Air Quality Monitors for VOCs

Air Quality Air Quality Monitors
42

Hello Ethan,

Thank you for sharing this detailed information and breakdown. I came across this thread searching for information about VOC monitoring. I am smack in the middle of the recent fires we had in Altadena, CA and of course air quality has become a major topic. I would like to know your thoughts on Purple Air monitors? In my little bit of research I see that they are also measuring VOCs, but officially they are saying the readings are “experimental”. They use the BME688 from Bosch. Many schools and homes in our area are making a push to have these installed. I’d appreciate any insight you might have, and how Purple Air may be better or worse than the other monitors you’ve recommended here. Thank you again for your time and information!
Michael

43

Hi @Mrryan76,

You’re welcome. I hope the information has helped, and I also hope you and your family are safe.

It’s very hard to tell with PurpleAir because, as you mentioned, they are still labelling the VOC readings as ‘experimental’. However, based on what I was able to find in this forum post, it looks like they use the Bosch default output (another index).

Based on this datasheet, Bosch also outputs an index by default (similar to Sensirion).

Bosch Gas Index
Bosch Gas Index1920×523 107 KB

AQMD also tested the accuracy of the VOC sensors in the PurpleAir monitors, and the results indicate the BME688 (and previous BME680) can pick up spikes and some short-term trends, but they shouldn’t be relied on to show the actual concentration and lose usefulness for identifying longer-term trends. With that said, AQMD’s testing seems to be carried out using the 0-500 index, whereas the PurpleAir map appears to have changed this index somehow.

While it still appears to be using an index (as opposed to trying to implement an ‘absolute’ conversion), it looks like PurpleAir has converted this index to be on a scale of 0-1500.

PurpleAir VOC Map
PurpleAir VOC Map1920×1245 223 KB

It’s worth noting that Bosch provides two indexes - IAQ and Static IAQ. IAQ is confirmed as being on a 0-500 scale, but I can’t find confirmation that Static-IAQ (which PurpleAir uses) is also on a 0-500 scale. I have heard it can go higher, but the AQMD testing linked above mentioned it also uses a 0-500 index. Therefore, it’s hard to tell if PurpleAir is using raw Static IAQ values or if they’re somehow converting the 0-500 scale to a 0-1500 scale for some reason. There doesn’t seem to be much information online explaining this. Frustratingly, I also can’t find any information as to what precisely ‘auto-trimming’ means in this case.

Bosch IAQ Index
Bosch IAQ Index1494×256 98 KB

Based on this, here are my takeaways:

  • PurpleAir is also using an index and not absolute values.
  • They’re either showing the direct index output (Static IAQ) on the map or somehow converting it to a 0-1500 scale. Without knowing the scale of the Static IAQ, I can’t definitively say which of these is the case.
  • Without knowing what auto-trimming means, it’s hard to tell exactly how the Static IAQ works. However, if I’m right in assuming that it means there are fewer algorithms dynamically adjusting the baseline, this could make it more useful than both the Bosch IAQ and Sensirion default output.

I would guess that, if the last conclusion about the Static IAQ is correct, the PurpleAir monitors are slightly better than most (non-manufacturer modified) Sensirion-based monitors. That said, as per AQMD’s report, even PurpleAir devices are not good for picking up longer-term trends, which seems to indicate some adjustments and algorithms are working behind the scenes.

Unfortunately, VOCs are just too hard to measure properly with low-cost equipment. It would also be great if the algorithms were more transparent so we could understand everything more confidently!

