CO2 can also reduce cognitive capacity. It's useful to monitor, other than just being a proxy for air quality.

Sidenote: it's a fun project to built your own. You can buy the good sensor from this test (Senseair S8) for about $20 from AliExpress. Add a ESP32, a case, and EspHome.io software, and have a high quality sensor for <$30. Same quality as the Aranet4 device you mention, at about a tenth the price. They also use a senseair sensor with the same accuracy rating, but slightly higher priced one designed for long term use on a battery.

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.

The Aranet4 sounds like a solid choice for monitoring

Aranet is pretty expensive for what it does, there are great NDIR based monitors on aliexpress for much less.

I'm not sure I believe the extremely dramatic claims made about airplane HEPA filters, having seen normal HEPA air purifiers and various dust collection systems, how much space they take, what they cost, and how loud they are, and seen airplanes & the cost-cutting they undertake.

What I heard in 2020, here as in many other areas, appeared to be a combination of lies, exaggerations, and wishful thinking that would allow the jet-set to justify not changing anything they were doing.

Based on chained analysis of R number (some grains of salt there), later variants of COVID were more contagious than measles or just about anything else we deal with using dramatically more intense precautions. There was a lot of cope.

All homes need fresh air ventilation systems - at least here, all new homes are required by code to have it.

ERV / HRV or a static one that goes into your air handler.

Personally, ERV is the way to go. Panasonic makes one that is perfect for the ceilings, and there's Nutone, etc.

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

It's crazy how optimized the Aranet 4 Home is. It runs on AA batteries for a whole year, no need to plug it in.

That's unique in the space, and a case where e-ink makes a real difference. And very convenient in practice.

I bought one a year ago for 200 bucks. I'm still happy with the purchase, and I haven't had to change the batteries yet.

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 02 sensors that I know of are around $50 each, and have a shelf life of 1-2 years. Average price for a decent sensor is closer to $90. And digital solid state O2 sensors go for $400+.

*Edit*: google dive gear express O2 sensors to see these.

I use them for technical scuba diving. In particular with a rebreather, which is capable of mixing oxygen with other gases such as nitrogen and helium. And as the gas is recirculated CO2 is scrubbed away.

CO2 sensors are also available for rebreathers but are not always reliable (same for O2 sensors, so a minimum of 3 are used). The human body is actually quite sensitive to CO2, and it determines our breathing rate. So there are ways to know if you're likely being exposed to elevated CO2 levels by being aware of your breathing rate, paranoia, etc.

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

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.

TFA Dostmann make an “Airco2ntrol mini” which is USB powered (and you can pull the readings over USB too) and has a nice little red-amber-green light system on the front (as well as the LCD) for easy reference. It’s about £85. It has a dual NDIR sensor (not sure which one exactly) which the article says are usually the most accurate type of consumer-grade sensors.

I’ve had mine for years, very pleased with it… just sits on my desk and lets me know when it’s time to open the windows.

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.

Yes. Look up quingping co2 monitor on Aliexpress, the white one with BW screen. It’s NDIR based, and can last a month on battery and then sync the measurements to the cloud, if you want to leave it somewhere. Around $70.

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.

Sure if you are already airing every hour you don't need it. Then for me it was surprising how often I have to air and for how long (also depends on draft and temperature difference). Once you take into account the more harmful particulate matter coming in from outside, it's about finding a compromise, and without measuring you are tapping in the dark. But in the end people get old without all that stuff.

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.

Winter is coming, some won't keep windows open regularly.

Higher levels of CO2 impair cognitive function [1].

In fact there's no floor - even small increases in CO2 have small but measurable effects.

[1] https://www.hsph.harvard.edu/healthybuildings/2021/09/09/imp...

What sensor in my body detects CO2?

Unless you mean headache-inducing levels of CO2 which, per my actual sensor, is upwards of 3000 because at that level I still didn't notice anything. The air feels warmer, but that's attributable to me breathing in a small and enclosed space

Genuinely curious if there's something in our bodies that has been shown to serve as CO2 detector, or if you just mean "I can smell stale air and why don't you just do the same" (which I'm pretty certain relies on proxy factors like dust, mould smells like in a moist cellar, temperature, a combination, and/or perhaps other stuff that didn't come to mind)

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.