original single-sided american depression era antique industrial cold-rolled porcelain enameled steel "not enough oxygen" danger or cautionary pennsylvania coal mining shaft sign designed and fabricated by the ready-made sign company, new york city, ny. the heavily corroded sign is very unique in terms of its message. the two opposed holes once contained steel grommets. the porcelain finish is discolored along the edges, but remains mostly intact within the center.