"Danger - Oxygen Storage, No Smoking, Open Flames" Sign By SmartSign | 7" x 10" Plastic DURABLE PLASTIC. Signs use 55 mil thick HDPE (high density polyethylene) and can last up to 2 years outside. Signs have a 100% recyclable base material. ROUNDED CORNERS. Signs have rounded corners and burr-free corners for safe handling, longer life and a professional appearance. PROMOTE SAFETY.Signs speak volumes about your commitment to everybodys well-being by promoting safety. INSTALLATION. Signs have pre-punched and pre-cleared mounting holes for easy installation. Signs mount to walls, fence posts and doors, and can be used indoors or outdoors.