Best-selling ActiveIce technology works more efficiently and effectively than ever before. Outdoor Researchs proprietary self-cooling technology now reacts to increases in your body temperature by dynamically cooling you faster when you need it most. A special additive drives this cooling process via phase changes, which keeps the wicking, breathable, and quick drying ActiveIce technology the ideal solution for anyone who spends serious time in hot, sweaty, sun-baked conditions.