Working out indoors I've noticed a pretty drastic correlation in measured splits/watts and CO2 over 1k ppm. I tend to notice it at the 1k mark and it falls off

Working out indoors I've noticed a pretty drastic correlation in measured splits/watts and CO2 over 1k ppm. I tend to notice it at the 1k mark and it falls off exponentially form there when working out with large numbers of other people in enclosed spaces.