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Effects of Exercise: Changes in Carbon Dioxide Output

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You might know 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? In this project, you will get a peek into the fascinating science of exercise physiology and find out—with the help of a color changing reaction.


Areas of Science
Time Required
Average (6-10 days)
Ability to physically exercise (doing jumping jacks). Knowledge of acid-base reactions is helpful, but not required.
Material Availability
Readily available
Low ($20 - $50)
Adult assistance recommended for construction and setup of respirometer. Use a safety valve in your experiment as recommended in the procedure to make sure that you do not accidentally suck up some of the indicator solution.
Andrew Olson, PhD, Science Buddies
Edited by Svenja Lohner, PhD, 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.


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


Every day, you need lots of energy for all the activities you do, even when you are just sleeping. To produce this energy, your body extracts energy from the food you eat. This process is called cellular respiration and takes place in the cells of your body. In a series of chemical reactions, the food you eat is broken down to glucose, which then reacts with oxygen from the air you breathe to carbon dioxide (CO2), water and energy as shown in Figure 1.

Cellular respiration diagram shows a cell taking in oxygen and glucose and creating carbon dioxide, energy and water
Figure 1. Cellular respiration—a process in which glucose reacts with oxygen to form carbon dioxide, water and energy—happens inside the cells of your body.

The chemical equation of cellular respiration is given in Equation 1.

Equation 1:

Water and carbon dioxide are waste products of cellular respiration that are usually not used by your body. Whereas the water ends up in your sweat or urine, the carbon dioxide is released back into the air when you exhale. In this project, you will learn a method for measuring the relative amount of carbon dioxide in the air that you exhale. To measure your carbon dioxide output, you will make use of the fact that carbon dioxide is an acidic gas. This makes it possible to use 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 tenfold. Pure water has a neutral pH of 7. pH values lower than 7 are acidic, and pH values higher than 7 are basic (alkaline). If you want to know more about acids, bases, and pH, you will find more information here.

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, it forms carbonic acid, which is acidic. This makes the pH of the water shift from neutral (7) to more acidic (somewhere near 6)—the pH indicator will change color to yellow. Figure 2, below, shows an example of the pH indicator solution bromothymol blue changing color over this pH range.

Three beakers are filled with liquids of different pH values and change color due to the pH indicator bromothymol blue
Figure 2. The pH indicator bromothymol blue changes color from yellow to blue over the pH range 6.0–7.6.

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 took for the color change to happen, the more CO2 there was in your breath.

The two conditions you will test are before and after a short period of exercising. You probably notice that your breathing gets faster and your heart rate increases when you exercise. You can do your own research to look into how your body controls heart rate and breathing during exercise, which is a topic called exercise physiology. What do you think will happen to the amount of CO2 that you exhale after exercising? Will it increase, decrease, or stay the same? And what does your result mean with respect to the cellular respiration reaction in your body?

Terms and Concepts



For information on respiration and exercise, carbon dioxide, and how the human lungs function, check out these websites:

Materials and Equipment

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Experimental Procedure

Building Your Respirometer

  1. Fill one clear plastic bottle a little less than one-third full with water and add one teaspoon (about 5 mL) of the 0.04% bromothymol blue solution. You should get a nice green or blue-green color (pH >= 7) as shown in Figure 3. If not, try using distilled water. This will be your diluted pH indicator solution.

Adding bromothymol blue solution to water to make a green or blue-green solution
Figure 3. Adding the bromothymol blue solution to the water should result in a green or blue-green solution.
  1. Cut about two inches from one side of the tube for the aquarium pump.
  2. Insert the safety valve in between both pieces of tube as shown in Figure 4. The tip of the red part inside the valve should be on the side of the longer end of the tube. Note: The safety valve will prevent you from accidentally sucking up indicator solution from the bottle.

Two ends of a tube are connected to a safety valve
Figure 4. The safety valve ensures that you do not accidentally suck up indicator solution.
  1. Cut a two-inch piece off your straw. You will use this as the outlet tube of your respirometer.
  2. Using the aquarium pump tube (with inserted valve and the airstone), the straw, and the modeling clay, set up your respirometer as shown in Figure 5. Make sure that the inlet tube reaches all the way to the bottom of the bottle. The outlet tube (the straw) should stay above the indicator solution.

Putty holds two tubes in the lid of a water bottle filled with a pH indicator solution
Figure 5. Set up respirometer with inlet and outlet tubes and diluted indicator solution.

Measuring Your Carbon Dioxide Output

  1. Fill the second bottle with the same amount of water as the first one and again add one teaspoon of bromothymol blue solution. Make sure the solution has the same color as the previous one and set the bottle aside as a control for color comparison.
  2. Now you are ready to begin. Your helper should tell you when to start, and start the stopwatch.
  3. On your helper's command, take a deep breath and start exhaling through the inlet tube into the indicator solution for as long as you can. Do your best to maintain your current, comfortable breathing rate, inhaling through your nose and exhaling from your mouth through the tube.
  4. When the indicator solution in your respirometer has turned yellow-green (as shown in Figure 6) and the color does not change anymore, your helper should stop the stopwatch.
    Indicator solution in a plastic bottle turns yellow after fully saturated with carbon dioxide
    Figure 6. At the end of your reaction the indicator solution should be yellowish.

  5. 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.
  6. Now aerate the pH indicator solution (in the respirometer) to return it to the starting pH. To do that, attach the inlet tube to the aquarium pump and switch it on (you can leave the safety valve attached) as shown in Figure 7. Aerate the solution until the solution matches the original color (compare to your control; it will take 5–10 minutes).

An aquarium pump is used to aeriate indicator solution in a plastic bottle
Figure 7. Aerating the indicator solution with an aquarium pump.
  1. When your respirometer solution is ready again, repeat step 3–6 until you have at least three measurements at rest (more is better).
  2. Next, collect at least three measurements right after exercising (doing jumping jacks) for one minute. Test how long it takes you to change the color of the pH indicator immediately after you finished exercising, 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. Average the results for each test condition and compare the results using graphs and data tables.
  4. How do your results compare to your expectations from your background research? Did the amount of carbon dioxide in your exhaled breath decrease or increase after exercising? Can you explain your results?
icon scientific method

Ask an Expert

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Global Connections

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.
This project explores topics key to Good Health and Well-Being: Ensure healthy lives and promote well-being for all at all ages.


  • 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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MLA Style

Science Buddies Staff. "Effects of Exercise: Changes in Carbon Dioxide Output." Science Buddies, 1 May 2021, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Zoo_p013/human-biology-health/exercise-changes-in-carbon-dioxide-output. Accessed 28 Nov. 2023.

APA Style

Science Buddies Staff. (2021, May 1). Effects of Exercise: Changes in Carbon Dioxide Output. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Zoo_p013/human-biology-health/exercise-changes-in-carbon-dioxide-output

Last edit date: 2021-05-01
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