Which statement about compressed gases is correct?

The statement regarding the necessity of keeping cylinder valves closed except when the cylinder is in use is a critical safety practice when handling compressed gases. This action helps to prevent accidental releases of gas, which can pose serious hazards, including fires, explosions, or the risk of asphyxiation in confined spaces. Keeping the valves closed when the gas is not being used ensures that the gas cannot escape accidentally, thereby maintaining a secure environment.

In contrast, mixing gases without proper training can lead to dangerous reactions. Using oxygen as a substitute for compressed air is highly discouraged due to the risk of combustion; oxygen can dramatically increase the intensity of a fire. Lastly, cylinder caps should be in place during both transportation and storage to protect the valve from damage and to prevent gas leakage, highlighting a misunderstanding in the statement about the need for caps during storage.