Air Quality Monitors for VOCs

[Ethan]

When it comes to indoor air quality monitors, there are generally three pollutants that people care about:

• Particulate matter (PM0.3-PM10)
• VOCs (volatile organic compounds)
• CO2 (carbon dioxide)

For some, CO and radon sensors are also important, but I’ve found that most readers ask me to recommend a monitor with the above three sensors. While I’ve discussed CO2 and PM sensors at length, I thought it would be interesting to discuss VOC sensors in air quality monitors because they have a few caveats that few people know. They shouldn’t be trusted with absolute values - only with identifying trends. Why? Let’s discuss.

At the moment, the two most popular VOC sensors you will find in many indoor (and outdoor) air quality monitors are the Bosche BME680 and Sensirion SGP30, SGP40 and SGP41. Across all of the monitors I’ve reviewed in the past few months, I’ve seen one of these sensors in every monitor that monitors VOCs. In other words, they’re very common and used all across the industry.

Many people don’t know that these sensors show an index by default - not an absolute value. Below is an explanation of the index used by default for the BM680.

From Bosche BME680 datasheet.

In short, since the absolute values from VOC sensors aren’t very reliable (more on that soon), the companies that produce the sensors have generally opted to use indexes instead. In Bosch’s case, the VOC sensor adapts and calibrates itself frequently to set new baselines. This means an IAQ of 50 should always mean ‘typical good’ and 200 ‘typical polluted’ regardless of your environment. In other words, this number helps identify trends but not absolute values.

Sensirion VOC index from SGP41 datasheet.

Above is an explanation from Sensirion that recommends that air quality monitor manufacturers don’t use the raw output from their VOC sensors but rather use the Gas Index Algorithm. This algorithm functions the same as Bosch’s and sets 100 as the baseline ‘average’ air quality over a set period (which manufacturers can adjust). Therefore, any number below 100 indicates improving air quality, and any number above represents worsening air quality.

So, why do these manufacturers recommend this approach? It’s because consumer-grade VOC sensors aren’t very accurate unless you know the exact VOC composition of the measured air. This is because VOC sensors are more sensitive to certain compounds and less sensitive to others.

The above image from Sensirion shows how a metal oxide sensor (all of the VOC sensors mentioned in this post are MOX sensors) has different selectivity towards individual VOCs.

As you can probably guess, since MOX sensors have different sensitivities to different VOCs, and we can never know for sure the exact composition of our indoor or outdoor air, an absolute VOC concentration is not particularly useful or accurate. While many older VOC sensors (and some current ones) do give an absolute value, and even the sensors mentioned in this post can be adjusted to show absolute values, they’re not accurate and hence the choice by Bosch and Sensirion to use a relative index instead.

Based on this information, I don’t think anyone should choose a consumer-grade monitor based on VOC performance. None of them are particularly accurate, and the VOC concentration recorded by the device should be used only to identify trends in indoor air quality.

The IKEA Vindstryka shows only an up arrow (increasing), a side arrow (stable), or a down arrow (decreasing) for VOC levels. While I didn’t realise it then, this makes a lot of sense with the limitations of VOC sensors in air quality monitors.

One final note is that since these sensors regularly readjust their baselines (depending on the period set by the air quality monitor manufacturer), overall trends shouldn’t be trusted - only short-term trends over a day or less. Some monitors will allow you to adjust the autocalibration period, but it’s not common.

At the end of the day, having a VOC sensor in an air quality monitor is useful, but it’s important to know the limitations and caveats of such sensors. If you have any questions, please don’t hesitate to ask below!

[oBi1]

Hello Ethan, thank you for the above! I stumbled upon this while searching for a solution. Honestly, I’m getting really anxious that we run some kind of huge health risk and I feel completely unable to change it!
I’m hoping you can share a thought or two if it suits you.

Background
Up to end of 23 challenges with dry eyes, throat etc. Some doctor visits but I thought maybe look at air quality inside (especially now we are working from home most of the time). This was some 2-3 years ago.
I bought a UHOO Indoor air quality device seeing it has quite a nice set of measurements. I saw mainly two things:

• very high CO2 readings (mid-70s house, ventilation manual) especially during night.
• TVOC levels going extreme into tens of thousands.

This was part of our deliberations to do a home renovation. Q1 of 24 this happened and I had them install a per room ventilation system (intakes) with per floor an outtake from a company called Climarad. It has soem filters in it and can get quite high in volume if pushed. I also installed an LG air conditioner that is supposed to be able to demoisturise and clean air.

Recently we have been moving back in and I found my Uhoo device again. Installed in our bedroom and you can probably guess:

• CO2 much much better. Two peoplesleeping it gets to 1500 or so at night, but that is a matter of tuning the device I believe
• TVOCs still going through the roof!. Very hard to get down.E.g this night climbed to 7,700 ppb. It took my ventilation at 4 out of 5 power and the LG airco with cleaning 7 hours to get it down to 1500. At that point it didn’t seem to go down anymore. Switching the vent to auto it climbs back quite fast (to 2,000 in 40 mins).

I’m very concerned especially as I read that it’s very hard to determine the origin or actual TVOC item (is it even a dangeruos one?)

