Another form of "filter" breathing is removing the regulator and reversing your hood to cover the hole and give somewhat of a "filter" (NIOSH 2000a). Not as effective as the option with the low pressure hose (NIOSH 1998) but as you said, that option is out the door. But if your out of air anything helps. I haven't heard of anyone using the last chance filters and i think the price tag is the reason. Thanks Capt. Be safe