How do compressed gases need to be marked?

Ensure that cylinders, containers, pipes, etc. are clearly marked so that the gas can be easily identified. Labels should not be defaced or removed. Do not accept or use containers whose content labels are not legible. Segregate these items and return them to the supplier. Do not use the container color to identify the cylinder content; do not repaint the container.

In addition, all gas lines leading from a compressed gas supply should be clearly labeled or identified (some states require specific labeling of cylinders, containers, and pipes).