What is CO2?

Carbon dioxide (CO2) in indoor environments is a colorless, odorless gas produced from activities like breathing, combustion, and use of motor vehicles in garages. While CO2 itself is not toxic, high indoor levels can indicate poor ventilation and the presence of other harmful pollutants. Elevated CO2 levels in buildings can directly impact human health. At levels between 1,000 to 2,000 parts per million (ppm), people may experience drowsiness and poor air quality. When CO2 levels rise between 2,000 to 5,000 ppm, it can lead to headaches, sleepiness, stagnant air, poor concentration, loss of attention, increased heart rate, and slight nausea. Exposure to CO2 levels over 5,000 ppm can cause serious oxygen deprivation symptoms.

Furthermore, recent studies have shown that even lower levels of CO2, such as 1,000 ppm, previously considered safe, can have harmful effects on the body. These include inflammation, reduced cognitive performance, and even kidney and bone problems. This finding is significant because such CO2 concentrations are commonly exceeded in many indoor environments, especially those with increased occupancy and reduced ventilation. Continuous exposure to increased atmospheric CO2 can be an overlooked stressor in modern environments, and it's crucial to mitigate exposure, especially for children and vulnerable individuals.

Overall, maintaining good ventilation in buildings is essential to keep CO2 levels in check and ensure a healthy indoor environment




Our personal observations

CO2 is generally considered safe up to 1000 ppm. Many studies report impairments only starting at 5000 ppm.

However, our personal observations contradict this. No study had previously ensured that the subjects were not already exposed to high CO2 concentrations in their everyday environment. Thus, for example, 1000 ppm in the test environment could actually represent a significantly higher exposure than in daily life.

In our own experiments, we ensured for weeks beforehand that the CO2 concentration was as low as possible, consistently under 600 ppm for at least a week. This led us to the following realization:

From approximately 650 - 750 ppm, the body begins to reduce functions and conserve energy. This manifested as fatigue, yawning, and reduced concentration.

While temperatures between 2 and 12 degrees Celsius were comfortable in a T-shirt without a jacket under 600 ppm after a week, from 700 ppm upwards, a room temperature of 18 degrees was needed after just a few minutes to avoid feeling cold. At 1000 ppm for more than 30 minutes, at least 21 degrees was required for comfort.

At 750 - 800 ppm, most people, without physical activity, such as during a movie or in school, begin to feel tired and their concentration decreases.

At 1200 ppm, impairments in vision begin after just a few minutes.

Physical activity and stress delay the reaction to high CO2 levels.

Observations in a cinema:

In a cinema without fresh air, we observed that the CO2 level quickly rose to 750 ppm, then slowly increased to a level of 1200 ppm. We assume that from about 750 ppm, the body reduces functions in a resting state to compensate for the increased CO2 level. From 800 ppm, the first people began to yawn. From 950 ppm, the first people began to feel cold. These had blankets with them, as they were probably accustomed to this in cinemas. However, the temperature in the cinema remained constant at 21.2°C throughout the screening, and the air flow did not change. At 1100 ppm, some people left the cinema shortly before the end of the show, as they could no longer follow the plot.

Observations in a shopping center:

In a shopping store, we measured 2400 ppm CO2. People waiting for their partner or parents were extremely restless and had the urge to leave the building. People who were not significantly in motion, such as the security personnel, were invariably inattentive and sleepy. Only people who were actively moving around the store remained partially unaffected by these effects until they joined the queue at the checkout.

Observations in queues:

In all queues of Magic Kingdom and Universal Studios in Florida, we made identical observations. While people in the outdoor queue communicated extensively at under 650 ppm CO2, just a covered area was enough to raise CO2 to over 800 ppm. Even people who had previously been actively talking reduced their conversations to a minimum. From 900 ppm onwards, hardly any ongoing communication took place. From 1000 ppm, 90% of the people who had been talking non-stop earlier turned to their phones and barely reacted to their conversation partners. Others, who only occasionally checked their phones, stared at their phones almost exclusively from this point and reacted very slowly to the movement of the queue. At this time, many already sat down on the ground or leaned against walls.

