The one resource you that you might find useful to further discuss my answers can be found in the OSHA eTool for Respiratory Protection.

Please "prowl around" the below website and imbedded links to uncover some more information on the subject topic.  Also, please read all of the disclaimers and warnings in the "Keep in Mind" and other blue highlighted sections.

It is interesting to note that when I was in the 3M Personal Safety Division's Technical Service Department 45 years ago, we obviously did not have the eTools or Service Life Software programs used today.  When customers inquired about the service life of our Organic Vapor Cartridge Respirators, for individual organic vapors, we had to calculate the results by using the Mecklenburg and modified Wheeler equations, which were the predecessors of the referenced Math Models.  

Also, from the period 1972 - 1998, NIOSH approved and certified chemical cartridges came with the following caveat always stated on the respirator manufacturers' NIOSH Approval Labels under the Use and Limitations Schedule, "For use for respiratory protection against chemical compounds with good warning properties."  Good warning properties meant that you could smell, taste or otherwise recognize by irritation, etc., the chemical gas or vapor breaking-through the cartridge bed at a safe level, generally below the Occupational Exposure Limit, a PEL, a REL or a TLV, generally and conservatively, which ever was the lowest before over exposure.  In the earliest of the days,  that was not a problem for the acid gases chlorine, hydrogen chlorine and sulfur dioxide and base bases ammonia and methylamine which all had good warning properties.  That was not the case for carbon monoxide (CO), ethylene oxide (EtO) vinyl chloride monomer (VCM) and, of course, mercury (Hg).  CO and EtO were available as gas mask canisters through MSA, VCM was available as a half facepiece respirator cartridge through 3M and Hg was available as a half- and full-facepiece respirator cartridge through MSA and American Optical Safety (now 3M), all with service life indicators.  Organic vapors was a special case, considering that there were just as many which had good warning properties as those without.  All of this changed in 1998 with the OSHA revisions to 29 CFR 1910.134, the Respirator Protection Standard which included: