The UnderWarmer® is a single-use, compression fitted base-layer shirt. 8 HEAT PACKS are built-in and will activate within minutes of removing from its packaging. When ready to use, simply remove The UnderWarmer® from its packaging. Upon opening, heat activation begins and will take approximately 30 minutes to reach its maximum temperature of over 100f/38c.

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UNISEX SIZING (inches): Youth sizes 60-100 lbs Small Chest: 30 - 34 inches Medium Chest: 35- 39 inches Large Chest: 40 - 44 inches X-Large Chest: 45 - 49 inches