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Pettenkofer’s Number – CO₂ and Indoor Air Quality
Indoor air quality is increasingly being linked to human health. In this context, carbon dioxide (CO₂) and air quality have become inseparable concepts.
CO₂ is not considered an air pollutant in the classical sense. At typical concentrations, it is not toxic. Nevertheless, its concentration is an important indicator of indoor air quality and ventilation efficiency.
The topic of CO₂ and air quality may seem ultra-modern. However, it is not new. As early as the 19th century, Max von Pettenkofer demonstrated that CO₂ concentration could serve as an indicator of indoor air quality.
This text is dedicated to Max von Pettenkofer on the occasion of the 125th anniversary of his death, in remembrance of his key contribution to understanding the relationship between CO₂ concentration and indoor air quality.
Air Quality and Health Before Max von Pettenkofer
We cannot see air, but we breathe it continuously. It is a fundamental prerequisite for life, health, and everyday functioning.
As something invisible and difficult to explain, air has always been the subject of reflection by philosophers, physicians, and scientists. Hippocrates pointed out that human health also depends on the environment in which one lives. Pliny the Elder warned that air can become a medium that threatens life.
Questions related to air accumulated and multiplied over centuries. In the 19th century, they began to receive scientific, measurable answers.
Pettenkofer’s Contemporaries: Air and Health in the 19th Century
Pettenkofer was not the only one thinking about the connection between air and health in the 19th century. His era was a period of major changes in medicine and public health.
In the 19th century, Florence Nightingale, Ignaz Semmelweis, and Joseph Lister demonstrated how environmental conditions, hygiene, and infection prevention could save lives.
In this context, Pettenkofer’s idea that air could be measured and controlled became part of a broader historical shift. Health thus began to be viewed as a matter of space, not just of the individual.
See also: Clean Air Heals: Florence Nightingale
See also: Ignaz Semmelweis: Savior of Mothers and Newborns
Max von Pettenkofer and Modern Hygiene
Max von Pettenkofer (1818–1901) belonged to the scientific elite of his time and is considered one of the founders of modern hygiene.
His key idea was that health is not only a matter of treatment, but also of living conditions. He was particularly interested in how space, water, and air affect human health.
Pettenkofer was among the first to demonstrate that indoor air quality could be regarded as an objective, measurable category. This marked a significant shift in understanding the relationship between space, health, and science.
CO₂ Concentration Levels and Air Quality: Pettenkofer’s Number
The concentration of CO₂ in outdoor air is typically around 350–450 ppm. Indoors, however, CO₂ levels are usually higher.
According to expert publications of the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA), CO₂ concentration is considered the leading parameter for assessing indoor air quality and ventilation levels.
- 600–800 ppm is considered a range of reliable indoor air quality.
- Around 1,000 ppm represents the upper limit of acceptable indoor air quality and is known as Pettenkofer’s number, as he defined it.
- Up to 5,000 ppm is the maximum permitted average concentration in the workplace during an eight-hour exposure.
- Between 6,000 and 30,000 ppm, concentrations become critical to human health and are acceptable only for short-term exposure.
Guidelines of the American Conference of Governmental Industrial Hygienists (ACGIH) recommend an average eight-hour CO₂ exposure limit of 5,000 ppm. Concentrations of 40,000 ppm and above are considered immediately dangerous to life and health.
How the Human Body Responds to Elevated CO₂ Concentrations
The human body reacts to increased CO₂ concentrations.
At CO₂ levels between 3% and 8%, breathing accelerates, headaches occur, and a pronounced feeling of discomfort develops. At concentrations above 10%, nausea, vomiting, and loss of consciousness may occur.
Concentrations above 20% lead to rapid loss of consciousness and pose a risk of fatal outcomes. Therefore, CO₂ is not only an indicator of ventilation but also an important safety parameter in indoor environments.
Sources of CO₂ Indoors
The dominant source of indoor CO₂ is people themselves. Through the physiological process of breathing, humans emit carbon dioxide. As a result, CO₂ concentration increases proportionally to the number of occupants and the duration of stay indoors.
