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Building Physicist Appeals for Carbon Dioxide Sensors in Public Buildings

12/27/2022 | Planet research

By Susanne Filzwieser

What smells and looks “clean” can be a major health risk. Christina Hopfe in an interview on indoor air quality, the connection between carbon dioxide and aerosols and why she thinks of wood stoves when jogging.

Slightly smiling woman
"I'd rather sit in a cold room with good air quality than the other way around," says Christina Hopfe, a building physicist at TU Graz. She advocates carbon dioxide sensors in public buildings. Image source: Lunghammer - TU Graz

News+Stories: What is good indoor air?

Christina Hopfe: For many people, indoor air is considered good when the temperature and humidity are perceived as pleasant. But of course there is much more to it than that. Good indoor air quality means a relatively low CO2 concentration in the air, no particulate matter, no mould spores and few volatile organic compounds, so-called VOCs. This also includes protecting the building from radioactive radon gas – which is very common in areas with granitic soils – with special membranes and ventilation ductwork over areas in contact with the ground.

What are “volatile organic compounds” (VOCs)?

Hopfe: VOCs are organic compounds (chemical that contain carbon-hydrogen or carbon-carbon bonds), which have a high vapour pressure at room temperature. High vapour pressure means that VOC molecules are volatile and easily escape into the surrounding air. Just about anything new in a room emits these pollutants. Parquet, laminate, furniture, wall paint, appliances, household products containing solvents, many adhesives, even organic paints and cleaning products. This is why ventilation in buildings is so important.

Can you smell VOCs? How do they make themselves felt?

Hopfe: People perceive VOCs very differently. Some like the smell of newly renovated rooms or cars because it smells so “clean”. Personally, I find this unappealing and want to inhale as little of it as possible.

Indoor air can actually be far worse in reality than we perceive it because humans adapt very quickly to stale air.

Why does the air in a room sometimes feel so stale?

Hopfe: This is very subjective. Some people notice immediately when the air in a room smells stale or is contaminated with pollutants. Many people do not notice the CO2 content right away because it is odourless, others quickly get a headache and feel like they are suffocating without fresh air. In any case, if the air feels stale, it’s definitely time to ventilate. This is also important to prevent mould growth. Indoor air can actually be far worse in reality than we perceive it because humans adapt very quickly to stale air once they have spent a few minutes in the space. If you have been exposed to bad air for years, your body will notice. The health consequences of fine dust and mould in buildings have been proven and the air quality inside is often much worse than the air quality outside. That is a frightening fact when you realize that 400,000 people die prematurely every year in Europe due to air pollution. Moreover, many people are very allergic to airborne chemicals and particulates and can suffer from life-long asthma and other serious respiratory illnesses as a result.

Let’s take a closer look at indoor CO2 levels. Why are you advocating CO2 sensors?

Hopfe: CO2 sensors can help to reduce the risk of infection from virus-laden aerosols in the air. The better the air exchange rate in a room, the lower the aerosol concentration and the lower the risk of infection. The CO2 content in the room, which results from people breathing in the room, is an indicator for this. A lot of CO2 in the air invariably means a lot of exhaled air containing aerosols will be in the room air. So the CO2 sensor can tell me when it is time for fresh air and when the window needs to be opened. Apart from that, a high level of CO2 has detrimental side effects in relation to concentration, academic performance and the like, and is a good indicator that other contaminants are also likely to be present in high concentrations.

I’d rather sit in a cold room with good air quality than the other way around.

Where do CO2 sensors make sense?

Hopfe: Wherever people are, in offices, medical practices school classes, lecture halls. We spend over 90 per cent of our time indoors, during the Covid lockdown it was probably even more. And yet we are very much at the beginning when it comes to understanding the importance of air quality. Everything we can feel, see and smell seems more important to us. Temperature, for example. If it is too cold or too warm by even a degree or two, we will quickly start to complain. But we don’t notice particulate matter, VOCs or CO2 immediately, and even if we do, other aspects such (like our thermal or acoustic comfort) are still more important to us. Yet it is precisely here that there is a connection to health. I’d rather sit in a cold room with good air quality than the other way around. But I am not the standard person though because I am a scientist and I am fully aware of the risks.

What can I do now if I want to avoid bad air on the road as well?

