Since 1900 CO2 has gone from 300 ppm to 420 ppm. That study looks at 900+ ppm CO2. They also controlled that by doubling the ventilation speed of the room, wh

Since 1900 CO2 has gone from 300 ppm to 420 ppm. That study looks at 900+ ppm CO2. They also controlled that by doubling the ventilation speed of the room, which would definitely have had secondary effects. More importantly, you can't infer anything about the CO2 levels when they were being measured from the global CO2. Big buildings would have had very high CO2 levels since there's no central air. Small buildings would have much lower CO2 than any current buildings when the windows were open, and much higher otherwise. Oil or gas lamps would raise the CO2 levels a ton. Proximity to coal furnaces would have had a huge impact. Fireplaces too. Bottom line: global CO2 levels have virtually nothing to do with how much CO2 people are exposed to.