Measure and optimize indoor air quality

Indoors, the main source of air pollution is humans. Its respiration and evaporation noticeably contaminate the air with acetone, various alcohols and odorous substances. In addition to humans, there are a variety of indoor air pollutants, including: chemical-laden cleaning agents, cigarette smoke and evaporation from furniture varnish. Since it is difficult to analyze and evaluate all these substances individually, the CO2 content in the air is taken as an indicator for measuring the indoor air quality.

People get tired when the indoor air quality is poor. Poor concentration occurs. The risk of infection increases with increasing CO2 levels. The higher the CO2 content in a room, the more germs can be found.

It is therefore essential for human health to ensure good indoor air quality.

What is CO2 and how is it created?

Carbon dioxide (CO2) is an odorless gas that makes up 0.04% of our air. It is a chemical compound of oxygen and carbon. As an important component of photosynthesis, CO2 enables life on earth.

CO2 is created by the breathing of animals and humans, by the decomposition of organic matter, and by the ripening of fruit and vegetables. However, most of the existing CO2 concentration is caused by the burning of fossil fuels, such as oil, petrol and gas.

As a greenhouse gas, high concentrations of CO2 have fatal consequences for the environment in the course of global warming. The reason for this is that it is not permeable to long-wave thermal radiation. It thus prevents the radiation of the short-wave rays of the sun that hit the earth. This increases the temperature on Earth.

CO2 in rooms

The modern human spends an average of 20 hours a day indoors. How much CO2 is in these rooms and how the air quality affects people basically depends on the following factors:

  • Number of people in the same room
  • Space volume
  • What are the activities
  • Duration of stay
  • Are there any combustion processes
  • ventilation

The CO2 content is measured in ppm (parts per million). Good air quality outdoors is around 350 ppm. Cities have an air quality of about 500 ppm. Indoors, a hygienically safe number is 1000 ppm. It is also called the Pettenkofer number. The first symptoms, such as headaches, appear above the Pettenkofer number. Some indoor spaces, such as classrooms, can exceed 5000 ppm. Such accumulations of CO2 are particularly common in modern buildings. Buildings are made airtight through energy-saving construction measures.

Who was Max von Pettenkofer?

Max von Pettenkofer was a German chemist and hygienist who recognized 150 years ago that the air in a room loses quality over a longer period of time. He identified the CO2 content as an important key component for measuring indoor air quality and uses the Pettenkofer number (1000 ppm) to set a guide value. CO2 concentrations that exceed the Pettenkofer number therefore lead to the first physical complaints.

What is the composition of the ideal room climate?

The ideal room climate requires a temperature of 20 to 23 °C and an air humidity of 30 to 70% RH. The CO2 content should be below 800 ppm. Values ​​above 1000 ppm are already unpleasantly noticeable.

Classification of indoor air quality according to DIN EN 13779:2007-09 (DIN 2207-09)
Room air category (indoor air) Description Increase in CO2 concentration compared to outside air (ppm) Absolute CO2 concentration in indoor air (ppm) Ventilation rate / outside air volume flow (l/s person (m³/h person))
IDA 1 High indoor air quality < 400 < 800 > 15 (>54)
IDA 2 Average indoor air quality > 400-600 > 800 - 1000 10 - 15 (>36 - 54)
IDA 3 Moderate indoor air quality > 600 - 1000 > 1000 - 1400 6 - 10 (>22 - 36)
IDA 4 Low indoor air quality > 1000 > 1400 < 6 (<22)

Which interiors are particularly affected?

Particularly affected are rooms with a high concentration of people in a small space, for a long time, with little ventilation. This affects the interior of vehicles, offices, apartments, day-care centers and schools.

