When Your Sniffer Snoozes, You've Got Olfactory Fatigue
AbstractThe holidays are a wonderful time, when lots of good foods and good smells come from the kitchen. But have you noticed that if you stay in the kitchen awhile, you no longer notice the delicious smells? Don't worry! Your nose is not broken, you are just experiencing olfactory fatigue—basically, that's when your nose takes a nap. But what is behind olfactory fatigue and what happens when you experience it? Does a person's sense of smell "get tired" in the same way for different smells? Put your nose to work and answer these questions.
Michelle Maranowski, PhD, Science Buddies
ObjectiveTo determine the time it takes to reach olfactory fatigue for different substances and smells and to establish if gender plays a role in the response.
Smell is one of our five senses. The different smells that our noses detect can help us to recognize things that are delicious and good for us, like warm baked bread or roast turkey, and things that are dangerous, like paint thinner or spoiled food. How does the nose work? And why can we smell some things easily and some things not as easily?
When we smell something, molecules from that item have reached our nose. That means that to be detected, the molecules from that item have to be volatile, or easy to evaporate. At the top of our nasal passages is a patch of sensory neurons the size of a postage stamp. This patch is called the olfactory epithelium. The sensory neurons, numbering approximately 50 million, are exposed to the air we breathe, and have hair-like projections called cilia. The molecules we breathe into our nose dissolve in the watery mucus of the nasal passages and then bind to the cilia. The binding sends an electrical signal through the olfactory bulb to the brain where the signal is identified. Researchers believe that humans can detect up to a trillion different smells; for some substances, it takes only a few parts per trillion to be detected. Another way to understand this parts-per-trillion concentration is to think of a room filled with a trillion marbles. Of those trillion marbles, some people can pick up a smell that may be associated with only 10 marbles of the trillion in the room.
Now that you know a bit more about how the nose works, let's look into the concept of olfactory fatigue. Olfactory fatigue results from a normal but temporary inability to pick up a particular smell after being exposed to it for a long time. Once you are no longer exposed to the smell, the ability to pick up that particular smell returns. Olfactory fatigue is a sensory adaptation. It enables us to get used to smells so that our nervous systems don't become overloaded, and we can be ready to respond to new smells. The interesting thing is that people smell things and reach olfactory fatigue differently. This is due to the fact that we respond to stimuli differently—and that response may have something to do with gender, too.
In this human biology and health science project, you will experiment with olfactory fatigue. You will test a few different substances—almond extract, cinnamon, ground coffee, and lemon extract—and see how the time to fatigue (when one can no longer smell the scent or the scent has weakened) varies.
Terms and Concepts
- Sensory neurons
- Olfactory epithelium
- Olfactory bulb
- Olfactory fatigue
- Sensory adaptation
- Scatter plot
- Where is the olfactory bulb and what is its role in the human detection of smell?
- What is it called when a person can't smell anything? What are the dangers associated with this condition?
- Why does having nasal congestion or a cold sometimes prevent you from smelling properly?
- Wikipedia Contributors. (2011, July 14). Olfaction. Wikipedia: The Free Encyclopedia. Retrieved July 25, 2010.
- Murray, M. (n.d.). Our chemical senses: Olfaction. Neuroscience for Kids. Retrieved December 10, 2010.
- BBC News. (2002, February 4). Women nose ahead in smell tests. Retrieved December 13, 2010.
- Cecire, K. (2002, March 22). Histograms: Construction, analysis, and understanding. Retrieved July 21, 2011.
- Time. (2014). Your nose can smell at least 1 trillion scents. Retrieved March 11, 2016.
The following Science Buddies document will help you understand how to deal with human test subjects for human behavior science projects.
- Science Buddies. (2011). Projects Involving Human Subjects. Retrieved July 15, 2011.
Materials and Equipment
- Test subjects (20). You can use your classmates or club mates as potential sources for volunteers. Try to use an equal number of boys and girls. Ask your school principal and your teachers if you can test your subjects at school. If you need another group to reach 20 subjects, ask at different schools in your area. Note: Make sure you work with children from the same grade for the whole experiment and that they can all read without assistance. You can also use yourself as a test subject.
- Location for testing. You should have a table and two chairs at this location.
- Small (4 oz.) plastic containers with snap-on lids (4). You can find these at restaurants or grocery stores where you purchase take-out food or salad dressing.
- Almond extract, 2 oz. bottle
- Cinnamon, 2 oz. jar
- Fresh ground coffee, 2 oz. jar
- Lemon extract, 2 oz. bottle
- Pitcher of water
- Disposable cups (20)
- Lab notebook
Working with Human Test Subjects
There are special considerations when designing an experiment involving human subjects. Fairs affiliated with Intel International Science and Engineering Fair (ISEF) often require an Informed Consent Form (permission sheet) for every participant who is questioned. Consult the rules and regulations of the science fair that you are entering, prior to performing experiments or surveys. Please refer to the Science Buddies documents Projects Involving Human Subjects and Scientific Review Committee for additional important requirements. If you are working with minors, you must get advance permission from the children's parents or guardians (and teachers if you are performing the test while they are in school) to make sure that it is all right for the children to participate in the science fair project. Here are suggested guidelines for obtaining permission for working with minors:
- Write a clear description of your science fair project, what you are studying, and what you hope to learn. Include how the child will be tested. Include a paragraph where you get a parent's or guardian's and/or teacher's signature.
- Print out as many copies as you need for each child you will be surveying.
- Pass out the permission sheet to the children or to the teachers of the children to give to the parents. You must have permission for all the children in order to be able to use them as test subjects.
Preparing for Testing
- Wait for a week to receive permission from your test subjects. Once you have collected the permission sheets, schedule testing time, and go to the next step.
