How does smell disapear? in the terms of a gas, since gas particles do not just dissapear.?

Do gas particles diffuse so much that the smell is no longer traceable to a human becasue of the now small concentration, or do the gas particles eventually condensate or change their state of matter along the way where it is also not as easily picked up? Also if you have any other theories feel free to share them thank you, i will pick a best answer. i also would like to add i was thinking of this either like you made dinner and the house smells good the next day or in the terms of a gas with an odor like ethanol.