44

Thank you so much for the quick reply and great info Ethan! Purple Airs scale, whichever they are using, does seem to have some consistency. If you look at the VOC’s in their map, it shows a reading of 50-ish at my home in Altadena, and a reading 250+ just north west of us in Santa Clarita, where a large fire just burned.
Our AQI has been amazingly good (consistently below 50) since 2-3 days following our fire. The obvious concern now is the continuous gassing of the burned structures. My little neighborhood was miraculously saved, but just 1/2 mile east of us, everything is destroyed. As the cleanup effort gets underway, I would really just be interested in monitoring those VOCs. It seems Purple Air being useful for spikes could fit this parameter? And it’s really outdoor VOC’s I would be concerned with at this time and over the next several months. The purple air Zen looks great because it can be moved both indoors and outdoors. I trust the AQI readings from places like arrow.gov and from Caltech monitors, etc. So would you have a recommendation for best, consistent monitoring of the outdoor VOCs? I am not concerned about the price, as I would rather pay more for something that is providing the most accurate results.
Thank you again for providing all this wonderful information! Michael

45

No problem! I hope it helped a bit and didn’t make things even more difficult! It’s so hard to know for sure when there’s little available information on how exactly these sensors and monitors work.

This would seem to imply that perhaps the baseline for the Bosch sensors (in Static IAQ mode) either doesn’t automatically adjust or has a long adjustment period. Either way, it’s good in these cases! This may also make it the best monitor suited to these situations.

I would love to give a definitive answer, but with so many unknowns, it’s tough. However, based on what we’ve discussed and how I believe the PurpleAir devices work, I think they might be the best fit for you.

The AirGradient Open Air also has a VOC sensor (Sensirion, SGP4x series), and you can adjust the baseline calibration period up to one month. This is another potential choice, but I don’t know how you would get a good initial baseline calibration if VOCs are prevalent in the area already. Also, again, just to be transparent, I work with AirGradient.

With that said, I think the PurpleAir Zen is a great monitor overall, and it’s the highest-rated outdoor monitor I’ve reviewed so far (I gave it 4.5/5 stars in my review below). It’s a very solid device, with the most significant downsides being price and PurpleAir’s restrictive access to things like historical data. Since you mentioned that price isn’t a big concern in this case, I think the PurpleAir Zen is your best bet.

This, too, is also a reason I really like the Zen monitor:

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Thank you so much for the in-depth response Ethan. And for sure, you are not complicating further at all!

Couple follow-up questions:

  1. If I was take something like the AirGradient Open Air away from my home, to my office, let’s say, which is totally clean and free from VOC’s, would I be able to set a good baseline there and then bring it back to my home for a more accurate readout?

  2. How would you expect Purple Airs “experimental readings” to evolve? Would they likely continue to study the results and make a solid determination moving forward? Would they be able to implement whatever they determine to be reliable theough some sort of firmware updates?

  3. Despite the unprecedented devastation we’ve just been through, places like NASA JPL and Caltech’s testings of our air have yet to return any major detections of VOC’s and harmful gasses. I imagine this could continue to change day by day, but even without VOC monitors, my understanding is that a low PM 2.5 reading would seem to indicate the air is generally safe overall.

From all that I’ve read and listened to in the past couple weeks, those “fire toxins” have to attach themselves to PM 2.5’s. So if the PM 2.5 levels are low, and we don’t smell smoke or see ash, common sense would seem to say that there is limited presence of the toxins as well. Do you have any insight as to how accurate this might be?

Thank you again for your time and insight. I find it to be exceptionally valuable.

Gratefully,
Michael

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I believe so, yes. However, one thing I’m actually not aware of (and I will ask about) is if the baseline is kept through power loss. I would have to assume it is, because my carbon dioxide values seem to retain calibration, but I am not certain on this.

It’s tough to call because they’ve had them labeled as experimental for years already - I can find mention of these ‘experimental readings’ from as early as 2022. That makes me question how much progress they’ve actually made on the readings, and I can’t really find any significant updates on the matter. Improvements could certainly be made through firmware updates, but I’m unsure how significant they would be.

I have a feeling that they’ve come to the conclusion that VOC readings only have limited use and I wonder if they will actually continue to try and make use of them. I hope they do, but it’s tough to tell!

