Stainless Steel Butterfly Vent - Center Knob - 5" Butterfly Vents promote air flow preventing mold, mildew, and associated odors. Features a center knob that allows for easy adjustment of the opening. Corrosion resistant 304 stainless steel construction. Installs with four #6 RH fasteners (not included).WARNING: This product can expose you to chemicals which are known to the State of California to cause cancer, birth defects or other reproductive harm. For more information go to P65Warnings.ca.gov.