The Nose Knows — Or Does It? Questioning Our Sense of Smell

Townsend Letter for Doctors and Patients,  Jan, 2001  by Barbara Eaton

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Almost everyone with chemical sensitivities has probably been in a situation like this: Someone's strong perfume is causing you to have a significant reaction. When you attempt to explain the problem, hoping to get some accommodation and respect, you get a response like, "but I'm not wearing any perfume" or "I can't smell anything."

I remember trying to explain to my neighbor that her son could come over to play with my son only if he would change his clothes into some clothing I had at my home that was free of all perfumes and other odorous chemicals. Since she was already aware of my condition, she had switched to what she thought was fume-free laundry products. She became indignant at my suggestion about the clothes-changing, insisting that her son couldn't have any perfume on his clothes. She said, "if he had perfume on, don't you think I could smell it?"

This dilemma sent me on a mission to investigate the workings of the human sense of smell. What I have learned raised many interesting questions about the alleged hypersensitive sense of smell that MCS patients exhibit. In this essay, I wish to share most of what I learned and the many yet unanswered questions I have found. Realize, please, that I am not a scientist, and I am offering much of this writing as speculation. Perhaps, some of you readers will brainstorm along with me and/or suggest more avenues of fruitful research. Perhaps, some of you will have some of the answers and will send them my way.

The first revelation I discovered was from a medical textbook devoted entirely to olfaction. It stated that there is a "well-recognized phenomenon of olfaction" called olfactory fatigue. To quote the author, "If we keep sniffing one odor continually, our ability to perceive that odor soon declines and disappears." (Takagi). The author goes on to explain further, "Even when we fail to smell one kind of odor due to the phenomenon of fatigue, we can still smell other odors. This is called selective fatigue. We seldom live in an absolutely odor-free atmosphere, but we still perceive the air to be odor-free, despite living in an atmosphere that has variety of odors."

This, of course, explains why cigarette smokers are unaware of the scent of their own cigarette smoke that is on their clothing and in their homes. It explains why even the non-smoker can't smell the cigarette smoke he has picked up while with smokers, but as soon as he returns home his family can tell he's been out with a smoking crowd. It explains why the strong smell of garlic in my grandfather's house is imperceptible to him but not to visitors. We adapt to the odors we live with. Since people carry these odor molecules in their hair and on their clothing, and even sleep with them on bedding that has been laundered with perfumed products, do they become permanently adapted, I wondered?

Now, I began to think of the sense of smell as similar to our other senses which adapt to a level of stimulation for a time and then require time without that stimulation for a return to a baseline level of acuity. Much like when you've been blinded by a bright light. Or when you've been at a rock concert and for those first few minutes after the music stops you cannot hear the soft sounds of conversation until your ears readjust. With this analogy these questions came to mind: how long would it take for the sense of smell to readjust back to a "normal" level after an exposure to an olfactory stimuli? In other words, how long does this fatigue response last, I wondered? And would having a very low level of the stimuli still in the air, say for example, the amount of perfume that one picked up in one's hair from being next to a person wearing perfume, interfere with this readaption to normalcy?

The next textbook I read, entitled Sensation and Perception (Goldstein), referred to the senses of taste and smell as "the chemical senses" since they operate via receptors that respond to chemical stimuli in the form of invisible molecules. This distinguishes them from the senses of hearing and vision which respond to waves instead of molecules. This text suggests that the human olfactory receptors are as acute as those of animals, contradicting the myth that humans have a much inferior sense of smell than animals. A study cited in this textbook demonstrated that human olfactory receptors "were excited by the action of just one molecule of odorant." The substantive difference between animals and humans is that animals simply have many more olfactory receptors.

Perhaps the sense of smell works much like our sense of taste, the other "chemical sense," and requires long periods of abstinence from certain stimuli in order to readapt. Anyone who has gone on a low-sodium diet knows that it takes some time, often weeks, for the taste buds to become adjusted. In that adjustment period, food appears to have no flavor at all, but lo and behold, after a time these low-sodium dieters will tell how they can now taste all kinds of flavors that were previously imperceptible. Doesn't this sound exactly like what happens to the olfactory acuity of the MCS patient when they begin to live in an odor-free environment?