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Despite conventional wisdom giving all the credit to canine snoots, we humans also have amazing sniffers that are highly sensitive to odor.
Any odor starts out as a volatile molecule—which means it can easily turn from liquid to gas—that’s looking for something to attach to. When you inhale, you pull in these molecules, some of which go to a small patch of tissue called the olfactory epithelia. This area contains about six million olfactory sensory neurons. If the scent molecule is at least somewhat soluble in water and lipophilic (that is, it likes fats), it’ll attach to a light layer of mucus over these neurons, completing a connection that tells your brain just what’s in the air.
Humans - Smells - Scent - Sources - Matter
However, to humans, not all smells are created equal. We’re more likely to catch a scent from biological sources, such as organic matter, because our minds consider them more important. For example, the smell of rain comes from bacteria in the soil that are doing nature’s trash-hauling by eating dead matter.
In another weird twist, because our sense of smell shares the same signalling channel as our sense of pain, we have particularly intense reactions to the malodorous. In other words, something really can smell so bad it hurts.
That said, scientists are still studying just why...
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