Why is understanding the indoor environment, its occupants, interactions and effects so important?

Why is understanding the indoor environment, its occupants, interactions and effects so important?
Swegon Air Academy

Swegon Air Academy

Written by members of the SAA team

While most people are aware of the importance of the outdoor environment, especially in relation to climate change, not that many people consider that indoor environmental quality could also be of such importance. Spending more and more of our time indoors also means that our exposure to the indoor environment is much longer. Through research, we have discovered that many diseases and disorders (such as mental disorders, obesity, cardiovascular diseases, lung cancer, and chronic respiratory illnesses, including asthma, and more recently COVID-19) can be related to staying indoors.

Swegon Air Academy

Swegon Air Academy

Written by members of the SAA team

Stress mechanisms and health effects

Indoor environmental quality is defined as the combination of lighting, acoustical, air, and thermal quality. Being indoors means that people are exposed to stressors, such as noise, smell, heat, or cold. The human body has several stress mechanisms available to cope with these indoor environmental stressors. These include the anti-stress mechanism, circadian rhythm, endocrine disruption, oxidative stress, inflammation, irritation, cell changes, and death. When the anti-stress mechanism is triggered by for example a noise, the brain sends a message to the adrenal glands, producing adrenaline and noradrenaline to deal with this stressor. Now, when these stressors are only present for a short time and are of low intensity, the levels of adrenaline and noradrenaline will soon return to normal levels. However, if the stressors last much longer and become chronic, other anti-stress hormones, such as cortisol, are produced. And eventually, it can lead to a chronic imbalance in the hormones that are released. Exposure to high levels of cortisol can contribute to changes in carbohydrate and fat metabolism and can lead to anxiety, depression and heart disease, while low levels can lead to fatigue, allergies, asthma and increased weight.

How do we assess indoor environmental quality?

Buildings must provide good indoor environmental quality (IEQ) for their occupants. To assess IEQ, we often use environmental parameters (also so-called dose-related indicators): concentrations of certain pollutants, indicators such as CO2, ventilation rate, temperature, and relative humidity. These parameters can be found in legislation, standards or guidelines.

First, let's consider the current standards.

The current standards are based on single-dose response relationships for an average person and mostly for comfort conditions, i.e. a performance assessment using only threshold levels for the single parameters.

We are basing our assessments on effect modelling. However, it seems obvious that we need a different view on indoor environmental quality (IEQ). A view in which possible stressors, interactions and effects are accounted for in different scenarios, focusing on situations rather than single parameters. A view in which we not only take the quantitative dose-related indicators into account but, in fact, all stressors and effects. Wherever the factors are physical, psychological, physiological, mental or social, positive or negative.

Second, it is important to consider other indicators.

To assess indoor environmental quality, we must use a combination of all available parameters – related to occupants and the building.

  • The dose or environmental parameters such as concentrations of certain pollutants, ventilation rate, temperature, lighting intensity, sound levels, etc.
  • The occupant or end-user indicators focused on occupants, such as productivity, sick leave, number of symptoms or complaints, health-adjusted life indicators, physiological indicators and physical indicators.
  • The building-related indicators focused on buildings and their systems, such as certain measures or characteristics of a buildings and their systems, using labels for buildings and even components, etc.

Thirdly, we also need another research model.

The single-dose response model used so far is obviously not enough. We are dealing with individuals in different situations. In every situation, building occupants are exposed to patterns of stressors, whether negative or positive. Possible invoking different effects for different people. Ideally, everyone would respond similarly, but that is just not the case. In real life, people are different and have different preferences and needs.

Studies and research

Several studies have been performed to determine patterns of stressors for certain health effects and preferences & needs for occupants in different scenarios and situations (offices, homes, schools, hospitals) and during the pandemic. In all of these studies, similar methods were used: checklists and questionnaires to collect information on stressors, effects, preferences & needs. To evaluate the results, multivariate regression analysis was used to identify patterns of positive & negative stressors. We also used cluster analysis to get profiles of occupants or so-called clusters.

The study showed that multiple stressors were associated with having a disease or disorder and the importance of including all stressors while assessing how people get a disease or disorder. Also, 2-step cluster analysis was performed in several studies; For example, we have identified 6 profiles based on comfort and IEQ preferences in primary schools. Four of those profiles showed concern with specific IEQ factors, while the other two did show a specific concern (the ‘all concerned’ cluster and the ‘nothing concerned’ cluster). The profiles we observed were found in all groups of the different participating schools, so they were not correlated to staying in one classroom. This confirms the fact that we are different, even though we are in the same environment.

To investigate whether home occupants can be clustered based on their comfort and energy behaviour, we studied comfort and energy behaviour at homes. The outcome showed that we could cluster this group of respondents in five clusters with different behavioural and energy attitudes towards comfort and energy use at home. The results confirm that we need to consider people's behaviours and attitudes in homes. We will not get the desired results if we consider them all as average people. Identifying clusters of occupants with similar behaviour/attitudes toward comfort and energy use at home is possible.


Assessing indoor environmental quality in buildings with dose, occupant and building-related indicators.

To assess indoor environmental quality, it is important to realize that using only the so-called dose or environmental indicators (concentrations of certain pollutants, parameters such as CO2, ventilation rate, temperature, and relative humidity) is insufficient. Two other categories are available: indicators related to occupants and buildings.

We need indicators providing information related to occupants, such as the effects of stress, information about changes that occur in people's bodies as a response to stress, and information based on experience, preferences and needs. We need to know occupant-related indicators to explain the integrated effects of exposure to different air, light, sound and thermal conditions over time.

For building-related indicators, we need certain measures and actions that give us information about stressors, not just the outcomes of those stressors. For example, for ventilation it is not enough to know how much ventilation is needed but also how, which ventilation type works best in certain situations. We still need to do more research on how building-related indicators positively affect the health and comfort of occupants in different scenarios and situations.

Occupants can be clustered based on their profiles, preferences and needs.

To determine these indicators and the criteria for specific situations, the outcome from a new research model based on several performed studies showed that occupants can be grouped into clusters with different profiles, preferences and needs. The patterns of stressors can be associated with different health end-points, leading to a better understanding of which building-related and occupant-related indicators play a role in the development of potential diseases or disorders and how (positive or negative).

Future challenges – ventilation systems provide a range of options for occupants.

The challenge for the future is to reveal the effects of changes in the interactions that occur at the environmental and human levels. Now, specifically for ventilation, this entails that the new generation of ventilation systems should not just focus on ventilation but also provide a range of ventilation options that meet the occupants' demands over time, whether these demands are related to health or comfort.


This text is based on the keynote presentation by Philomena M. Bluyssen, Professor of Indoor Environment at the Delft University of Technology. The presentation on Understanding the indoor environment, its occupants, interactions, and effects was given at the RoomVent Conference in April 2024.

Bluyssen PM (2014) The Healthy Indoor Environment How to assess occupants’ wellbeing in buildings? Taylor & Francis.

Bluyssen PM (2020) Towards an integrated analysis of the indoor environmental factors and its effects on occupants. Intelligent Buildings International 12(3):199-207.

Bluyssen PM (2022) Patterns and Profiles for understanding the indoor environment and its occupants. CLIMA 2022, Rotterdam, the Netherlands.

Bluyssen PM (2023) The need for occupant-related and building-related indicators in our guidelines for indoor environmental quality. Healthy Buildings Europe 2023 conference, June 11-14, Aachen, Germany, ID 1240.