- CO₂ levels may also increase due to:
- indoor plants, which release CO₂ at night, although this effect is relatively small under typical conditions,
- cooking and the use of gas appliances,
- gas or solid-fuel heating,
- candles, fireplaces, and smoking indoors, among others.
Where Are We Most Exposed to Elevated CO₂ Levels?
When discussing elevated CO₂ concentrations, classrooms, offices, and meeting rooms are most often mentioned. These are spaces with many occupants and often insufficient ventilation.
However, one space is frequently overlooked, even though we spend the most time there: the bedroom.
At night, windows are usually closed. Air exchange is minimal, and CO₂ gradually accumulates. According to some scientific studies, its concentration can reach several thousand ppm overnight. As a result, people often wake up feeling tired, even after sufficient sleep.
In modern, energy-efficient buildings, windows are tightly sealed. Air infiltration is minimal, and air exchange is often insufficient, causing CO₂ to accumulate relatively quickly.
At the same time, natural ventilation by opening windows is not always a solution, especially in urban environments where outdoor air is polluted.
Can Air Purifiers Help Reduce CO₂?
Air purifiers have become widespread, especially since the Covid-19 pandemic. They can play an important role in improving air quality by effectively removing particles, dust, pollen, smoke, and certain pollutants.
However, CO₂ is a gas produced by breathing and cannot be removed by standard filters. The same applies to nitrogen and sulfur oxides, and often to odors.
Applied Science in Modern Interiors: Marquis Intelligence Solutions
Marquis Intelligence achieves high indoor air quality through a combination of:
- controlled ventilation – supplying fresh air and extracting polluted air,
- filtration and purification – removing particles, microbes, odors, and other pollutants from the air,
- energy recovery, which is a prerequisite for energy efficiency and sustainability.
Air purification alone does not solve the problem of CO₂ in indoor air.
Likewise, ventilation without air purification loses its purpose, as outdoor air is polluted.
The United Nations point out that only 1% of people on the planet breathe healthy outdoor air, while the U.S. Environmental Protection Agency (EPA) states that indoor air is 2–5 times more polluted than outdoor air.
Marquis Intelligence integrates CO₂ and VOC sensors into automated ventilation systems that maintain optimal air quality—discreetly, precisely, and energy-efficiently.
That is why the solution developed by Marquis Intelligence, based on scientific knowledge, is truly tailored to human needs.
New Standards Confirm Marquis Intelligence Solutions
It is therefore no coincidence that modern European standards, such as the EPBD Directive (Energy Performance of Buildings Directive), increasingly impose requirements that buildings must not only be energy-efficient, but also ensure healthy indoor air.
Energy efficiency without ventilation and air quality control is no longer a sustainable concept—neither technically nor from a health perspective.
This development of standards confirms the direction in which Marquis Intelligence has been developing its solutions for years.
See also: EPBD: Energy-Efficient Buildings Must Have Healthy Air
Pettenkofer’s Idea Is Still Alive Today
Pettenkofer’s message is perhaps more relevant today than ever.
The modern concept of healthy buildings confirms that buildings are not neutral environments in which we spend 90% of our lives. They actively affect our health. Joseph G. Allen of the Harvard T.H. Chan School of Public Health summarizes this powerfully:
“The people who design and operate our buildings have a greater impact on our health than doctors.”
In a world where we spend most of our lives indoors, the way we ventilate and purify air becomes one of the most important preventive measures of modern society.
Health begins where air begins—and Marquis Intelligence is here to help.
Source
- Küçükhüseyin Ö. CO₂ monitoring and indoor air quality. REHVA Journal, February 2021.
- Environmental Health and Preventive Medicine 12, 238–245 (2007). https://doi.org/10.1007/BF02898030
- Max Joseph von Pettenkofer – Wikipedia: https://en.wikipedia.org/wiki/Max_Joseph_von_Pettenkofer
- Joseph G. Allen (Harvard T.H. Chan School of Public Health) – Healthy Buildings: https://healthybuildings.hsph.harvard.edu/
- https://www.co2meter.com/blogs/news/carbon-dioxide-indoor-levels-chart
This text is part of the “Learn More About Air Through History and People” series from the Marquis Intelligence archive.
Document: MI-LAH-009.26 – Pettenkofer’s Number – CO₂ and Indoor Air Quality
