ELI5, how do certain smells seem to travel faster than air currents?


I’m thinking of my middle school science class where the teacher opened a bottle of butyric acid and within a second everyone in the room could smell it even though the air was still. It made our whole school reek for weeks.

I remember being shocked at how quickly the smell traveled. How does that happen?

In: 5

Air is basically always moving and running into things. You might not feel it because the motion is inconsistent and random, as opposed to a breeze that blows **all** the air molecules in an area in the same direction at a similar speed.

Consider that if you sucked the air out of an empty plastic water bottle, it would instantly crumple inwards. What is causing the force to crush the bottle, other than the air molecules that are constantly hitting the outside of the bottle? Once you suck the air out from inside, there is an imbalance of force on the outside air molecules pushing inwards and the inside air molecules pushing outward; this imbalance crushes the bottle. Therefore, there must a ton of air molecules hitting everything all the time.

This is what causes air pressure (or fluid pressure in general) and enables other gaseous molecules like butyric acid to diffuse rapidly through a space.

Our sense of smell does not treat everything equally. We’re very sensitive to certain smells and it takes fewer of those molecules getting into our nose to trigger a response. Butyric acid is a byproduct of bacteria breaking down food. That’s good when it’s inside of our guts and they’re helping us digest what we’ve eaten. It’s *bad* when it’s outside our guts because it means either something died and it’s rotting, or something pooped. Either way, it’s better for our health if we avoid it.

We have similar strong reactions to molecules with sulfur, especially sulfate compounds. They are often the product of nasty anaerobic bacteria which show up when things are rotting. Those compounds are the “rotten egg” smell.

Our strong sensitivity to these compounds evolved to protect us, so that we avoid eating rotten food that might be full of harmful bacteria and parasites and whatnot. The result is that only a tiny amount of these compounds in the air is enough for us to notice them. Compare that to, say, the smell of grass that is everywhere, all the time but we don’t really smell it because we don’t need to. It’s only when there is a *lot* of it that we notice.

Air and molecules are always moving, the air movement you sense and feel is just a portion of the actual flows always happening — the portion that happens to be enough to trigger senses.

Molecules in the air are moving around 1000 miles per hour. They can cross a room in no time at all.

In still air the directions are random and cancel out so you don’t feel like you’re in a hurricane.

Noses can be quite sensitive to certain things, so you don’t need much butyric acid to reach you.