What Could Be The Feeling Of Smell And Kind Of Smoke That Comes And Goes?

mo coworker is a 67 yr old lady in general good health, she keeps complaining of a smell, kind of smoke, that comes and goes. another lady about her same age also can smell it from time to time. me and my other younger co-worker dont smell it, we evn had to move to another office that was used as a conference room but still there they complain about the smell. i just feel it in my thoat and eyes sometimes, like burning sensation but we are really worried right now and we cant determine what its about. the maintenace guy and the manager for the building have been trying to find out if any incense is being lighetd up but nothing found.