Guide to air change

A frequent air change becomes necessary in closed rooms to extract moisture and pollutants, and to ensure a high quality of the breathing air. However, not only ventilation in itself is crucial for a healthy indoor climate, the efficiency and hygiene of the ventilation process are important, too. All you need to know about monitored air change and the myths surrounding “breathing walls“ are detailed in this guide. The type of construction, the motor housing, blades and impeller determine the achieved air pressure and the direction the airflow takes after passing through the device. The device should consume as little energy as possible to provide the area with a steady pressure and airflow. The design also decides whether the unit is suitable for handling large or small volumes of air. Currently, there are two distinct main types of fans: axial and centrifugal fans. The differences, fields of application, benefits and disadvantages are explained further down this guide.
Reading time: 11 min


  • Required change of air
  • Ventilation efficiency
  • Hygiene and carbon dioxide pollution of indoor air
  • Health effects

Required change of air

Up until the last few decades nobody really attached great importance to proper thermal insulation of buildings. Traditional buildings and period properties experienced a brisk and uncontrolled air change due to leaky joints and gaps between windows and exterior walls. Recently, more and more countries begin to enact laws concerning conservation of energy. In terms of construction that includes the insulation and sealing of all heat-transferring surfaces (building envelope, exterior walls, roof, windows, etc.). New insights and scientific research about our climate influence guidelines and technologies, such as the concept of low energy houses. On the other hand however, insulated building envelopes require a controlled air change to ensure hygiene and well-being. 

➥ Recommendation for common air change rates (kitchen, bathroom, fitness room, …)

The general recommendation is 0.5 air changes per hour for domestic environments. However, this value was determined under ideal conditions, meaning that it can differ from reality. In reality, the supply air from the outside mixes with the existing indoor air so that pollutants remain at least partially.  

Ventilation efficiency

Depending on the way air is supplied the efficiency of the air change can be reduced or increase. The place of installation of the planned inlets (at the top, bottom, side, ground level), degree of circulation (turbulence) and the direction of the exhaust.

Uncontrolled airflow:

Short-time ventilation
Short-time ventilation is rated with a 0 (poor) ventilation efficiency, the masses of air are not moved at all. This is happens if two opposite windows are opened. Supply air comes in through one window and immediately streams out through the other window, it remains unused and does not create an air change with the existing indoor air. As there is no mixing of air, short-time ventilation causes more of a cooling of the building than a desired air change.

Controlled ventilation:

Dilutive ventilation / Turbulent ventilation process
Dilutive ventilation with an efficiency rating of 0.5 is suitable for loosening uop the concentration of pollution in residential, conference and working spaces without any particular odour contamination. In this case incoming air is transported to the room by a fan, ventilation system or strategically placed ventilation shafts. The air velocity is relatively high in comparison to other methods, causing the existing indoor air to be swept away (induction).

Low turbulence ventilation
In industrial environments where one encounters free vapours, this type of ventilation is ideal. It is rated with an efficiency value of 1. Due to parallel airflows indoor air and dissolved contaminants are literally pressed out of the room. A mixing (turbulence) is not taking place because of the low velocity of supply air. Contaminants do not blend in with the incoming fresh air, instead they are transported to the outside. 

Displacement ventilation
This powerful type of ventilation describes the process of fresh air coming in through supply openings on the ground. The air is heated up and rises to the top where old, used masses of air are pressed out through exhaust ventilation shafts. This type of ventilation works best in buildings with inside cooling, meaning the sensible heat of persons, devices and lamps. Examples of application include cinemas and theatres. In order to remove particles, leaves, insects etc. from the incoming fresh air, air inlets in the building envelope are protected by weatherproof covers and fins. They are not only able to prevent cold draught, but also keep rain and snow from infiltrating the shaft. The construction of these openings depends on their place of installation, in domestic environments round valves for pipes and square grilles for shaft openings are most common. Cleaning the incoming air is the first step towards high hygiene standards on the inside. You will find that it is often possible to add a filter to the inlets which rid the air from fine dust and flower pollen. When using pipes to recirculate air it is necessary to employ grease filters that are also able to remove smells. The inserted filters must be checked regularly and replaced if required.

The term thermal comfort describes a situation where the indoor temperatures of the air, surfaces and floor, the air velocity (draught) and the relative humidity have all reached a balanced, harmonious level. As the individual perception of the residents can vary greatly, general reference values have been determined, for instance the temperature difference between floor and head level should not exceed 3° Celsius. On top of the content of water vapour and room temperature, the CO2 concentration in the room air is a decisive factor that contributes to our well-being.

 Hygiene and carbon dioxide pollution of indoor air

Carbon dioxide (CO2) is a metabolite of the cells in the human body, an adult produces about 700 grams of the scentless gas per day. Outdoors, this metabolic process is non-toxic, carbon dioxide is already part of breathing air by nature (0.04%). In a space with closed windows (and without any form of ventilation) the concentration reaches a level that we perceive as unpleasant within about one and a half hours. That is why experts generally reccomend to change the air three to four times a day (by opening the windows). However in most households this simply cannot be accomplished due to lack of time and because nobody is home during the day. Furthermore, there is a reluctance to open windows, particularly during the colder seasons of the year – even though about five minutes are enough per interval in winter. In order to ensure sufficient ventilation the windows should be opened completely and not just left ajar.

