Night air quality monitor for child respiratory issues (at high RH)

[Ethan]

Hi @Guilherme,

Thanks for the update!

@Guilherme

The AirGradient dashboard actually had nice features to control the light intensity of the LEDs and the display, and turn them off if desired. Quite handy.

I’m glad you found this! Yes, they can be dimmed or even disabled if you want.

@Guilherme

So looking at this, I would believe that I opened the window between 1AM and 2AM, when the CO2 curve stopped to increase. It still decreased slowly likely because of the window shutter - it has just tiny holes, so it doesn’t let much air come through. And I would guess that at around 3:30 the wind picked up and made the CO2 decrease quite quickly.

Interesting findings! I am surprised, however, that the CO2 concentration really didn’t decrease much at all until around 3:30 am. However, it makes sense if there are only tiny holes in the shutter.

@Guilherme

Anyway, maybe the timestamps and hypothesis are not right and it’s difficult to know now, but I was impressed to see how quickly the CO2 increased (and likely it was 16% higher due to altitude), and how it can potentially have affected so often our sleep quality in the past.

I know how hard it is to balance outdoor pollution and conditions (like temperature), but this is the reason I almost always sleep with at least one window open (even if just slightly) now. I’ve found that decreasing the CO2 concentration from < 2000 ppm to less than 1000 has vastly improved my sleep quality. However, this isn’t always possible if you’re running AC or heating due to outdoor conditions. My wife has also noticed a massive difference in sleep quality (and cognitive performance) so now she will also insist we always have a window at least slightly open.

@Guilherme

I found also quite interesting to observe the opposite pattern here. In the closed and controlled environment, the PM2.5 readings decreased over time until reaching some plateau in the middle of the night. And likely when the CO2 mostly decreased (air coming in), we also observe the PM2.5 increasing. I believe the outdoor air quality was poorer than the indoor one, leading to such behavior. And this may be tricky for us to balance (although I believe in this particular case the impact of CO2 was worse than the moderate PM2.5).

I would love to hear more about this once you receive the outdoor air quality monitor, as I wonder how bad (or good) outdoor air quality is where you live. Based on how this graph is inverse to the CO2 graph, I believe your conclusion that the outdoor PM2.5 concentration is worse is spot on.

I think this is where having an outdoor monitor will really help you. It makes it much clearer to know when opening a window is a benefit or detriment.

@Guilherme

This for me was the most intriguing behavior… I have not yet read about TVOC other than it’s a relative metric. But I was not expecting it to positively correlate with the CO2 in that way, and I would like to hear your thoughts on this But one thing I started to consider is whether there is some source of VOCs in my bedroom that would lead to increased TVOC numbers while it’s closed… We have painted the apartment one year ago, not sure if it could (still) be related.

This is particularly interesting. You are right in that VOCs can influence CO2 measurements from NDIR sensors, however, CO2 should not greatly impact metal-oxide semiconductor readings (which the SGP40 is). Also, the NDIR sensor should not be impacted much at all unless the VOC concentration is very high. In fact, from what I’ve read, the S8 should have little to no sensitivity to VOCs, and the SGP40 should not be significantly influenced by CO2. As such, I would say that you do actually have a source of VOCs in the room.

It sounds like you’re already very aware of the relative nature of VOC sensors, but this post explains the Sensirion index in more detail:

Air Quality Monitors for VOCs

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 cav…

Luckily, on AirGradient monitors you can change the baseline calibration period to 720 hours (look under ‘place’ settings), meaning it is far more useful than a sensor that adjusts every 24 hours.

One thing to consider here is that humans exhale trace amounts of VOCs. While a very small amount, they can accumulate over time, and this could explain the increase if no other VOC sources are present. While the increase would be relatively small (I would think), it may be that your usual VOC levels are so low that a 500 index reading is possible. To test this, I would try putting the monitor in the room (without people) and completely close the room so there is no ventilation. Does the VOC index still increase? If so, there is another source that you might want to look into - if there is nothing obvious, I would guess it is off-gassing.

@Guilherme

And regarding the TVOC I’m still puzzled… this night I observed the same behavior. Then I tried to turn on the air purifier in the room in the morning, but TVOC stayed quite high regardless. Only solution was to indeed open the windows, then it decreased rapidly.

Does your purifier have a carbon filter? If not, it won’t decrease VOCs, only particles.

@Guilherme

Another experiment I tried was during the day to just close the bedroom window and door again, but this time without anyone in. But the TVOC didn’t increase then. I’m not sure whether it could be potentially impacted by the CO2 concentration itself, or even by some other physiological source… I tried to look it up but couldn’t find an explanation yet, other than that it seems to be indeed tricky to understand it.

I only just read this now, but I think what I mentioned above may be the case. Humans can exhale some VOCs. For example, we exhale acetone and isoprene (among others) and this could build up enough to trigger the SGP40 if there is no ventilation. Since you’ve already tried the empty room experiment, I think this is likely the reason for your increasing VOCs at night.

These are all really interesting findings. Thanks so much for sharing!