Oil refinery safety signs. The industrial-scale refining of oil requires high temperatures and high and low pressures, which can be dangerous if not tightly regulated. These warning signs tell workers that they must wear the appropriate protective equipment to protect their heads, eyes, feet, hands and ears. There is also a warning that poisonous gases may be present, and that many of the substances in oil are carcinogens (substances that induce cancer). Photographed at ExxonMobil's Fawley Oil Refinery and Chemical Plant, Hampshire, UK, which is one of the largest petrochemical plants in Europe.