cold body core is the cause of icy extremities. Best of all, unlike all those bulky coats and layers you’ve tried, the Underwarmer is extremely lightweight and won’t slow you down. You CAN stay warm for the entire day, even in negative digits. Just grab your UnderWarmer! Instant Air Activation Single Use No Batteries Compression Fit 8 Built In Heat Packs Reaches up to 100°F/38°C Unisex Lasts up to 10 hours. SEAL AND REUSE The UnderWarmer is essentially a single-use product. However, If only worn for a few hours place it in an air tight bag and it will stop producing heat. Essentially going dormant.