HOW COME? Nostrils' role in smelling

How does having two openings on our nose helps us smell things? -- A reader

When you smell just-popped popcorn, thank scent molecules, drifting through the air and up your nose. These tiny chemical fragments dissolve in the wet mucus of the epithelium membrane, several inches up from each nostril. There they make contact with odor receptors.

The smells around us are complex, and we each have some 40 million odor receptors to help decode wafting-in scents. These fiber-studded nerve cells are connected by longer fibers (axons) to the brain's olfactory bulb, located behind the nose.

Scientists say it's the unique structure of each scent molecule that enables the brain to figure out what exactly we're smelling. And as each molecule is mapped by the brain, it files away the information, allowing us to instantly recognize scents ("lemon!" "lilacs!") in the future. In fact, the average person's nose and brain can detect and decode more than 10,000 separate odors.

But if the same scent molecules drift up both nostrils when we take a sniff of, say, roasting turkey, why isn't one nostril enough? Just as two ears help us figure out what we're hearing, and where the sounds are coming from, we also smell in stereo.

Unlike our widely separated ears, nostrils are side-by-side at the end of the nose. But experiments show each nostril sniffs in its own sample of air. The brain compares the scents arriving from each nostril, putting together a picture of what's being smelled.

Our nostrils also have varying airflows, depending on how nose tissue contracts or expands with blood flow. When one side is slightly swollen, the other is free-breathing. Scent molecules that dissolve slowly in mucus may be whisked quickly through the more open side, with less chance for detection. But fast-dissolving molecules make their biggest scent impact when swept into contact with a large swath of neurons. Having two nostrils with different airflows allows us to detect both kinds.

And in experiments at the University of California, Berkeley, researchers found two nostrils are definitely better than one. They dipped a 33-foot length of string in chocolate essence and lay it on a lawn. Like kids on an Easter egg hunt, volunteers wearing eye masks and ear muffs crawled along the ground and tried to pick up the chocolate trail using only their noses and a zigzag sniffing pattern. Out of 32 people, 21 -- an impressive two-thirds -- were able to follow the trail from beginning to end.

But with one nostril taped shut, the volunteers found it harder to pick up the scent. Likewise, when wearing a device that pumped the same air into both nostrils, the human bloodhounds were slower -- and only about half as accurate.

Go to newsday.com/health to watch an attempt to follow the chocolate trail.