What Is This????????????

Recently, I have noticed that after I have stayed in a closed-door(house, office) environment for a few hours, and then I go outside, when I come back I feel an unpleasant smell. It is a sweet, pungent, slightly "rotten" smell that I can only feel on the back of my nose. This is the same smell that my 67 year-old father leaves in any closed-door environment. Some people would call this "old people smell". I am 31 now and recently I have getting this same smell every time I stay indoors for many hours in the same space. Even when I leave all windows open, I still sense the smell. I cannot sense it outdoors. What is this? Is it caused by bad breath? I think it is my own respiration which is causing it, since I only feel it indoors after I have stayed in the same place for hours and never outdoors, and this smell follows me and becomes apparent in any indoor environment I stay for many hours. The other reason why I think it is caused by my own respiration is because it is a slightly "rotten" smell of putrefaction. But it is not really a bad breath smell, but more of an old people bad breath smell. Is this smell caused by my own exhaled air? If so, is it Carbon dioxide I am smelling or a putrefaction odour caused by bacteria? Please answer me this question. I really need to know the answer to this one. Thank