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The emission of carbon dioxide (CO₂) from human activity is a well-known threat to our planet and is often considered one of the main contributors to the greenhouse effect.

That's true!

But what happens when you breathe in carbon dioxide? Is carbon dioxide dangerous to an individual person? Yes, exposure to high CO₂ concentrations may cause drowsiness, increased heart rate, blood pressure, unconsciousness, and even life-threatening complications. Scientists have also shown that carbon dioxide can make you dumber!

Although the carbon dioxide level in outside air is still relatively low, the problem arises in closed spaces where people constantly exhale CO₂ as a natural consequence of breathing - classrooms being the perfect example of such space. To investigate this problem, we have decided to create this CO₂ breathing emission calculator, which estimates how much the CO₂ concentration can increase in your room. Is it at an acceptable level, or does it require additional ventilation? Check it out with the CO₂ breathing emission calculator!

Do humans breathe out carbon dioxide?

In 2020, the major constituents of dry air (by volume) were nitrogen (78%) and oxygen (21%). The remaining 1% consists of argon (0.9%), carbon dioxide (0.04%), and other gases (0.06%). You inhale this mixture of air all the time. As you can see, the relative CO₂ level is extremely low compared to the other air components. Using the unit of parts per million (PPM), we can say that the CO₂ concentration equals 400 PPM: you can learn how to do it with Omni's PPM calculator.

Exhaled air is a mixture of around 3,500 compounds, including nitrogen (78%), oxygen (16%), argon (0.9%), carbon dioxide (4%). The rest of the components are present in microscopic amounts. So, do humans breathe out carbon dioxide? Yes, we use 5% of oxygen we breath in to increase the CO₂ concentration by nearly 4 percent points, i.e., 40,000 PPM. That's a hundredfold increase!

The adult human lung capacity is usually 3 - 5 liters (you can learn how to calculate with our lung capacity calculator), but we usually inhale only about 0.35 liters of air during one breathing cycle. Of course, this is next to nothing compared to the average room volume of approximately 50,000 liters, but that's only one breathing cycle. In reality, an average adult breathes around 12 - 20 times a minute at rest, which is what might actually change the room's air composition.

🙋 Although humans emit carbon dioxide, we can't compete with the industrial emissions, and the one of transportation and food production: learn more with Omni's smog calculator and flight emissions calculator.

What happens when you breathe in carbon dioxide? Is carbon dioxide dangerous?

Although CO₂ gas is generally non-toxic for humans, it can be dangerous in excessive amounts for two reasons:

  1. Chemical - CO₂ dissolves in the body's water, enters the bloodstream, forms carbonic acid, and eventually makes the blood acidic. That lowers the blood pH, and too low blood pH is dangerous for health. Interestingly, too low CO₂ concentration results in elevated blood pH, which causes alkalosis, which is also a dangerous state.
  2. Mechanical - CO₂ acts as an asphyxiant, a gas that displaces the normal oxygen in air. As a result, the oxygen content is reduced, which can lead to death by suffocation in extreme cases. What's more, CO₂ is odorless, and we might not even notice if its present in high concentrations.

If you are still in doubt about is carbon dioxide dangerous?, the US Food Safety and Inspection Service prepared a health hazard information sheet for carbon dioxide, where you can find the following table describing symptoms of different levels of CO₂ exposure. Keep in mind these values vary between people and the length of time a person is exposed. We use the following table in the CO₂ breathing emission calculator:

CO₂ concentration


< 5,000 PPM (1%)

Permissible exposure limit (for 8-hour exposure)

5,000 - 15,000 PPM (1% - 1.5%)

Typically no effects, possible drowsiness

15,000 - 30,000 PPM (1.5% - 3%)

Mild respiratory stimulation for some people

30,000 - 40,000 PPM (3% - 4%)

Moderate respiratory stimulation, increased heart rate and blood pressure

40,000 - 50,000 PPM (4% - 5%)

Immediately dangerous to life or health

50,000 - 80,000 PPM (5% - 8%)

Strong respiratory stimulation, dizziness, confusion, headache, shortness of breath

80,000 PPM (8.0%)

Dimmed sight, sweating, tremor, unconsciousness, and possible death

How to use the CO₂ breathing emission calculator?

The CO₂ breathing emission calculator can estimate the CO₂ emission in your room depending on a few inputs. Here is a short instruction on how to use this tool:

  1. Select the room type from the wide range of available options. Alternatively, select the Custom option and input the ACH value (air changes per hour), which is the number of times the room's ventilation system replaces the air volume in the room in an hour.
  2. Decide whether this room was previously empty or occupied. It is crucial for estimating the initial CO₂ concentration level.
  3. Specify how many people are in the room, what they do (sleeping, resting, or working), and for how long. From the Do humans breathe out carbon dioxide? section we know that people produce CO₂ and it turns out the production rate depends on the activity level.
  4. Fill in your room dimensions or volume. You can also directly input the room's floor area if you know it.
  5. As a result, you will obtain the CO₂ concentration level expressed in percentages and parts per million (PPM) for the room .

