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Comment by matznerd | Hacker News Reader

Not sure what people are interested in CO2 sensors for, but I use them as a proxy for ventilation in indoor spaces, which is then a proxy for covid risk.

It won't tell you if the air is being filtered (need MERV13 or better), but a lot of places for efficiency and/or due to old HVAC equipment don't bring in fresh air, which increases the risk in a more confined and/or crowded space.

Also, some places like airplanes, which have HEPA filters and have 10-20 air changes per hour (ACH), don't run those systems at full power or bring in outside air at fast enough rate until airborne, so PPM can't build up to 2000 ppm vs the ~420 ambient outdoor CO2 level and approx 800 "good" indoor air rate.

I use an Aranet4 device, which uses 2 AA batteries that last like a year.

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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.

I tried this but somehow my sensor got stuck at 500 ppm and never recovered. My Airgradient devices have not yet suffered from this problem.

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.

Devices like the Aranet4 are mainly useful for telling you how well ventilated a space is relative to how many people are breathing into it, which is a very good proxy for viral load and viral risk. It's pretty much only in spaces with high quality air filtration (aka high CADR for cleaning an enclosed environment) or significant far-UVC protective measures that the proxy relationship breaks down, and the breakdown will always be an overestimation of risk from the CO2 monitor, never an underestimation. For those reasons it's a very useful tool to have to put an upper bound on the risk of the space you're in and help you make informed decisions about whether you can e.g. hold your breath and take a drink under far-UVC cover or not.

PM2.5 sensors or other things like what you're thinking of do have their place, but they also have a bunch of other issues - they'll be detecting VOCs, brake and tire dust that's fine enough, any aerosol whether it's coming out of a mammal or not. I think it's inaccurate to say they're just a generically better solution than CO2 monitors for viral risk.

Exactly. CO2 is an imperfect proxy, but it's a pretty decent one. And, even for spaces that have high quality air filtration (e.g. a strong commercial central air system with good HEPA/MERV filters), the CO2 measurement can also tell you if the system is configured with a good recirculated to outdoor air ratio (residential systems usually have no such thing, unfortunately).

But yeah, as you said, the best aspect of a CO2 monitor is that it gives you a useful upper bound for the general respiratory virus risk in a space.

The Aranet4 sounds like a solid choice for monitoring

I'd say the AirGradient products are a better choice since they includes VOC, PM and NOx sensors (Aranet4 doesn't), are open source, and Achim Haug who did this study is really dedicated to air quality.

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.

In modern planes, advanced HVAC systems end up being cheaper than older air management methods.

The old way of keeping a planes air “fresh” was to bleed air from the compression stage of each engine, because the air there will be at high pressure, and also reasonably high temperature (outdoor air temps at cruising altitude is -40C to -57C). You then just inject that air into the cabin, and let the old air bleed out.

But it turns out, pulling air like that from the compression stage of a turbine has a material impact on efficiency (you badly disrupt the airflow through the engine), and engine efficiency is so high these days, that removing those air bleeds, and replacing them with sophisticated HVAC systems with dedicated compressors, heaters and filters ends up being cheaper, at least from a total cost of ownership perspective. As a consequence, modern planes from the past 5-10 years have had some pretty capable air management systems. Of course there’s a much larger stock of older planes out there, which you’re more likely to be travelling on. But the claims regarding plane HVAC aren’t crazy when looking at modern planes.

The dubious claims made were about how aggressive and effective internal filtration was, not about actual fresh air circulation.

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