Okay so 4 ferns would do it in 250 +- 100 hours then. I'm not saying one home garden will solve global co2 buildup I'm saying its significantly cheaper and potentially more rewarding as a hobby than trying to play god with air quality and a 1k budget. You're also not accounting for gaseous exchange rates related as a function to current air quality. Yes a terrible PPM is 2000 but your body produces less co2 as a result of reduced function in such an environment, therefore increasing the effect of any filtration.