[RBS] gas mask disposal??

[Rivka Radonsky SLP]

shavua tov
We have a bunch of outdated gasmasks - is there a special way to dispose of 
them, or do i need to save them in order to get new ones the next time (we 
should never need them!) they are issued? Do you only get a new one if you 
swap in the old one, or do they just look at your teuah zehut and give them 
out based on that?
Please advise
thanks
Rivka