bambū werx running hats are the world's lightest, most breathable, and eco-friendly training caps on the market today. These feather-light training caps are constructed with extra-large, laser-perforated ventilation holes and feature a proprietary "vented" brim which allows airflow over-and-through to one's forehead. Built with bamboo and recycled nylon, our running hats are naturally hypoallergenic and antimicrobial; so, they won’t smell or cause skin irritation like those constructed with man-made materials. bambū werx running hats are "one size fits many" and can accommodate head sizes between 52-58 cm. And as with all bambū werx products, our running hats come with a 30-day satisfaction guarantee. If for any reason you are not 100% satisfied, simply return the (unworn) hat for a full refund!