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?
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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.
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