For example, a couch that smells like a household animal, a stuffed animal having that “grandma” smell, or a carpet peed on and not properly cleaned: it seems like these sorts of smells seep into the object and become nearly impossible to remove economically. You can mask smells with air fresheners, but those usually fade, and they just impart their own opinionated odor to the object. What about odors makes them so inherent to an object over time?
It is because you are not able to clean it thoroughly. On things with volume, or multiple layers such as a couches or carpet you might only be cleaning the top layer. While the inner layer will still have the stain, smell, as well as bacteria and dust (skin cells, dirt) that the bacteria eat and produce the smells.
I believe it has to do with how porous or not much of our surroundings and the objects within our environment may be. The more porous the more likely to absorb and maintain the odor introduced to it.
Items made of less porous materials such as glass don’t often carry or hold odors within them once they’re cleaned very well.
It’s all down to three factors : smell retention potential of the material, smell sensitivity of the olfactive receptors to said smell, and chemical stability of the smell molecules.
Two different smells will be detected at two VERY variable concentration levels. Then some materials will be hard do clean (think carpets, textiles) as they are on a microscopic level acting like a sponge or a system of caves, compared to order surfaces (glass, PVC flooring, etc.). And finally some molecules will degrade very easily, some others are impossible to break using common cleaning substances or even just UV rays or air oxydation