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Effects of Exercise: Changes in Carbon Dioxide Output

Time Required Average (6-10 days)
Prerequisites Interest in exercise physiology; the deeper your knowledge of acid-base reactions, the further you can take this project
Material Availability Readily available
Cost Low ($20 - $50)
Safety Adult assistance recommended for construction and setup of respirometer and data collection


Everybody knows that your body needs oxygen to keep going, and that you breathe out carbon dioxide as waste. What happens when you exercise? You have probably noticed that you breathe faster, and your heart beats faster. What triggers your body to respond in this way? How does it "rev up" to keep your muscles going? Here is a project that gives you a peek into the fascinating science of exercise physiology.


To measure changes in carbon dioxide levels in exhaled air before and after physical exercise.


Andrew Olson, Ph.D., Science Buddies


  • Investigating CO2 in Breathing. Bronx High School of Science.
  • How Much Carbon Dioxide Is Produced During Exercise? Teacher's Notes. Faculty of Education, The Chinese University of Hong Kong.

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Science Buddies Staff. "Effects of Exercise: Changes in Carbon Dioxide Output" Science Buddies. Science Buddies, 13 Oct. 2016. Web. 25 July 2017 <https://www.sciencebuddies.org/science-fair-projects/project_ideas/Zoo_p013.shtml?from=Blog>

APA Style

Science Buddies Staff. (2016, October 13). Effects of Exercise: Changes in Carbon Dioxide Output. Retrieved July 25, 2017 from https://www.sciencebuddies.org/science-fair-projects/project_ideas/Zoo_p013.shtml?from=Blog

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Last edit date: 2016-10-13


The air that you exhale contains carbon dioxide (CO2), a waste product from the energy-producing biochemical reactions in your cells. You should do background research to find out how your body produces and gets rid of carbon dioxide, including how your lungs and heart (cardiac system) function. You will also want to look into how your body controls heart rate and breathing during exercise, which is a topic called exercise physiology.

In this project you will learn a method for measuring the relative amount of carbon dioxide in the air you exhale. You can compare the amount of carbon dioxide you produce when resting and when exercising, both moderately and briskly.

To measure your carbon dioxide output you will be using a colorimetric pH test. pH is a numerical (specifically logarithmic) measure of how acidic or basic (also called alkaline) something is. Technically, pH is the negative logarithm of the hydrogen ion concentration:

What this equation means is for each 1-unit change in pH, the hydrogen ion concentration changes ten-fold. Pure water has a neutral pH of 7. pH values lower than 7 are acidic, and pH values higher than 7 are basic (alkaline). The table below has examples of substances with different pH values (Decelles, 2002; Environment Canada, 2002; EPA, date unknown).

pH Value H+ Concentration
Relative to Pure Water
0 10 000 000 battery acid
1 1 000 000 sulfuric acid
2 100 000 lemon juice, vinegar
3 10 000 orange juice, soda
4 1 000 tomato juice, acid rain
5 100 black coffee, bananas
6 10 urine, milk
7 1 pure water
8 0.1 sea water, eggs
9 0.01 baking soda
10 0.001 Great Salt Lake, milk of magnesia
11 0.000 1 ammonia solution
12 0.000 01 soapy water
13 0.000 001 bleach, oven cleaner
14 0.000 000 1 liquid drain cleaner
Table 1. The pH Scale: Some Examples

A colorimetric pH test means that the color of the solution changes when the pH changes. Here is how it works. When you add the pH indicator solution (specifically bromothymol blue) to plain water, it turns blue, or greenish blue, indicating that the pH is near 7. Carbon dioxide is very soluble in water. When it dissolves, CO2 reacts with water, producing carbonic acid. The pH of the water is shifted from neutral (7) to more acidic (somewhere near 6). Over this range, the pH indicator will change color to yellow. Figure 1, below, shows an example of the pH indicator solution bromothymol blue changing color over this range.

bromothymol blue pH change
Figure 1. The pH indicator bromothymol blue changes color from yellow to blue over the pH range 6.0–7.6, as illustrated above.

To compare your CO2 output under different conditions, you will exhale through a tube into a bottle partly filled with the pH indicator solution. The CO2 that you exhale will dissolve in the water, and gradually acidify it. You will be able to see the pH indicator change color as this happens. By measuring how long it takes for the pH change to occur, you will have a relative measure of the amount of CO2 in your breath. The less time it takes for the color change to happen, the more CO2 there was in your breath.

Terms and Concepts

  • Lung function
  • Heart (cardiac) function
  • Carbon dioxide (CO2)
  • Heart rate
  • Exercise physiology
  • pH
  • Neutral pH
  • Acidic pH
  • Basic pH
  • Colorimetric pH test
  • pH indicator
  • Bromothymol blue


  • How is oxygen used and carbon dioxide produced in cellular respiration?
  • How do cells in the body obtain oxygen and get rid of carbon dioxide?
  • How does the body sense and respond to increased cellular respiration rate?