- Prepare four small containers and lids.
- Label the first container "AE" for almond extract. Then place 2 teaspoons (tsp.) of almond extract in the container and snap the lid on.
- Label the second container "CIN" for cinnamon. Place 2 tsp. of cinnamon in the container and snap the lid on.
- Label the third container "COF" for coffee. Place 2 tsp. of ground coffee in the container and snap the lid on.
- Label the fourth container "LE" for lemon extract. Place 2 tsp. of lemon extract in the container and snap the lid on.
- Place the containers on the table along with your stopwatch, the pitcher of water, and a disposable cup.
- Make a table in your lab notebook which you can use to collect and organize data.
|Test Subject||Gender||Olfactory Fatigue Time (Seconds)|
|Almond Extract||Cinnamon||Ground Coffee||Lemon Extract|
Table 1. Olfactory Fatigue Data
- As stated in the Introduction, the purpose of this experiment is to determine how much time it takes each subject to reach olfactory fatigue for different substances. To do this you will ask each test subject to hold the container 6–8 inches away from his or her nose and then gently waft the aroma with their hands to their noses. (Chefs waft scents from their cooking pots to their noses in the same manner so that they can tell if the dish will taste as it should. Practice this a few times yourself so that you can describe how to do it to your test subjects.)
Testing Your Subjects for Olfactory Fatigue
- Bring in the first test subject. Ask him or her to sit down at the table in front of the containers while you sit down on the opposite side. In your lab notebook, write down the name and gender of the test subject.
- Using the "Working with Human Test Subjects" information at the top of this section as your guide, inform the test subject about the experiment, what you hope to learn, and what she or he will be doing.
- Tell your test subject to hold the container 6–8 inches away from his or her nose, showing how to waft the scent to the nose. Instruct the subject to tell you when she or he can no longer smell the scent or when the scent has weakened significantly.
- Ask the test subject to pick up the first container and open the lid. As soon as the test subject opens the lid, start timing with the stopwatch. Stop timing when the test subject informs you that he or she can no longer smell the scent or the scent has weakened. Record this "fatigue time" in seconds in your lab notebook. Ask the test subject to close the lid.
- Now ask your test subject to drink some water and walk around for five minutes. This should clear her or his nose. When five minutes have elapsed, ask the test subject to come back to the table.
- Repeat steps 4 and 5 with the second, third, and fourth containers.
- After testing all of the substances, excuse the test subject and thank him or her for participating.
- Repeat steps 1–7 with each of the test subjects. Remember always to record the name and gender of each subject along with their fatigue times for each substance in your lab notebook.
Analyzing the Data
- Sort the data in Table 1 according to gender by making two tables similar to Table 1—one for males, shown in Table 2, and the other one for females, shown in Table 3.
|Male Test Subject||Olfactory Fatigue Time (Seconds)|
|Almond Extract||Cinnamon||Ground Coffee||Lemon Extract|
Table 2. Olfactory Fatigue Times (Seconds) for Male Test Subjects
|Female Test Subject||Olfactory Fatigue Time (Seconds)|
|Almond Extract||Cinnamon||Ground Coffee||Lemon Extract|
Table 3. Olfactory Fatigue Times (Seconds) for Female Test Subjects
- Average the data from Tables 1, 2, and 3 by substance and record the values in a table like Table 4. Average the data for the entire population and then average the data for males and females separately.
|Substance||Average Fatigue Time for All Test Subjects (seconds)||Average Fatigue Time for Male Test Subjects (seconds)||Average Fatigue Time for Female Test Subjects (seconds)|
Table 4. Average Olfactory Fatigue Times (Seconds) by Overall Population and by Gender
- Now plot the raw data from Table 1 on a bar graph. This data is for the entire population, male and female. You will make a bar graph for each of the four substances. Label the x-axis Time (Seconds) and the y-axis Number, with "number" being how many test subjects reach a particular fatigue time. Make "bins" of time on the x-axis. For example, mark the first bin for 1 second to 10 seconds. Mark the second bin of time from 11 seconds to 20 seconds, and so on, in sequence across the x-axis. Count how many fatigue times fall into each bin. This kind of a plot is called a histogram. A histogram is a bar chart that shows the frequency distribution of a set of data. You will have a histogram for each substance for a total of four histograms. To learn more about how histograms are constructed, check out this website: Histograms: Construction, Analysis, and Understanding
- Make another four histograms, one for each substance, based on the data from Table 2, the data for males. As in step 3, label each x-axis Time (Seconds) and the y-axis Number.
- Make four more histograms, one for each substance, based on the data from Table 3, the data for females. As in step 3, label each x-axis Time (Seconds) and the y-axis Number.
- Plot the average data from Table 4. Plot the data on three separate scatter plots: one for all of the test subjects, one for the male test subjects only, and one for the female test subjects only. Label the x-axis Substance and the y-axis Olfactory Fatigue Time (Seconds).
- When finished, you will have a total of 12 histograms and three scatter plots of data.
- What do the plots tell you? How does olfactory fatigue vary from substance to substance? How does olfactory fatigue vary for the entire population? Does olfactory fatigue differ by gender?
Ask an Expert
- How does age affect olfactory fatigue? Do children fatigue faster than adults or is it the other way around? Do the experiment again and find out. Try to find adult test subjects of various ages, such as 30-year-olds, 60-year-olds, and 90-year-olds.
- Can your test subjects distinguish the individual scents made from the mixture of two substances? Mix together two different substances like bananas and onions or garlic and cinnamon and ask the subjects to smell the mixtures. Is it easy to distinguish the two scents?
- If you are interested in learning more about your sense of smell, try this Science Buddies project, The Nose Knows Smell but How About Taste?
If you like this project, you might enjoy exploring these related careers: