Most of us use respirators that utilize chemical cartridges to purify the air coming in. These cartridges get saturated with contaminants after a while and have to be discarded, but anyone who uses a respirator already knows that.
Many don't know that leaving a respirator out on the counter or hung on a hook causes the cartridge to become saturated. Air moves in and around the cartridge and, over time, a perfectly good cartridge becomes unusable. That's why replacement cartridges are kept in sealed containers.
We find that the best way to prevent premature cartridge failure is to keep the respirator in a large ziplock freezer bag. These bags are inexpensive and do a very good job of sealing the respirator from outside air and other chemical contaminants. It also keeps the respirator free from dust while it sits around the workshop.