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Helly Hansen says:
A unique, lightweight, 2-in-1 base layer with 100% merino wool exterior and 100% LIFA® Stay Warm Technology interior.
LIFA® MERINO LIGHTWEIGHT is perfect for any type of activity in cold weather. A unique 2-in-1 base layer with 100% merino wool combined with LIFA® Stay Warm Technology in a extremely lightweight 2 layer construction. LIFA® Merino LIGHTWEIGHT delivers all the great benefits of merino wool fibers plus the unique properties of technical LIFA® fibers.