I had a similar experience. I got a cheap CO2 sensor from AliExpress, I basically would open a window or the door to my office if it got over 1000 during the day. I don't consciously notice a difference, but it also measures pm2.5 levels and I was surprised at how high they get while cooking.

Then for shits and giggles I brought it with me to the inlaws and after turning it on there was shocked the CO2 levels were 4300ppm!

Suddenly the headaches they would get at home made sense, and they've started opening windows in the mornings to help circulate some air.