I wonder if this is primarily due to wind blowing the gasses and particles away from the city? It seems highly unlikely that there weren’t a lot of toxic gasses and particles released. But yes, PM2.5 is probably more important and it can also indicate if pollution is being blown in your direction - in which case, VOCs likely are, too.

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Hi Ethan,

I’ve read some of your stuff on here. Very informative! I have an issue that I would like ur take on.

We are building our office in our house. I finished building the wood countertops. I did a dumb thing by staining the countertops with oil based stain, and then finished the wood with a water based finish all inside the office with all the windows closed in fear of the humidity and temperature change would warp the wood at the seams. Also reading that countertops, floors, etc are mostly stained or finished inside the home, I thought nothing of it. Well the countertops look great! But as u already guessed my entire home seems to be unlivable from the VOCs now!

After the first 2 days of still not opening any windows hoping the smell would go away by itself. We finally opened up with windows and added fans intaking fresh air and exhausting dirty air at the other end of the house. This as expected helped greatly. Although we live in FL and opening the windows all day everyday in 88-95° high humidity weather is getting very old and uncomfortable. Thankfully the smell went away the first few days. But we have a cheap Amazon AQ monitor that I got to measure VOC while I 3D print, we have been keeping track of the VOC that way.

Although opening windows does the trick and make the house extremely uncomfortable. Once we close up the windows and let the AC take over, as expected the VOC start to climb up again almost immediately. When we wake up in the morning the living room seems to be at around .8-1.1mg/m3, when venting the house we get it down to .1-.2 then slowly starts to go up again after we close up. We do feel it in our throats when we wake up. This has been going on to almost a month now. We have since then temporarily moved back to my parents house until we figure out what to do…We have spent hundreds trying to correct this. Buying about every charcoal filter we can find on the market for the AC unit, also making our own charcoal filters with pellets and a box fans, even getting a commercial grade dehumidifier, since I read high humidity hurts the effectiveness of extracting VOCs and have found that to be true in our case also.

We have recently just given up and just decided instead of opening windows every day to just leave the home and let the environment go back to its old self and see if it fixes itself. Since then we started logging all home data 24/7 for the passed few days and found that between roughly 1pm-3am with no one home disturbing the environment the VOC are in an acceptable level of .48-.57, then between 4am-12pm it goes up to .6-.8mg/m3. Of course this is all if the cheap monitor is accurate. I have place it outside and when it’s outside in fresh air, it reads a constant 0.0-.1.

Do u think the VOC of the stain and finish soaked into the walls? Or what do u think I should do? We also contacted a local air quality company that will do a 2hr VOC test in my home that then gets sent out to a lab for extremely in depth results for $400. I’m just unsure what that test will tell me and if it’s worth it or if I can put that money towards something more productive in this process.

Thanks!

49

Hi @fernandez924,

Thanks for joining the forum, and I’m sorry to hear about your situation. Hopefully, you can move back in again soon!

I guess you’ve already read some of the content in this thread, so you’re likely already aware of the caveats of VOC monitoring. In many ways, VOC sensors act like a nose, and they can detect VOCs initially, but over time, as the baseline updates, they will lose sensitivity and no longer be able to identify the higher concentration - at least, not until they’re exposed to fresh air for a certain period again. Unfortunately, almost all lower-cost sensors have this same caveat.

If you were feeling health impacts from the VOCs, then I think moving out temporarily is the best choice. It’s very hard to give a timeline, but the off-gassing will decrease and eventually stop; unfortunately, it’s not really possible to expedite this process much.

Often I recommend professional testing for VOCs, but I’m not sure if it would help much in your case. If you’re worried about VOCs being present or not, these tests can help. However, it sounds like VOCs (and probably in higher concentrations) are almost certainly present in your case, so a test would only confirm this, but would not resolve the issue. With that said, if you’re questioning how serious the issue is, then a VOC test could certainly be worth doing. When you’re in the house, do you feel any symptoms that you believe are due to the VOCs?