The purpose of ventilation is to freshen up the air inside buildings in order to achieve and maintain good air quality and thermal comfort. Ventilation also has important psychological aspects, which can be illustrated by the feeling of being in control, having odour management and creating a link to nature.

2.1 Indoor air quality

2.1.1 How to achieve good indoor air quality

As we spend 90% of our time indoors, it is crucial to understand what the quality of the indoor air we breathe is. Indoor air quality is influenced by the generation of pollutants indoors but also depends on the outdoor air around the building. Indoor air quality has a considerable impact on health and comfort. It is under pressure due to constant tightening of the building envelope, and introduction of many new materials that may emit harmful pollutants. Indoor air quality is also about human perception. Good indoor air quality may be defined as air that is free of pollutants that cause irritation, discomfort or ill health to occupants (AIVC, 1996). Generally, rooms have different needs for ventilation; bedrooms, for example, experience more intense emission of bioeffluents/CO2 than kitchens or living rooms. This could make demand controlled ventilation based on room type a good way to achieve the right indoor air quality. The quality of indoor air influences humans in several ways (Sundell, 2004a):

• Comfort: the pleasantness of the air is immediately felt when a person enters a building.
• Health: breathing poor indoor air can have negative health effects.
• Performance: high-quality indoor air can improve mental performance and general well-being.
• Other: fresh air creates a link to the outdoor environment, and fresh air through windows is a valued aspect of ventilation. An analysis by Navigant Ecofys published in The VELUX Healthy Homes Barometer 2017 (VELUX, 2017) showed that one out of six Europeans live in unhealthy buildings, i.e. buildings that are damp, overheated, have a lack of daylight or inadequate heating.  The analysis also showed that 1,7 times as many Europeans report poor health when living in damp homes. Similarly, when living in a home with a lack of daylight or overheating, 1,5 times more Europeans report poor health. 

Source control
Indoor air contains many different compounds,  some of which have a negative impact on health or comfort (Bluyssen, 2009):
• Gases; e.g. formaldehyde, organic chemicals (VOC) and inorganic chemicals (NOX, SOX, etc.).
• Particles; e.g. house dust and combustion products.
• Radioactive gas; radon.
• Biological; e.g. mould, fungi, pollen and dust mites
• Water vapour (humidity). 

Most of the pollutants come from sources indoors (Bluyssen, 2009):
• Human beings and their activities; e.g. tobacco smoke, particles from cooking, products for cleaning and personal care, consumer electronics and electrical office equipment like laser printers.
• Building materials; e.g. thermal insulation, plywood, paint, furniture and floor/wall coverings.
• Outdoor sources; e.g. pollen, traffic and industry. Radon exists naturally in the ground and enters the house through the floor construction.  It is important to use the principle of source control to minimise the concentration of pollutants in the indoor air. An example of source control is to use building materials and furniture with a well-documented low emission of chemicals. Another example is to use low-emitting cleaning products, and to avoid smoking indoors.  Source control can also be used as a principle to limit moisture indoors. Showering, cooking or an evening with guests raises humidity in the home, which needs to be removed by ventilation – at best at the source (e.g. a cooking hood in the kitchen or a roof window in the bathroom).
​Figure 2.1.1 The main reasons for ventilation.
Outdoor air quality

Indoor air is affected by other means than the indoor generation of pollutants – outdoor air also has an influence on indoor air quality. Particles are either directly emitted into the air (primary) or formed in the atmosphere from gaseous precursors such as sulfur dioxide (SO2), ammonia (NH3) etc. (secondary) (WHO, 2013). Primary particles are emitted by e.g. combustion engines (diesel and petrol) - they are spread to the outside air and may eventually penetrate into buildings, thereby affecting the indoor climate. Ambient or outdoor air quality has been shown to be improving - pollution levels in cities have fallen in recent decades due to regulation of industrial pollution and less polluting vehicles. 

Particles are differentiated in size (ultra-fine, fine and coarse) and the size determines how they spread within buildings and outside buildings. Particle size and their chemical composition are important factors for their health impact. Fine and coarse particles are measured by their weight in μg per m3 while ultra-fine particles are measured in particle count (number) per cm3. 

Fine particles (also called PM2.5) can travel for thousands of kilometres across borders, while coarse particles (also called PM10) are spread over only shorter distances up to 100 km (Schmidt, 2003). Ultra-fine particles are mostly generated from diesel cars and are concentrated locally, spreading over only short distances and decreasing with height above street level. There are limited studies in literature on the difference in particle levels between rural, urban background and urban street settings. For example, measurements from Denmark in 2012 show that urban background levels of fine particles (PM2.5) are 10% higher and urban street levels 40% higher than those in rural backgrounds (Ellermann et al., 2014). 
Other sources of pollution of indoor air should be included, along with ways of controlling them. 

​Langebjerg school.

» Children are particularly vulnerable to poor air quality «

Airing with windows in the morning, afternoon and before bedtime will help create good indoor air quality in the house. Airings can also be controlled via sensors, which is the most efficient way of providing good indoor air quality with windows and natural ventilation.

2.1.2 Indoor air quality indicators

As described earlier, indoor air contains many pollutants. For many years, discussion has continued as to which indicator for indoor air quality is the most suitable. Carbon dioxide (CO2) is probably the most commonly used indicator, measuring the CO2 produced by human breathing and emitted by appliances such as gas cookers and boilers (CIBSE, 2011). Other indicators are humidity and volatile organic compounds (VOCs), both of which are possible indoor air quality indicators. 

