Airlines probably class all extinguishers as dangerous goods, as they assume they are stored pressure containers
If dropped an extinguisher could explode, hence why it needs to be securely fitted in a bracket or clamp, so it doesn't roll around
Perfectly safe when correctly fitted, but you wouldn't want it rolling around or dropped in an airline cargo hold
From looking at the data sheet of the Element extinguisher, they are not stored pressure, but the airlines and carriers probably just apply a blanket exclusion and won't carry them, perhaps Element need to educate them
"Element is composed of stable, solid minerals; it does not contain gas and is not
pressurized. The aerosol-like jet is only produced when the charger is struck with its
base. The produced aerosol jet is free of thrust and is essentially an inert salt that emits
gas already present in the atmosphere"
