What hazards do compressed gas cylinders present?

Compressed gas cylinders pose significant hazards primarily because of the potential for them to be knocked over. When a cylinder is knocked over, the pressurized gas inside can escape rapidly. This sudden release of gas can lead to the cylinder behaving like a missile, potentially causing severe injury or damage.

The violent release of gas occurs due to the high pressure contained within these cylinders, which can result in the cylinder becoming a projectile if the valve is broken or removed. This risk emphasizes the importance of proper storage and handling of compressed gas cylinders to prevent accidents.

In contrast, the other choices present misleading interpretations of the dangers associated with compressed gas cylinders. For instance, a cylinder cannot harmlessly leak gases if it is compromised; instead, it can very much be a source of danger. Additionally, while cylinders may be difficult to move manually when full, they are indeed transportable when proper protocols are followed. Lastly, stating that they are not hazardous overlooks the significant and well-documented risks associated with improper handling and storage.