From : http://encarta.msn.com/encnet/features/columns/?article=bn_smell
How smells travel
Smells travel in the air by what's called diffusion. Molecules are always moving, bumping and jostling each other, so they mix. The rate of natural mixing or diffusion depends on how often molecules encounter one another. The same with how long it takes a smell to dissipate. If the wind is blowing, there will be more mixing. That can either make the smell reach you sooner, or it can spread the smell out so that you don't notice it at all. Predators such as lions, tigers, and bears always approach their prey from downwind to keep their own scents from alerting whatever they're hunting to their presence.
The speed with which a smell travels depends on how fast the molecules are going, how massive they are, the relative temperature of the molecules making the smell, and how many molecules there are in a given volume, their density. We express all this mathematically as a gas's temperature and pressure. By the way, our brains are quite used to air, so for the most part, we don't smell oxygen, nitrogen, or carbon dioxide; we smell the other molecules instead.
