Plain old atmospheric CO2. We've upped the amount in our atmosphere to well over 400ppm. Drowsiness becomes noticeable at over 1000ppm but with the now higher a

Plain old atmospheric CO2. We've upped the amount in our atmosphere to well over 400ppm. Drowsiness becomes noticeable at over 1000ppm but with the now higher atmospheric CO2 and the fact that many of us now live and work indoors where poor ventilation can increase CO2 levels you have a recipe for subtle cognitive problems similar to what lead caused.