Carbon contents in the room depend on

  • preceding contamination of the fresh air
  • area in cubic metres
  • number of people / animals
  • active or passive tasks
  • how long the room is used for
  • combustion process, e.g. smoking
  • air change rate

The carbon monoxide emission is the first indicator during the evaluation of indoor air hygiene as it correlates to other organic exhalation such as smell. German hygienist Max Josef Pettenkofer (1818-1901) coined the “Pettenkofer value“ that continues to be of significance even today. The value determines the threshold between clean and “used“ indoor air. Starting with a CO2 value of 0.1 (1000 ppm) the air has reached a critical, that is unhygienic, concentration. In his calculations Pettenkofer assumed the outside air to have a 500 ppm CO2 concentration, which is definitely realistic. Pettenkofer experimented with required air change rates for keeping indoor air clean and gave recommendations to school buildings and other instituations where the ACH depends on the quantity of people.

Nowadays a value of 1000 ppm and above is described as critical enough to begin ventilating. Values above 2000 ppm are considered hygienically inacceptable. Even though the human does not have any sensory organs for the perception of CO2 content, the body begins to react by means of fatigue, lack of concentration and headache. The air appears to be used, muggy and stuffy. Take these signals seriously and air out the place. 

Overview air quality 

CO2 content indoor air Evaluation
< 800 ppm very good
800-1000 ppm good
1000-1200 ppm satisfactory
1200-1500 ppm sufficient
1500-1800 ppm barely sufficient
1800-2400 ppm poor sanitariness
>2400 ppm reduced
CO2 content outdoor air Evaluation
380-420 ppm Very clean for a natural composition of air
420-440 ppm ppm very good
440-480 ppm good
480-520 ppm satisfactory
520-620 ppm sufficient
620-700 ppm poor
> 700 ppm insufficient, polluted

The legend of the breathing wall
One of Pettenkofers discoveries, the permeable wall, even said to be “breathing“, has turned out to be a mistake. The measured air exchange is likely attributed to insufficient insulation of the building in question. Even though all gaps and joints were masked carefully, the opening of the fireplace was not considered.

By now it has been proven that hollow building materials like bricks and concrete are permeable to air on each side under when faced with different air pressures. However, even in case of strong wind this naturally occurring pressure is too low to actually cause an air change

Health effects  

If a higher dosage of CO2 is inhaled it expands the bronchia, increases the breating rate and promotes release of adrenaline. This causes irritation of the mucosa and diseases of the respiratory tract. Keep in mind that an adult spends more than ¾ of the day in closed rooms, therefore it stands to reason that a special importance is attached to indoor air quality.

In full classrooms and day-care centres (without ventilation systems) the carbon dioxide content quickly rises above 3000 ppm, as sufficient ventilation is not ensured or is underestimated. This is a problem as it decreases performance of the students. Apart from the mental stress carbon dioxide causes it can also lead to physical illness: Independent groups of researchers were able to prove that the flu virus spreads faster in closed rooms if the CO2 content rises. Among a class of 30 one student infected four others once the CO2 content was equal to or above 1000 ppm. During a concentration of more than 3000 ppm (0.3%) already half of the class was infected with the virus (Rudnick, Milton 2003). In comparison: Unconsciousness or even death occurr only with a a CO2 content of 10-20 %.

Open-plan offices or the own home often experience a less than ideal composition of indoor air, making it hard to concentrate on working or to have a proper sleep. In private cars without air conditioning concentrations as big as 10,000 ppm were measured during tests. Those extreme values are reached quite easily in small spaces, ventilation and fresh air need to be ensured individually. There are a couple of useful tools you can utilise to check the concentrations, such as red warning decals that are stuck to the armature as a reminder. They can also be glued to the window crank. At home a sensor-controlled ventilation system or air passage prove to be helpful. 

Quick check: Contaminated indoor air?

  • Fatigue, light-headedness
  • Decrease in productivity, slowed thinking
  • Attention deficit
  • Impaired visual perception
  • Irritation and infections of the respiratory tract
  • Asthmatic diseases, cough
  • Sore throat, headache
  • Swelling and irritation of the eyes and mucosa
  • Eczema, allergic reactions
  • Depressive mood
  • Feeling short of breath, dizziness
  • Insomnia

Pettenkofer saw humans and their breathing process as the main cause for pollution of the indoor air. It wasn’t established yet back then that natural building materials such as conifer timber also emit ethereal scents containing organic solvents. The toxic pollution adds to the pollution of breathing air, too. As soon as you repeatedly notice the same symptoms upon entering a certain building / room that seem to appear despite frequent ventilation, conducting a test to check the air quality is a wise decision. The pollutant analysis however is very complex due to the many possible causes, and should only be done by a specialised professional. If you already have a certain cause in mind, simple self-testing kits are a great alternative.
Read more about this topic in our blog about pollutants at home.

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