What indication suggests a gas cylinder may be unsafe to use?

A gas cylinder that has visible rust or damage is an indicator that it may be unsafe to use. The integrity of a gas cylinder is crucial for safety, as any corrosion or physical damage can compromise the cylinder’s strength and lead to leaks or even catastrophic failure. Rust can weaken the metal and make it more susceptible to bursting under pressure. Therefore, cylinders should always be inspected for physical condition, and any signs of deterioration warrant immediate consideration of the cylinder as potentially unsafe.

In contrast, a cylinder marked "FULL" does not indicate safety but simply communicates its contents' status. Labeling the contents is standard practice for proper identification, contributing to safety during handling but does not imply that the cylinder itself is safe. Storing a cylinder in an upright position is necessary for most applications, as this is the generally accepted and safe way to store gas cylinders; however, this positioning alone does not address whether the cylinder is in good condition. Hence, while other choices provide key information about the cylinder, none indicate a direct safety concern as clearly as visible rust or damage does.