Engineering expert witness Philip J. O’Keefe, PE, writes that pressurized vessels can pose a danger for various reasons.
Suppose for instance that the substance leaking from it is flammable or toxic. An example would be when propane gas leaking from a storage tank mixes with air surrounding the tank. This can create an explosive mixture, readily ignited by static electricity or a nearby ignition source, such as a spark from a worker’s tool. When a toxic substance is released by a leak into an occupied area, it can be inhaled or come into contact with skin eyes, nose, and mouth, where it can enter the body and injure or kill.
It might be obvious that toxic, flammable substances can prove threatening, but it is not quite as obvious that some nontoxic, nonflammable substances can be just as dangerous. For example, a substance can be heavier than air, and as it leaks out of the pressure vessel, it will roll along the floor, sinking into adjacent low spots such as basements and tunnels. This substance would then displace the air, creating a suffocation hazard for anyone who is unlucky enough to be there.