TVOC - When it’s useful and when it’s useless

by Achim Haug

At AirGradient, we could say that we have a love-hate relationship with TVOC measurements. Part of it is due to inherent limitations of the sensor technology, but another part of it is also due to a lack of useful information on this topic. This often makes it difficult for people to truly understand TVOC measurements.

As a result, the first few generations of the AirGradient ONE did not contain any TVOC sensor modules, and we only introduced them after gaining a more complete understanding of their performance and identifying clear use cases where TVOC measurements can bring distinct benefits and indicate specific problems in air quality that should be addressed.

To better understand the issue with TVOCs, we need to first examine two things: (a) What TVOCs actually are, and (b) How TVOC sensors work.

What are TVOCs

Total Volatile Organic Compounds, commonly referred to as TVOCs, are a diverse group of organic chemicals present in both indoor and outdoor air. These compounds easily vaporize into the air at room temperature, thus influencing the quality of the air we breathe. TVOCs encompass a wide spectrum of chemical substances, comprising both harmful and harmless variants, playing an important role in evaluating indoor air quality. Researchers estimate that there may be more than 10,000 distinct volatile compounds in existence.

Harmful TVOCs encompass pollutants such as formaldehyde, benzene, and toluene, which emit from various sources, including building materials, furniture, and household products. Extended exposure to harmful TVOCs can result in adverse health effects, ranging from respiratory irritation and allergies to more severe conditions such as headaches, dizziness, and, in extreme cases, even cancer.

On the other hand, some TVOCs are harmless and naturally occurring, including compounds like ethanol and acetone, emitted by plants, trees, and certain foods. While these TVOCs generally pose no significant concerns, they can still contribute to overall TVOC levels in indoor air. Distinguishing between harmful and harmless TVOCs is crucial when evaluating indoor air quality and implementing strategies to maintain a healthy living or working environment.

Problem 1: TVOCs are a mix of very harmful but also completely harmless substances.

How do TVOC sensors work

All indoor air quality monitors use MOX (Metal Oxide) sensors for VOC measurements. Typical brands are Bosch or Sensirion. MOX sensors operate based on changes in electrical resistance when exposed to VOCs in the surrounding environment. These sensors consist of a semiconductor material typically composed of metal oxides such as tin dioxide (SnO2). The MOX sensor is heated to a specific temperature, enhancing its sensitivity. In the absence of VOCs, the sensor maintains a stable baseline resistance. However, when VOCs are present, they adsorb onto the heated MOX material’s surface, causing the electrical resistance to change. The degree of resistance change is proportional to the concentration of VOCs in the air. Due to this inherent principle, VOC sensors are generally not highly specific and cannot distinguish between individual chemicals with precision.

Problem 2: TVOC (MOX) sensors cannot distinguish between harmful and harmless substances.

This begs the question, are there better sensors available that would be able to distinguish between substances and yes there are but these are gas chromatography-mass spectrometry instruments which are extremely expensive and large and thus not suitable for permanent and affordable air quality monitoring.

Absolute measurements with older TVOC sensors: “ppb ethanol equivalents”

The older generation of TVOC sensors (e.g. Sensirion SGP30) provided values in parts per billion (ppb) and are calibrated using ethanol. This raises questions about the practicality of these readings in real-world indoor environments. While ethanol calibration serves as a reference point in the laboratory, it most likely does not reflect the diverse mix of VOCs commonly found in your home.

Indoor air quality can vary significantly depending on factors like the age of the building, the presence of ventilation systems, the types of flooring and furniture, and various other variables. These factors contribute to a complex and dynamic mixture of VOCs, making it challenging to establish a fixed VOC mix that universally characterizes indoor air quality for all buildings.

As a result, while the SGP30 sensor provides ppb values based on ethanol equivalents, it most likely will not be able to give a precise representation of the actual VOCs present in indoor air. It’s important for people to be aware of this limitation and interpret the sensor readings in the context of their specific indoor environment.

My first experience with the SGP30 TVOC sensor

I vividly remember when I tested the SGP30 TVOC sensor from Sensirion the first time a couple of years ago. I connected it to a small microcontroller and printed out the TVOC values on the screen in high frequency. First I put some aggressive paint near the sensor and as expected, the TVOC just shoot up, and I thought “Hey, great. I can detect dangerous substances!”

Then, I had a sunscreen laying on the table and I thought, cool lets see what happens with sunscreen and I opened the tube and brought it next to the TVOC sensor. Now the TVOC also shoot up into similar dimensions and I thought “Hey, great it can detect sunscreen!”. But how useful is the sensor if it shows me a spike, and I don’t know what causes it? How do I know if I’m breathing something harmful or harmless?.

Some scientists come to a similar conclusion

Berkley Labs writes in its Indoor Air Quality Scientific Findings Bank: “However, there are two main limitations to TVOC measurements. First, different TVOC measurement methods can yield substantially different TVOC concentrations and the differences between measurement methods will depend on the mixture of VOCs present. Secondly, the toxicity and the odor thresholds of individual VOCs within the VOC mixture may differ by orders of magnitude; therefore, the total concentration is not likely to provide a useful measure of total toxicity or total odor level.”

From the beginning we wanted to only put sensor modules into the AirGradient where we can clearly know what causes the measurements and clear actions can be derived from the values. With our above initial experience this was not the case and as a result we did not use TVOC in the first generations of our monitors.

AirQuality Certifications, newer Sensor technologies and specific use cases

What brought us back to the TVOC sensor modules was a mix of reasons. Firstly we wanted our monitor to get RESET Air certified, one of the few performance oriented 3rd party air quality monitor tests. For this certification measuring TVOCs is a mandatory requirement.

Secondly, both Sensirion and Bosch came up with newer generations of VOC sensors. Probably having recognised the inherent problem with absolute measurements as pointed out above, both companies have now moved on to provide only index based measurements focusing on the relative change / development of VOC’s in the air. The Sensirion SGP41 that we use in the AirGradient product defines improving air quality with numbers below 100 and worsening air quality with numbers above 100.

Thirdly, we came across a number of use cases where monitoring of VOCs made sense and led to clear improvements for people inside these buildings. Here are two:

  • Due to environmental concerns, a large office building asked its cleaning company to use less aggressive cleaning chemicals. After a few months, suddenly the VOC levels increased dramatically at the same times when the cleaning company was working. With this information, it turned out that the cleaning company changed back to more aggressive cleaning chemicals without informing their client.

  • Another example was an art classroom in a school that suddenly showed high VOC levels that stayed up for quite some time. After the facility manager went to that classroom and took a closer look he found an open pot of aggressive paint in a corner polluting the air and potentially harming the students.

There are many more examples like above that show clear benefits monitoring TVOC levels. What they do have in common is that they do not try to measure absolute values of TVOCs but they look at changes and spikes and then try to identify the causes of these spikes. Often the causes can be identified but -unfortunately- also sometimes you might see spikes that cannot (and TVOC measurements can also be influences by e.g. humidity or temperature).

Love or hate?

We at AirGradient see the TVOC measurement as the most challenging from all the parameters we measure. Not only from the accuracy issues of the measurements but also from the awareness side of our customers and the expectations they might have on the performance.

However, we also experienced very clear use case where measuring TVOCs clearly led to a reduction of harmful substances in the indoor air so we believe using this sensor is a net positive.

We have now a very strong in-house science team and this year one of our focus will be to provide much more information on these air quality parameters in form or blog posts and webinars. So stay tuned for more information. Also, if you have any questions, feel free to contact us or take part on the discussion about this topic on our forum.

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