Hopfe: I take my CO2 sensor with me everywhere. I recently measured 3000 ppm at the hairdresser’s and spoke to the owner about it. She personally didn’t care because her comfort is more important and the risks seem somehow distant or uncontrollable. She has air conditioning, which keeps her and her clientele very comfortable, and she doesn’t open the windows because she thinks that will only waste energy. It is then inevitable that people get headaches and infect themselves with diseases in the high aerosol concentration. So there is no fresh air unless I ask for a window to be opened. I am probably one of her most difficult customers, simply because I want to breathe fresh air. But we should not blame the hairdresser for this problem. The problem is widespread simply because there is a complete lack of public health information and legislation around the issue.

Christina Hopfe and her team from the Institute of Building Physics attracted attention in summer 2022 with a DIY exhaust system for school classes. Under the direction of Robert McLeod TU Graz showed that effective exhaust air systems in schools and kindergartens can be installed very easily and inexpensively using materials from a DIY store.

Do we need a legal requirement for the use of CO2 sensors? What’s this like in other countries?

Hopfe: Not yet, but I think it’s coming. In Belgium, CO2 sensors will be mandatory in all public buildings from 2024 and in some countries if a limit is exceeded, people are advised to immediately leave the room. When legislation comes that will change things a lot. Maybe we should think about that. Even though I think it is going to be difficult to raise awareness among people just through guidelines. In any case, my appeal is to use our common sense now and install CO2 sensors in all public buildings and educate people in their use. I want everyone to be aware that over 1000ppm is completely unacceptable indoors and 800ppm would be an acceptable upper limit. Buildings and rooms must be ventilated accordingly so that the limit values are not exceeded.

How clean can the indoor air be if the outdoor air is polluted?

Hopfe: If the outside air is contaminated with pollutants such as fine dust during the heating season, then the pollution inside is often even worse (and can be twice to a hundred times greater). If we ventilate with the windows open, get the outside air in and the building is also well insulated, then the polluted air outside is added to the polluted air inside. Mechanical ventilation with filters systems can help here and should be used where the outdoor air is highly polluted.

We are sometimes unaware of the scaling of the problems and think that if we heat with sustainable raw materials, everything is fine. We simply need to stop burning things as a society, whether that be fossil fuels, waste products or biomass.

So no good indoor air without good outdoor air?

Hopfe: Attempts are being made to reduce outdoor air pollution, as well as establishing appropriate indoor CO2 and VOC limiting values. Unfortunately, we are geographically poorly positioned in the Graz area, even if the use of district heating has improved some things. When I came to Graz from England, I immediately thought to myself, due to the basin location here, a lot of polluted air will get trapped, especially on overcast still days with temperature inversion (a reversal of the normal behaviour of temperature in the troposphere in which the air temperature increases with height). Ireland, the UK and Scandinavia in particular have an advantage there, as they are located by the sea to the west of the main European land mass, the prevailing wind blows through there and there is little pollution burden from the neighbouring countries. Heating with wood or biomass pellets is also problematic. The particulate pollutants from heating with wood are higher than those from a diesel vehicle. I like to jog in the north of Graz, where you can clearly smell the many wood-burning stoves in operation. We are sometimes unaware of the scaling of the problems and think that if we heat with sustainable raw materials, everything is fine. In general terms, we simply need to stop burning things as a society, whether that be fossil fuels, waste products or biomass.

This article is part of the dossier "Pollutants in the air". Would you like to receive the latest stories, research stories, interviews or blog posts from TU Graz directly on your smartphone or in your email inbox? Subscribe to the TU Graz Telegram newsletter for free.


Christina Hopfe is a member of the Kommission Innenraumlufthygiene (IRK; Indoor Air Hygiene Commission) in Germany and sits here in the working groups for ventilation and mold, member of the Future Operations, which was founded in the context of the COVID-19 crisis, director of IBPSA (International Building Performance Simulation Association) World, board member of IBPSA Germany/ Austria, External expert member of the Belgium Research Panel FWO in the field of building physics and member of the scientific-strategic advisory board of AEE INTEC.


Christina Johanna HOPFE
Univ.-Prof. Dipl.-Ing. dr
TU Graz | Institute of Building Physics, Services and Construction
Phone: +43 316 873 6240