I have a a AOS Safety respirator with P100 cartrideges and i think it is time to replace the cartridges, i have used it for hobbies when airbrushing, lathe working, fiberglass sanding, and enamel spray painting. How do i tell when the cartridges need to be replaced? Also i was wondering, would it be possible to convert it to a forced air respirator, by taking an old pair of cartridges and cutting out all the filters and stuff and attaching to air hoses, one to each cartridge and using dryer duct alluminum tape to make it air tight. Then attaching some type of fan to it, or attach it to the houses a/c vent? Would this work?