According to that article Ricky posted, a deer can smell in parts per million levels. Your scent might be spread out, or less dense in the air, but a deer knows the 1 molecule it just smelled is recent enough to be alarmed.
That's not how it works. A molecule is a molecule. The strength/intensity of a smell is dependent on how many molecules the olfactory receptors pick up. Deer have amazing noses but, I seriously doubt they can pick up a single odor molecule. If they could they would be smelling people four county's over.
"The perception of odors, or sense of smell, is mediated by the olfactory nerve. ... Odor sensation usually depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant is usually recognized by many receptors."
https://en.wikipedia.org/wiki/Odor