What precaution should be taken regarding cylinder valves?

Ensuring that cylinder valves are closed when not in use is a crucial safety precaution. This action helps prevent accidental release of gas, which could lead to hazardous situations such as leaks or explosions. Compressed gases can be under high pressure, and an open valve increases the risk of gas escaping unintentionally, potentially causing asphyxiation or fire hazards.

The other options may not align with best practices for handling gas cylinders. Keeping valves open, for instance, contradicts safety protocols as it poses a risk of unintentional release. Regularly lubricating valves is not typically recommended for most types of gas cylinders, as certain lubricants can react with the gases or create dangerous conditions. Storing cylinders with valves facing upwards does not specifically address valve safety and could be misleading, considering proper storage involves secure placement with appropriate protective measures. Thus, closing the valves when the cylinders are not in use is the optimal choice for safety and risk management.