This site explains the pH scale: For information on respiration and exercise, carbon dioxide, and how the human lungs function, check out these websites:

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Materials and Equipment

  • pH test strips, available at Amazon.com.
  • Clear plastic bottles (4). Pint-size water bottles work well.
  • Water
  • Bromothymol blue. This will be your pH indicator solution. You can purchase bromothymol blue through an online supplier such as Amazon.com.
  • Homemade respirometer, as shown in Figure 2, below, which uses the following:
    • One bottle, with cap
    • Plastic tubing, approx. 1 cm internal diameter
    • Modeling clay
Diagram of respirometer setup.
Figure 2. Diagram of respirometer setup. Drill holes in the bottle cap for the tubes to pass through and seal with modeling clay. Pay attention to tube lengths, only the inlet tube should be below surface of the pH indicator solution liquid.
  • Permanent marker
  • A clock with a second hand or stopwatch
  • A helper to time you
  • Aeration setup for de-acidifying the pH indicator solution. The setup should include an aquarium pump, tubing, and an airstone that can all fit together, although the airstone is optional. Such a setup may be purchased at a local aquarium store, or through an online supplier such as Amazon.com.
  • Lab notebook

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Experimental Procedure

  1. Do your background research to find out about how your body produces and gets rid of carbon dioxide. You should also understand why carbon dioxide makes water more acidic when it dissolves.
  2. Fill one of clear plastic bottles nearly to the top with water. (Use distilled water if your tap water has pH < 7. You can test the pH using a pH test kit.)
  3. Add 20-25 drops of the pH indicator solution (bromothymol blue), cap the bottle, and mix gently. You should get a nice green or blue color (pH >= 7). If not, try using distilled water. This will be your diluted pH indicator solution.
  4. Take the bottle that you will use as your respirometer and fill it a little less than one-third full with the diluted pH indicator solution. Mark the level on the side of the bottle with tape and permanent marker.
  5. Set up the inlet and outlet tubes of the respirometer, as shown in Figure 2 in the Materials section.
  6. Fill a second bottle about a quarter full with the diluted pH indicator solution and set it aside as a control for color comparison. Reserve the remaining diluted indicator solution for later.
  7. Now you are ready to begin. Your helper should tell you when to start, and mark the time (or start the stopwatch).
  8. Do your best to maintain your current, comfortable breathing rate, inhaling through your nose and exhaling from your mouth through the tube.
    1. Caution: Do not inhale through the tube! You do not want to suck up the pH indicator solution!
    2. Observe the pH indicator solution for change in color.
  9. When the indicator solution in the exhalation bottle has turned yellow, as shown in Figure 3, below, your helper should note the time (or stop the stopwatch). In your lab notebook, record the number of seconds it took to change the color of the solution. You may need to practice this process with your partner several times before actual testing to determine the exact color at which to stop the time.
respirator bottle
Figure 3. Stop breathing into the respirometer when the solution has turned yellow. Record the time in your lab notebook.
  1. Now aerate the pH indicator solution (in the respirometer) to return it to the starting pH. Remove the cap of the exhalation bottle and aerate the solution using the aquarium aeration pump, as shown in Figure 4, below, until the solution matches the original color (compare to your control, it will take 5–10 minutes). (Alternatively, you can pour out the solution into an empty bottle and aerate it there, and pour in the same amount of fresh indicator solution from your reserve. Be careful to fill to the same level as before.)
aerate the pH indicator solution
Figure 4. Aerate the pH indicator solution in the respirometer until the solution returns to its original green-blue color.
  1. When your respirometer solution is ready again, repeat steps 7–10. Do this until you have at least three measurements at rest (more is better).
  2. Next, collect at least three measurements right after exercising moderately (walking at a comfortable pace) or briskly (for example, run in place, raising your knees up high) for 2–3 minutes. See how long it takes you to change the color of the pH indicator, then rest for 10 minutes while you re-aerate the buffer. Then repeat the measurement until you have at least three data points for each condition (more is better).
  3. Finally, while at rest, try breathing into (and out of) a paper bag for one minute (or as close to one minute as you can), and then testing your CO2 output (again for a total of three measurements).
  4. Average the results for each test condition and compare the results using graphs and data tables.
  5. How do your results compare to your expectations from your background research? For increasing CO2 output, how does breathing into a paper bag compare to exercise?

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  • Correlating CO2 Production with Other Measures. How does CO2 output correlate with other measures of increased physical activity such as breaths per minute and pulse rate?
  • Making Your Results More Quantitative. For a more advanced project, you can actually calculate the amount of CO2 produced. You can do this by adding a known amount of NaOH to the indicator solution, and calculating how much CO2 would be required to change the pH to 6. Then, measure how long it takes to acidify the indicator solution. Your measurements will allow you to calculate an estimate of the CO2 exhaled per second. Of course, to make this work you will have to work with solutions of known concentration and volume.
  • Effects of Training: Athletes vs. Non-Athletes. If you wanted to get really ambitious, you could see how conditioning affects CO2 output. Do conditioned athletes take longer to start producing additional CO2 with moderate exercise? Do they recover to normal levels faster after exercise?
  • Compare CO2 production after anaerobic and aerobic exercise. Compare respirometer results from subjects who run, walk, or bike for 4-7 minutes to those from subjects who do push-ups, lunges, or squats for the same duration.
  • For another Science Buddies project on exercise physiology, see: Heart Health: How Does Heart Rate Change with Exercise?.

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