Just got a Carbon Dioxide meter a few days ago. Walked over to a park with Kayimbo to calibrate it--400 PPM of CO2 is currently the average outdoor concentration globally.
While walking around the neighborhood, CO2 concentrations were pretty steady around 400 - 420 PPM, depending on the breeze. The following is a list of our readings taken in various places:
Pedestrian bridge over the highway: 450 ppm with regular amounts of flowing traffic
Living room: 550 - 650 ppm
Kitchen: 700 ppm
Devnill's room: 850 ppm
Kayimbo's room: 900 - 1000 ppm (he always keeps the door shut)
Rachel's room: 650 - 700 ppm
Bamboo greenhouse: 400 ppm
It's really interesting to see how much the CO2 concentrations change based on human activity. For example, the initial readings in Kayimbo's room of 1000 ppm are only when he's inside the room. When he first came home from work the concentration was around 550 ppm, but it quickly increased as he spent more time in the room.
Some of the most interesting readings I noticed were when the oven was turned on. I was in my room (about 60 feet away from the kitchen) and over the course of a couple minutes the CO2 concentrations suddenly spiked up to 850 ppm. I went down into the kitchen to check the readings and it was over 1200 ppm. To be fair it kinda smelled like there was some pizza cheese that melted and dripped into the bottom of the oven and it was burning a bit. Still tho.
The furnace running also measurably increases the CO2 concentrations in the room by 100-200 ppm while it's on, but it quickly dissipates after the desired temperature is reached.
Conclusion: We need more plants in the house and y'all should probably buy some too. Especially if you live in Canada and never have the windows open.
Edit: If you'd like more information on the health effects of elevated CO2 levels, read this article:
https://medium.com/@joeljean/im-living-in-a-carbon-bubble-literally-b7c391e8ab6