General Question

rebbel's avatar

Why does a scent fade away in time?

Asked by rebbel (35553points) August 26th, 2011

Yesterday when I entered the (empty) elevator I smelled a strong scent of chinese food.
It was pretty strong, i suspect the one carrying it had just left the lift before I came in.
I didn’t like it very much I don’t know why exactly, because I do like it when it is on my plate, but half way to my floor the scent had ‘faded’to a mere whiff, pretty bearable.
I find the same thing when I enter a toilet where the person before me just did a number two.
Which made me wonder:
Why does a scent seem to get less strong in (a relatively short) time, while with other senses that doesn’t seem to be the case the statue that you see doesn’t fade, the cd that you hear doesn’t fade, the fabric of your favorite sweater doesn’t fade when you touch it?

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13 Answers

Nullo's avatar

Smells are physical things. They wear off in time.

ANef_is_Enuf's avatar

Well the scent disperses into the air. Your sweater doesn’t.

Blondesjon's avatar

What we are smelling are molecules. They eventually dissipate to smaller and smaller ppm in the air you are breathing until there is nothing left to smell.

rebbel's avatar

I think I understand that all three of you say similar things, right?
And it does make sense to me, thank you!
But in my question there is the mention of an elevator which is a small space, does the dispersing thing still apply then?

Christian95's avatar

@rebbel the elevator has vent systems which refresh the air,the air in an elevator is not a closed medium,otherwise you would die of CO2 poisoning

ucme's avatar

Well, the fart in an astronaut’s suit, now that’s a keeper!!

Nullo's avatar

There’s also the fact that your brain recalibrates to keep the novel at the forefront of your attention. You tune out that CD, you don’t pay attention to the colors in teh sweater, you forget about the rock in your shoe, and the statue fades into the background.

WestRiverrat's avatar

You also have the ability to process scents and eliminate them from your perception.
I used to work in a print shop…and the chemicals and inks we used STINK.

After a while I didn’t even notice the smells unless someone mentioned them.

rebbel's avatar

Interesting stuff, guys…, thanks for that!

Judi's avatar

in 9th grade science I think they called it dispersion

gasman's avatar

Many phenomena like the dissipation of odor molecules have approximately first-order kinetics, which means that the more there is the faster it dissipates. The concentration will fall with a characteristic half-life as it approaches zero over time.

This is different in character from light (vision) and sound (hearing), not just because they follow inverse-square laws for distance, but because there’s no time period as with odors. Light and sound both blow past you very quickly.

Diffusion of gases across stagnant air approximately follows Graham’s Law, inversely proportional to square root of molecular weight. I’m not sure how much bearing this has on the smell of Chineie food, however. Besides gas-phase odorants there are probably aerosolized droplets carrying some of the smell. Greasy droplets stick to surface, so this probably helps clear them from the air faster. Also, when people get on and off an elevator there is significant air exchange (convective mixing) so it’s conceivable that all smells will quickly dissipate down to a certain level, unless it’s something rotting in the elevator.

Eventually the concentration goes below your olfactory threshold, which is not so remarkable compared to some animals’ sense of smell. Gypsy moths have been shown to emit nerve signals from their antennae in response to individual pheromone molecules.

LostInParadise's avatar

I would guess that complex molecules eventually break down. Otherwise we would be surrounded by the smells of all the things that have ever given off scents.

jrpowell's avatar

Mr Wizard did a thing on this. Put on a blindfold and have someone cut a onion in half and hold it up to your nose. Raise your hand when you think they moved the onion away. This was a science show for elementary school kids and he explained it as “You get filled up on a certain smell”.

The same reason smokers don’t realize that they stink.

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