As you said it won’t tell you if the air is being filtered. I think you’re better off measuring the small particle content of air if your concern is viruses, or both.

FWIW, Sensirion's AEs told me they have empirical data showing that CO2 drops to outdoor levels at least once a week even in closed environments. Based on that they recommend using the auto calibration.

Anecdotally, I see my CO2 levels hover around 600ppm inside even over night.

I chose the AirGradient[0] One. It's slightly more expensive than the aranet4, but has more sensors. It's open source, reports CO2, PM10, PM2.5, temperature, humidity, and more I'm probably forgetting offhand. Loaded it with the esphome firmware, so Home assistant integration is seamless.

And since it's open source, when a sensor inevitably wears out in a couple years (like the PMS5003), I can just replace the sensor instead of of buying a whole new unit.

Highly recommend it.

[0] https://www.airgradient.com/

Edit: I completely missed that AirGradient blog article

This. There are plenty of co2 but what I really need is to measure o2 level (plus co presence).

The MH-Z19 is similar from the performance of the S8 but we do see more noise in the readings and the MH-Z19 also seem to have less life time. We see some of them dying after around 5 years whereas the S8 has a ten year warranty and so far have been rock solid for us.

The cheapest one that is not eCO2 (estimated) will do the job of telling you when you’ve not ventilated enough just fine. No need to spend hundreds to thousands on lab-grade tech.

+- 50ppm, or even 100 or 200 is nothing. You will mainly care if the ppm is around 500 or around 1000-2000 (depending on preference).

The devices that have alarms are quite nice. Leave them on a desk and they’ll tell you when to open the window. The ergonomics of how you’ll use it are maybe more important than accuracy when it’s <200ppm off.

Aiegradient is great and they make indoor and outdoor monitors. Fully open source. They are the authors of this article and to a lot of research and advocacy which is great.

For NDIR sensors, the Aranet4 Home is an often recommended one. I have a couple, and they are great little units. A number of scientists recommended them during the heart of covid as a proxy for covid risk in indoor spaces (obviously, an imperfect proxy, but since there are no cheap, portable devices to measure viral concentrations in the air, you work with what you've got).

I use the Airthings (the $300 one). It detects more than just CO2, has an e-ink display, and has a decent app.

When I looked a few years ago the Await element was best consumer grade one on the market at a good price. I’ve been happy with it so far.

I like my qingping - works with both - homekit and home assistant.

In a small enough unventilated room you can fill it with enough co2 to make it unpleasant to be in with an hour. E.g. a study in the usual suburban house.

Let's be honest - the AirGradiant people/person are submitting these.

Which is fine - they seem like a good company with a strong open source commitment, but they do go a little overboard on HN submissions.

I don't see how it is a repost. That submission was to an article that talks about a new model of photo-acoustic sensor from Infineon.

This submission is to an article that compares photo-acoustic sensors to NDIR sensors. Its only connection to the earlier submission is that the author of the comparison participated in the discussion of about the new Infineon sensor, updated the comparison article, and linked to it in one of their comments.