WAHM Forums

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I just came across this incase anyone is intersted on learning more about our sense of smell.....


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A Column done by Bill Nye, The Science Guy:


Dear Bill,


Does smell weigh anything? (Bonus stinker: What is the speed of smell?)


Thanks,
Nosy
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Yes, smells have weight, or mass. When you smell something, your nose is detecting a few, a very few, molecules floating around in the air. Think about that next time you wake up and smell the coffee, or walk into the hall and smell dinner in the kitchen, or walk into a bathroom and smell something else. These molecules have mass and thereby weight, albeit a tiny amount. The key to your sense of smell is that your olfactory faculties--your nose, taste buds, and brain--are so very sensitive. Just a few micrograms (millionths of the weight of, say, a mosquito) of gasoline or french-fry oil in the air, and we're in business detecting chemicals. We humans can distinguish about10,000 different smells. And we do it in just a few milliseconds.


How we (perceive) smell
As you breathe in, air goes up your nose and then curves down toward your throat. Right there inside your head at the top of your nostrils where the air is moving relatively slowly, molecules in the air dissolve or mix into the mucus in your nose. Epithelial olfactory receptor cells (smell cells) embedded in the lining of your nose with tails (cilia) dangling in the mucus can then detect different types of molecules mixed in with air. They're so sensitive that they can detect as few as1 molecule in about10 million.
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Sometimes the mucus-borne molecule fits right into the olfactory cell's receptor, and we pick up a smell right away. Other times, the incoming molecule doesn't fit quite as well, and we don't sense the smell right away. Most smells are somewhere in between, with the airborne molecules dissolving and fitting well in some receptors and not so well in others. We get signals of varying strength from millions of smell cells. Then, our brain sorts out the patterns of stimulated receptions, and we identify the pattern as a smell.


Ever notice how fast smell can bring back a memory? "Oh, this smells like my classroom." Or, "This smells like my kid's shampoo." That's your brain's base getting a fresh signal. We have about a thousand genes that control our sense of smell. If each gene does about seven smells, with a few combinations, our brains can sort tens of thousands of smells in no time.
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Why smell matters
For us Earth creatures, smell is our oldest and perhaps most important sense. It has come down to us through billions of years of evolution. You can see it when you look closely at the brains and noses of fish, flies, and finches. Olfactory cells are connected by olfactory nerves right to the base of an animal's brain--yours included.


That this setup is so common in nature and that it goes right to the brain lead us to suppose that the detection of airborne chemicals, smell, is a key to survival. Just think of the trouble you could get in if you couldn't tell whether food is still good or has gone bad. You can detect sour milk in a moment. You know if you've stepped in something, and you know that there might be trouble in less time than it takes to say, "Do you smell something burning?"
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Incidentally, natural gas companies add methyl mercaptan, an "odorant," to the methane. It's a compound like alcohol with an atom of sulfur where there is often an oxygen. It comes from decaying things in nature, and you can smell it--like a rotten egg! It doesn't help the gas burn or anything. The gas companies put it there just so we can smell the gas and be awareof it if it's coming into the room unburned and dangerous.


If you've ever gone hunting with a gun or a camera, or if you've ever walked into a house with a dog, you know that we can detect other animals, predator or prey, in what seems like an instant. And it is very reasonable that we choose a mate based on his or her smell. There is strong evidence that perceived scents are somehow connected to the nature of a person's immune system genes.
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How smells travel
Smells travel in the air by what's called diffusion. Molecules are always moving, bumping and jostling each other, so they mix. The rate of natural mixing or diffusion depends on how often molecules encounter one another. The same with how long it takes a smell to dissipate. If the wind is blowing, there will be more mixing. That can either make the smell reach you sooner, or it can spread the smell out so that you don't notice it at all. Predatorssuch aslions, tigers, and bears always approach their prey from downwind to keep their own scents fromalerting whatever they're hunting to their presence.


The speed with which a smell travels depends on how fast the molecules are going, how massive they are, the relative temperature of the molecules making the smell, and how many molecules there are in a given volume, their density. We express all this mathematically as a gas's temperature and pressure.By the way, our brains are quite used to air, so for the most part, we don't smell oxygen, nitrogen, or carbon dioxide; we smell the other molecules instead.


Try this at home!
Try this smell test. Put a piece of bread in a toaster, and toast it till it burns. See how long it takes you to smell it. Better yet, get a few people and try the same thing with people positioned different distances from the toaster. Closer people will smell it sooner. How much sooner? Add a fan, if you like. Well, try it.
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<DIV =Section>If you have access to a school gym and some kids, you can run a similar experiment with popcorn popping in a microwave at the center circle of a basketball court. Notice that hot popcorn gets perceived much, much faster than cold corn where there's no difference in temperature. (No Delta-T in science-speak.) That's diffusion. Have people raise their hands as they sense the smell. It's big fun with tiny molecules.
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Who smells best?
We humans are not bad at smelling. But we use our brains for all kinds of other things, like interpreting images from our eyes and controlling fingers while we type. Sharks, for example, don't have those kinds of distractions. Their sense of smell is about 10,000 times more sensitive than ours. Using about a third of their brains for smelling, they can detect about one molecule in a trillion. That's a tiny, 50-microliter drop in an Olympic-size pool.


These things get tricky to measure, but salmon seem to have underwater noses that are 30,000 times better than ours. Many scientists believe that's how they smell their way back home to their birthplace when they spawn. For fish like these, the whole world must be pattern after pattern of scents, the way ours is pattern after pattern of sights.


Smell is the fundamental sense. It protects us and helps us enjoy the wonderful scents in nature. Our olfactory system is sensitive nearly beyond belief, and it's all neatly packaged with our brains. Smell on!


Bill Nye


http://www.billnye.com

Oops! I forgot to log Jonathan out! I've been showing him how all this works.

"Bill Nye the Science Guy" I used to watch his show. I wonder if his show is still on.





Thanks for the info. smileys/smiley17.gif