Here's what other sites are saying:
Focus on winter fun without worrying about the cold in the Hot Chillys midweight long underwear zip top. Moisture Transfer Fibers move moisture away from the body for maximum dryness and comfort. Form fitting, anatomic design provides support and keeps you warm. Permanent, antimicrobial silver ions incorporated into fibers minimize odors. Versatile zip T-neck styling lets you zip up for maximum heat retention and unzip for ventilation. Flatseam construction is soft and non-chafing next to skin. Fabric provides UPF 30 sun protection, shielding skin from harmful ultraviolet rays.
- REI refers to the men's version