I was motivated to bottom up research vs your top down and apparently technology has advanced and the cool kids these days are using obscure semiconductors that drop in resistance as reductive (as opposed to oxidative) gases adsorb on the surface, then periodically they heat up and cook off the adsorbed stuff (otherwise they'd be permanent total dosage sensors).

http://www.ewinsen.com/Admin/uploadfile/201209/2012916105693...

It ends up being the same argument on a high level that for both operational and practical reasons the sensors usually respond to anything that burns / reduces in the presence of oxygen regardless of specific chemistry and design.