I was thinking maybe my UHOO is broken, so I could buy another device like the one you have been recommending (qingping) but I fear that will not really tell me a lot as it seems to only use running average of the current situation.
let alone it showing me the source or direction I have to look.

I’m talking about the attic, not cooking even close to the room. Not a painter or sculptor or something with beauty products. just an office clerk working at my computer.

any thoughts would be so welcome!
KR Owen

[Ethan]

Hi @oBi1,

Firstly, thanks for joining the forum! I hope I can help with your question. I can understand your worries, and it’s certainly possible that air quality is impacting your health and wellbeing.

@oBi1

I’m very concerned especially as I read that it’s very hard to determine the origin or actual TVOC item (is it even a dangeruos one?)

Firstly, yes, this needs to be kept in mind. As you’ve already read my first post in this thread, it becomes quickly apparent that low-cost VOC sensors are very lacking as it’s impossible to tell exactly what VOCs constitute the final reading, and therefore, if it’s actually something to worry about or not. With that in mind, I personally would still be concerned about VOC levels that high as they could point towards a bigger ventilation issue.

@oBi1

Recently we have been moving back in and I found my Uhoo device again. Installed in our bedroom and you can probably guess:

I forget exactly how the uHoo monitor works, as it’s been a long time since I used mine (it’s in a different country right now!), but I believe there is a calibration period when it’s first turned on, and I wonder if that could have influenced the final readings. That said, if VOCs were present during calibration, and they’re climbing even more now, it still seems to point to an issue.

@oBi1

TVOCs still going through the roof!. Very hard to get down.E.g this night climbed to 7,700 ppb. It took my ventilation at 4 out of 5 power and the LG airco with cleaning 7 hours to get it down to 1500. At that point it didn’t seem to go down anymore. Switching the vent to auto it climbs back quite fast (to 2,000 in 40 mins).

Concentrations like this are still very high and far higher than I’ve ever seen with my device. While I don’t fully trust low-cost VOC sensors (for the reasons stated above), I, too, would be worried about such high concentrations. Out of curiosity, what happens if you place the device outside (or near enough, such as on a windowsill with an open window)? Do the concentrations decrease significantly as we would expect? With consistent readings this high, I would be suspicious of a faulty or incorrectly calibrated sensor. Another idea is to try multiple rooms around the house and see how the device reacts.

@oBi1

I was thinking maybe my UHOO is broken, so I could buy another device like the one you have been recommending (qingping) but I fear that will not really tell me a lot as it seems to only use running average of the current situation.

Unfortunately, this is correct. There are some devices out there that could be helpful, but I think it would first be worth looking into whether or not the uHoo is working as expected (which I’m hoping the above tests might help conclude!).

If you have a chance to test the device outside and in a few different rooms, please let me know how you get on!

[oBi1]

Blockquote Out of curiosity, what happens if you place the device outside (or near enough, such as on a windowsill with an open window)? Do the concentrations decrease significantly as we would expect? With consistent readings this high, I would be suspicious of a faulty or incorrectly calibrated sensor. Another idea is to try multiple rooms around the house and see how the device reacts.

Hello Ethan, so I ran the tests as you suggested. Interesting outcomes.

1. Outside placement for some 4 hours:
   TVOC - dropped to 36ppb
2. Living room for 1.5 days
   TVOC - during the day - 100-350,
   evening (family sitting there, tv) - climbs to 640 (orange section uHoo 400-800)
   night - drops again to around 100
3. Child’s bedroom 1st floor
   night - climbs to 960
   daytime - 0-100
4. Back to our bedroom in the attic (3 days)
   daytime - 25-150
   night - range from 1000 - 4000 at peak climbs up when we both sleep there and then peak in the morning when we get out of bed.
   The 4000 was at a night where I deliberately did not use the ventilation system at all. With ventilation running for the first 4 hours of the night at slightly higher than normal speed we reached 500-1000 at peak.

Conclusion to me:
machine seems to function properly
it appears it needed a recalibration as you said
I got nowhere near the extreme numbers as before (though I guess the 4,000 is still very high).
But… this clearly is when people are sleeping in the room at night (both bedrooms saw this pattern).

Tbh - this kinds of puts me to ease. It is clearly associated with humans sleeping. I note too that the CO2 levels rise quite a lot during the night. The ventilation usually counters that but only to around 1500 ppb at peaks (Co2).

[Ethan]

Hi @oBi1,

Thank you for the update! These readings sound much more reasonable, and I believe the device is working correctly, even if the values still seem a bit high. For the outdoor placement, how near was it to a door and do you live near a busy road/other potential outdoor VOC source? I’m just wondering as the 36 ppb seems a bit high from my experience still, and I’m wondering if leaving it outdoors in a sheltered area for a bit longer could allow for a more complete calibration (as it seems to have some kind of auto calibration on).

@oBi1

Tbh - this kinds of puts me to ease. It is clearly associated with humans sleeping. I note too that the CO2 levels rise quite a lot during the night. The ventilation usually counters that but only to around 1500 ppb at peaks (Co2).

This is very true! While minimal, humans do exhale VOCs and over a night in an enclosed space they can build up. I have no idea if they could build up to these levels, but the device is definitely picking up trends properly as the level is expected to increase in an enclosed room with people inside.

This is something I will have to test myself!