Following a few days with all the windows closed (thanks cold weather), I've started looking into devices I could integrate for (mainly) Co2 monitoring.

I thought other people might benefit from the information I've gathered, since it's not much more effort beyond me just doing the research.

I haven't purchased quite yet, and may add an update when I do.
Prices are in GBP.

Option 1: Random-name amazon/alibaba jobbies.

• Price: £20-40
• Connectivity to hass: WiFi or Zigbee.
• Sensors: Co2, VOC, Temperature, Humidity, PM2.5 (allegedly...)
• Pros: Very cheap.
• Cons: Data returned is effectively useless, when it gets returned at all.
• These sensors generally don't have a proper Co2 sensor, and instead estimate it from a VOC reading.

I nearly bought one of these a few times, fortunately there is now more data about them.
Once I have my proper solution, I may buy one just to chart how off they are!
As a main device, I would not buy any of these.

Option 2: Awair Element

• Price: £160 new, £70-110 second hand
• Connectivity to HASS: Local API or network API over WiFi.
• Sensors: Co2, VOC, Temperature, RH, PM2.5
• Pros: Decent sensors, integrates with HASS via a local API. AQI read-out on the device itself.
• Cons: Users report wifi issues, which the manufacturer isn't acknowledging.
  This can mean periods where the data is not reported.
  Firmware is closed source, specifications of sensors are not stated (but have been found unofficially)

These are not a bad solution, so long as you are happy dealing with a WiFi device and an API back-end that is closed source.
There is a slightly insane history to the Awair .
When they first released, they could contribute to a system called PlanetWatch, which paid tokens in exchange for readings.
These tokens (somehow) had value, so people started...Yep, they started buying multiples, and mining with them.
This has now ceased, afaik, so there is good second hand supply.
I'm hesitating a little, just because of the reports of wifi issues, which coupled with a potentially problematic wifi connectivity, could leave me with a brick one day.

Option 3: AirGradient (Open Air and One)

• Price including shipping: £120 for the Open Air, £135 for the One
• Connectivity to HASS: ESPHome via WiFi/BT.
• Sensors: Co2, VOC, Temperature, RH, PM2.5
• Pros: Open Source, maintainable, specifications and accuracy clearly stated.
• Cons: A little pricey, availability in Europe not known.

I like this one, and honestly, I could see myself buying it.
I rather like the OLED display on the ONE, too.
They also offer a more basic build-your-own without VOC detection for about £60

Option 4: Roll-your-own, with an ESP32 and an SCD30

• Price: £70 for the SCD30, £10 for the esp.
• Sensors: Co2 and RH
• Connectivity to HASS: ESPHome via Wifi/BT (or whatever you want)
• Pros: Open source, maintainable, specifications and accuracy clearly stated. Most accurate.
• Cons: A little work required, only tests Co2 and RH.

I would definitely consider this, however, it's not that much more for the more featured AirGradient. A cheaper option would be using a SensAir S8, at around £40, which is still as accurate as the AirGradient.

Option 5: Air-q

• Price: £280 for Light, £390 for Basic, £540 for Pro

• Sensors:

• Light: Co2, VOC, Temperature, Noise, RH, AH.

• Basic: As Light, plus CO, KPa, PM1/2.5/10.

• Pro: As Basic, plus O2, O3, SO2, N02

• Connectivity to HASS: Native integration over WiFi, local network.

• Pros: Very comprehensive sensor array. Specifications and accuracy clearly stated. Simple Good/bad LED readout on the device.

• Cons: Oof, that's expensive. Closed source.

If I had unlimited money, I'd probably get one of these, because it's pretty damned comprehensive.
And I definitely respect them for making a turn-key "it does everything out of the box" solution, and completely understand that's why it's expensive.