The body reacts to cold by adjusting the blood flow to different parts of the body, like hands, fingers and feet. The body can not adjust the blood flow to your head. This is to ensure brain function at all times. The colder it gets the more heat you lose from the head by radiation, or by conduction in the water. You stay warmer with an effective hood. Waterproof's H1 5/7 mm Nylon ll Neoprene Rubber Hood with Waterproof's Exclusive Hood Air Venting System (HAVS) is constructed with these scientific facts in mind and are proven to keep you warmer when you need it most. That's what Waterproof refers to as Facing Reality". Waterproof's Exclusive Hood Air Venting System (HAVS) uses Double One-Way Valves to Eliminate Annoying Trapped or Build-Up Air caused by your Regulator's Exhausted Air entering the Hood and features a 7 mm Double Layered Protection in Critical Heat Loss Areas. The Hood has an Anatomical 3-D Shape for a great fit with Glide-Skin Seals, Reflective Logo, I-Span Super-Stretch Nylon for Easy Donning and all Seams are Glued and Sewn with Bonded Hi-Quality Nylon Thread for Strength and Durability. Hood is available in 7 sizes from X-Small to 2X-Large and is covered by a 12 month limited warranty.