Additionally, we will inform you whether this concentration is safe for you or not. If the level of CO₂ becomes too high, we will explain what happens when you breathe in that concentration of carbon dioxide and what you can do to reduce the CO₂ concentration.

How to reduce CO₂ concentration indoors?

After learning how to estimate the CO₂ level generated by humans, the next question is how to reduce it indoors. There are several ways:

  1. Install and maintain a ventilation system that will bring in fresh outdoor air. The CO₂ particles will dilute in this air keeping a low CO₂ concentration.
  2. If you don't want to invest in expensive systems, you can ventilate your home manually by opening a window. Do it for 10 - 15 minutes, and the CO₂ levels will be drastically lower. This is especially important when you cook, light candles, or a fireplace (open flame consumes O₂ and produces CO₂). It is also a good idea to open a window or at least a bedroom door while sleeping. Scientists have shown that fresh air will help you sleep better.
  3. Smoke outside or open a window to prevent smoke from seeping back indoors.
  4. Studies have shown the highest CO₂ readings were found in crowded areas during commuting (e.g., train) or indoors. If you're in a room with many occupants, their collective exhalations might make the CO₂ levels skyrocket. Try to avoid such situations or let some fresh air inside by opening windows or doors.

Reducing CO₂ levels might offer many benefits including boosting mental health, sleep quality, immune response and a reduction in headaches and stress. So, if you ever feel sluggish or tired, that might be an indicator of a too high CO₂ level. Try to breathe with some fresh air - it should help in most cases.

Why do plants need carbon dioxide? Can they reduce the CO₂ level?

Generally, green plants capture water (H₂O), carbon dioxide (CO₂), and minerals, and by using sunlight, convert them into energy during the process of photosynthesis. That's why plants need carbon dioxide. The simplest chemical equation for photosynthesis looks like this:

6CO2+6H2OlightC6H12O6+6O2\footnotesize6\mathrm{CO_2}+6\mathrm{H_2O} \xrightarrow{\mathrm{light}}\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}

The products of this process are glucose (C₆H₁₂O₆), which is the source of energy, and oxygen (O₂). So, plants reduce carbon dioxide, right? Well, yes, but it is not that effective because plants also need to respire according to the equation:

C6H12O6+6O26CO2+6H2O\footnotesize \mathrm{C_6H_{12}O_6}+6\mathrm{O_2}\rightarrow6\mathrm{CO_2}+6\mathrm{H_2O}

And they do it in the presence and absence of light, while photosynthesis occurs only during the day.

Still, the former process remains the dominating one, and much more CO₂ is reduced than created. Research studies show that plants and soil microorganism are capable of removing CO₂ and such pollutants as formaldehyde or benzene, e.g., in smog. There is even a list of the top 10 houseplant air cleaners.

Unfortunately, to achieve satisfying results, we require hundreds of plants to make a substantial difference in O₂ concentration in a home. There are also people saying that plants don't clean indoor air at all because they do it too slowly. Indoor air is replaced by air from outside in about an hour.

Nevertheless, household plants can improve our well-being by improved concentration and stress reduction. Lowering CO₂ level in the environment is a task for bigger "plants", such as forests.


What level of CO₂ is dangerous?

CO₂ becomes dangerous when its air concentration exceeds about 3%. The symptoms are shortness of breath and increased heart rate and blood pressure. They may vary between each person and depends on how long they breathe in this air.

What is the basic role of CO₂ in photosynthesis?

Photosynthesis is a process involving two substrates, carbon dioxide (CO₂) and water (H₂O), that are converted into oxygen (O₂) and glucose (C₆H₁₂O₆). The latter product is a source of energy for plants. Since photosynthesis requires sunlight to occur, this process usually takes place during the day.

Where does carbon dioxide come from?

Carbon dioxide comes from two sources, natural and human. The examples of the former are:

  • Decay of organic materials;
  • Venting volcanoes; and
  • Animals and human breathing.

The human sources are, for example:

  • Fossil fuels combustion;
  • Clearing of forests; and
  • Cement production.

Do we breathe out carbon dioxide?

Yes, humans do breathe out carbon dioxide in each respiratory cycle. The exhaled air is a mixture of many compounds, including carbon dioxide at a concentration of about 4%. Red blood cells collect carbon dioxide from the body's cells and expel it through the lungs.

Kenneth Alambra and Dominik Czernia, PhD
Ever wonder why sometimes you get tired easily even though you've hardly moved? Maybe the CO₂ concentration in your room is getting high. Estimate how much CO₂ your room or office collects from your breathing and see how it affects your health, energy, and productivity.
Room function
Room type
Business office
This room...
was previously empty.
I'm staying here for...
regular non-strenuous work.
Average room population
I usually stay here for...
Room size
Room length
Room width
Room's floor area
Ceiling height
Room volume
cu ft
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