Smaller children and infants began to whine and occasionally cry or scream from at least 900 ppm. From 1100 ppm, all infants in the queues cried at least briefly or until their parents left the queue, while under 800 ppm nothing unusual occurred.

In the example given, there was a CO2 level under 600 ppm for about 45 minutes. This then rose within 3 minutes due to a cover to 800 ppm and remained there for another 20 minutes. Then, within 30 seconds, the CO2 level rose to over 1100 ppm as people entered the interior. The reactions to the different CO2 levels were almost immediate, within 30 to 120 seconds, in all observed people.

However, after this exposure time, at least an hour under 650 ppm is required to recover from it. Also, events experienced at more than 900 ppm CO2 are not stored in memory as detailed. More people remembered details during the waiting time than the actual attraction, if it took place indoors.

These examples are not isolated cases. In all stores without fresh air supply, we found CO2 concentrations between 2000 and 2400 ppm. In all areas where people were predominantly inactive, such as in a cinema or in a queue, we always found CO2 levels around 1000 to 1400 ppm. In rooms with a lot of activity, such as community halls, the CO2 content settled between 2500 and 5000 ppm.


When comparing the CO2 concentration in a room with the given values, one crucial aspect must not be overlooked: air circulation.

Even if a low CO2 value is measured at one spot in the room, the air actually breathed can contain much higher CO2 concentrations. This is because we humans exhale CO2 in large quantities. In environments such as schools, cinemas, or offices, without active ventilation to distribute the exhaled CO2, the concentration can rise to over 2000 ppm after just a few breaths.

This effect is even more pronounced when sleeping. Even with ideal room air conditions, healthy sleep is hardly possible without slight air circulation, such as from a fan. A gentle breeze passing over the head can significantly improve sleep quality. In contrast, absolute still air in the room, caused by one's own exhaled air, can be detrimental to health, even if the room has been well ventilated during the day.

Furthermore, wearing masks can increase the amount of CO2 re-inhaled, potentially affecting health. However, many studies show that wearing surgical masks or cloth masks does not significantly increase the CO2 concentration in the inhaled air for most people.

Carbon Dioxide levels

200 - 250 ppm Normal outdoor air before 1911
413 ppm Normal outdoor air 2013
< 600 ppm Our personal recommendation for sleeping
700 ppm From here, the yellow area indicating 'insufficient ventilation' begins on newer measuring devices.
800 ppm Max. Recommended for Sleeping
1.000 ppm US Gov. limit for good air quality indoor
Cannadian max. limit for Buildings.
1.400 ppm From here, the red area indicating 'bad air quality' begins on newer measuring devices."
1,000-2,000 ppm Common complaints of drowsiness or poor air quality.

US Gov.: Not enough ventilation in Room.
Causes Inflamation.
2,000 -5,000 ppm Headaches, fatigue, stagnant, stuffiness, poor concentration, loss of focus, increased heart rate, nausea.
>50,000 ppm Toxicity due to oxygen deprivation occurs
>100,000 ppm Oxygen deprivation in seconds: convulsions, coma, and death

Please be aware that these values are predicated on the assumption of an absence of any other pollutants in the air, which is seldom the situation.

How CO2 levels raising

In a bedroom, two people can raise CO2 levels to over 1000 ppm within about 5 to 10 minutes.


Without air circulation, the concentration at head level can exceed 2500 ppm within seconds.


A classroom can also reach the 1000 ppm mark in about 5 minutes and much more than 2500 ppm after 20 minutes.


In a tightly packed crowd outdoors with a light breeze, levels can reach around 1500 ppm.


In a shopping mall in the USA, we measured 2500 ppm.


Most indoor spaces have CO2 levels between 1200 and 1400 ppm.


While driving with the ventilation set to low and windows closed, 1000 ppm is easily reached within 5 minutes. Even with the ventilation on the highest setting, most vehicles do not go below 1000 ppm if more than one person is present and the windows are closed.


A stationary vehicle with slightly open windows and one person inside exceeds 2500 ppm after 10 minutes.