For a home #CO2 monitor, I ended up buying an #IKEA #ALPSTUGA for AU$50.

tl;dr: If you just want to keep an eye on relative CO2 levels in your home, this is the thing to get. It's only accurate to within 100ppm-ish, but that's enough for my use. It also does PM2.5, humidity, temperature, and the time - and the display dims very intelligently at night, making it an excellent bedside clock. Recommended.

The ALPSTUGA can connect to a Matter network, so it can do some smart home stuff, but Matter hubs are 2-3x the cost of an ALPSTUGA, so my research into that aspect of it stopped right there. My eyes also glazed over reading the instructions to turn a Raspberry Pi into one.

It's weird and annoying that my phone can detect its existence but not actually connect to or do anything with it without additional hardware. That's not very "smart" to me.

My two biggest complaints about the ALPSTUGA are that it has no internal battery whatsoever - if you unplug it to move it around to a different room, it immediately loses the time and has to be reset - and the button on top to change the display mode is very loud and clicky and echoes in the plastic case, making it sound loud and cheap (especially loud in the dead of night if I'm sleepy but curious). That's it.

It's built down to a price but actually does what it says on the tin very well.

If either of the environmental readings go over a certain level (1000 for CO2, 15 μg/m³ for PM2.5), and it's showing the time, a little orange dot appears warning you that something's awry. If you leave it on the clock mode, you can use this as a simple warning to open some windows. Or to stop exhaling directly at it.

Also every photo I take of this thing makes the display look super blurry, but it's not - that's just my phone blowing out the bright colours. It's much sharper in real life.