CO2 as an indicator of air quality 

Carbon Dioxide is often used when talking about global warming as it is one of the main greenhouse gasses causing global warming. But, CO2 is a good indicator of the indoor air quality in houses, where the occupants and their activities are the main source of pollution, as CO2 is emitted by all humans while breathing and not by many other sources. However, CO2 is rarely a health issue in itself. It is nevertheless a very good indicator of human presence and the level of ventilation. Outdoor air contains approximately 400 ppm of CO2; breathing generates CO2, so the indoor CO2 concentration will always be at least 400 ppm and usually higher, especially in bedrooms. 

According to European standard, EN 16798-1:2019, there are given 4 categories of the expected indoor air quality, ranging from category I (high expectation) to category IV (low expectation). This standard is not necessarily legislation unless referred to in national legislation. 

For CO2 these levels are category I (950 ppm), category II (1200 ppm), category III (1750 ppm) and category IV (above 1750 ppm). Though for bedrooms these levels are lowered in the standard. An indoor CO2 level of max 1200 ppm, therefore, provides a medium IAQ expectation which is generally acceptable and above 1750 ppm will give a low expectation of the IAQ  (CEN, 2019; Active house Alliance, 2020). CO2 is most relevant as an indicator in rooms where the need for ventilation is linked to the presence of people, e.g. in bedrooms, children’s rooms, living rooms, dining rooms, classrooms and offices.

Humidity as an indicator of air quality

The relative humidity indoors will vary on a yearly basis in correspondence with the humidity level outdoors. A high level of humidity in indoor air can increase the presence of house dust mites. So in climates with cold winters, the relative humidity inside should be kept below 45% during winter (Richardson et al, 2005). Generally speaking, high relative humidity levels should be avoided in order to limit the risk of mould growth, with negative health conditions such as asthma and allergies as a consequence (Liddament, 1996). 

As humidity is considered to be the main pollutant in homes, it can be relevant to keep the indoor humidity level under observation. For some rooms, this can be done via the relative humidity but, in more advanced systems, the difference in humidity content between the indoor and outdoor air can be evaluated and used as an indicator. 

Measuring relative humidity has been done for many years and is now a market standard. The indoor levels in cold climates are generally high during summer and lower during winter. With the same ventilation rate during summer and winter, the indoor relative humidity will be very different from summer to winter. In other words, a fixed relative humidity as an indicator for Indoor Air Quality has some limitations and is most useful in wetrooms, where the objective is to avoid very high levels of humidity. Relative humidity is very relevant as an indicator in bathrooms, and in kitchens.

In terms of absolute humidity, however, the difference between indoor and outdoor humidity content may be the best indicator, even though this will require indoor and outdoor sensors. In this case, a difference of 3.5 g of water vapour per m³ of air is a reasonable level and may be used all year to check if the humidity production in the home is balanced correctly with the ventilation rate. Measuring the difference in absolute humidity is not a market standard, so there are few products on the market. 

VOC as an indicator for air quality

Volatile Organic Compounds (VOCs) are substances that evaporate easily and are a mixture of many different chemicals such as benzene, formaldehyde and trichloroethylene (TCE). The effect on humans ranges from experiencing unpleasant smells to severe health effects, e.g. as a cause of cancer.

There are two kinds of VOC sensors on the market: one that measures the actual VOCs in the air, registering odours, cooking and smoking fumes, and solvents; and one that correlates VOC levels with CO2 levels coming from human activity which also generates VOCs. This fact, combined with the ability to detect smells, could make VOC sensors an alternative indicator for air quality to CO2 as the VOC sensor is often cheaper in price.

It is generally difficult to quantify the limit levels of VOCs, which is more commonly often used in scientific circles; whereas VOC sensors correlating with CO2 levels could be a good alternative to existing CO2 sensors that evaluate human occupancy in buildings.

Active House Alliance (2020) Active House – the specifications.
Bornehag, C. G., Blomquist, G., Gyntelborg, B., Nielsen, A., Pershagen, G. and Sundell, J. (2001) Dampness in Buildings and Health between Exposure to ‘ Dampness ’ in Buildings and Health Effects (NORDDAMP) Indoor Air, 11, 72–86.
CEN (2019) EN 16798-1: Energy performance of buildings - Ventilation for buildings - Part 1: Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics
Ellermann, T., Brandt, J., Hertel, O., Loft, S., Andersen, Z. J., Raaschou-Nielsen, O., Sigsgaard, T. (2014) Luftforureningens indvirkning på sundheden i Danmark. Nationalt Center for Miljø og Energi – DCE
Liddament, M. W. (1996) A guide to energy-efficient ventilation, AIVC
Nazaroff, W. W. (2013) Four principles for achieving good indoor air quality. Indoor Air, 23(5), 353–6. 
Richardson, G., Eick, S., Jones, R. (2005) How is the indoor environment related to asthma?: literature review, Journal of Advanced Nursing, vol. 52, no. 3, pp. 328-339
Schmidt, L. (2003) Ultrafine partikler. Gasteknisk center.
Sundell, J. (2004a) On the history of indoor air quality and health, Indoor Air, vol. 14, no. 7, pp. 51-58.
VELUX (2017) Healthy Homes Barometer.
WHO (2000) The right to healthy indoor air.