In order to improve indoor air quality, ventilation is mandatory:

Hygienic assessment of the CO2 concentration of indoor air with recommendation for short-term remedy
CO2 concentration (ppm) Hygienic Rating Recommendation
< 1000 Hygienic harmless No action
1000 - 2000 Hygienically conspicuous Increase and improve ventilation measure
> 2000 Hygienically unacceptable Check the ventilation of the room and take further measures if necessary

The following CO2 measurement serves as an example for ventilation (2 people, bedroom (16 m²)):


Classrooms are affected by poor indoor air quality

The American scientists Milton and Rudnick have dealt with CO2 in indoor air and how poor indoor air quality affects health. As a result of their research, they examined in 2003 by what percentage the risk of influenza infection increases under certain conditions. A classroom was used as a test environment.

Assume 30 people in a classroom over 4 hours. One of these people had acute flu. The result of this study is as follows:

  • 1000ppm CO2 followed to 5 infected people
  • 2000 ppm CO2 followed to 12 infected people
  • 3000ppm CO2 followed to 15 infected people

Between 2001 and 2007 tests were carried out in German classrooms regarding the CO2 content. The measurement period was also an important indicator here. Accordingly, the CO2 content was significantly higher in winter due to the poor ventilation conditions.

CO2 concentrations in classrooms of German schools
Number of classrooms Median (Min-Max) % overrun > 1000 ppm during class time % overrun > 2000 ppm during class time Measurement period Location Remarks
90 1412

(367 - 5359)

82 30 Winter Munich and surroundings measurement during class time; 1 full day of lessons
75 728

(314 - 2742)

29 5 Summer
39 1600 (300 - 6000) 80 17 Winter Berlin measurement during class time; 1 full day of lessons
220 1654 (335 - 4998) 86 32 Winter Erfurt measurement during class time; 1 lesson; Window closed during measurement
19 1459 (431 - 4380) 79 23 Winter Baden-Württemberg (mainly Stuttgart) measurement during class time; 1 full day of lessons
18 820 (304 - 3554) 36 3 Summer
36 1510 (730 - 4177) 89 Winter Hannover and surroundings Summer and winter measurements were taken at the same (7) schools; Measurement over a period of 48 hours
22 581 (339 - 1270) 32 Summer

Consequences of poor indoor air quality

Poor indoor air quality leads to many troublesome complaints. In addition to headaches and tiredness, there are also dizziness and poor concentration. The performance and attention are reduced. The heart rate increases. You put yourself at a higher risk of becoming a victim of an infectious disease. There is a higher incidence of acute illnesses.

All of these ailments are summarized under the Sick Building Syndrome. It is also called the building disease, which means that the building makes you sick. Poor air conditioning and poor air hygiene are the main causes of this syndrome.

Indoor CO2 Guidelines

There are no laws in Germany that must be observed regarding indoor air quality. However, guidelines are provided. SO the hygienic guide value according to DIN1946 Part 2 is 1500 ppm.

Our European neighbors take a somewhat stricter view. Finland's hygienic standard is 1200 ppm. In Norway and Sweden it is 1000 ppm. In Denmark, according to the guidelines of the occupational health and safety authority, the guideline value of 1000 ppm should not be exceeded in day-care centres, schools and offices. However, if the reference value exceeds 2000 ppm several times a day, the air quality is said to be poor.

Improve indoor air quality

There are a number of ways to improve indoor air quality:

  • intensive ventilation
  • Use little or no candles
  • do not smoke in the room
  • Avoid cleaning agents containing chemicals
  • Turn off electronic devices not in use
  • Use plants as air purifiers
  • electric air purifiers
  • Ventilation automation

CO2 meters are used to detect when air quality is deteriorating. This measures the CO2 content in the air, which, as already mentioned, is an important indicator of poor indoor air quality.


  • Ideal for monitoring indoor air quality (IAQ)
  • Blends in perfectly with the surroundings thanks to an optional, customizable deco-cover
  • Simple to install, integrate and operate – particularly user-friendly with air quality displayed in the form of a traffic light system
  • Perfectly networked: data storage in the Testo Cloud, real-time alarm via email or SMS (optional in conjunction with an Advanced licence)

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