You write: "If 90% of the air is drawn through the mask from the outside and 10% is drawn from inside the mask (which I estimated by measuring the volume of air I exhale compared to the volume inside the N95 mask) , then we can estimate the CO2 level he’s breathing in is at (90%*600+10%*38,000)=4,340 ppm which is in the danger region for…
You write: "If 90% of the air is drawn through the mask from the outside and 10% is drawn from inside the mask (which I estimated by measuring the volume of air I exhale compared to the volume inside the N95 mask) , then we can estimate the CO2 level he’s breathing in is at (90%*600+10%*38,000)=4,340 ppm which is in the danger region for headaches, sleepiness, loss of attention, increased heart rate, and slight nausea."
This is a flawed analysis. You are assuming that you breathe as deeply with the mask on as with the mask off. With the mask off you can exhale and inhale deeply, i.e. with a greater volume of gases. With the mask on the tendency is to shallow breathe, so the volume of air from outside the mask will be reduced relative to that behind the mask. Conclusion: you may be suffering more hypercapnia than you think.
You write: "If 90% of the air is drawn through the mask from the outside and 10% is drawn from inside the mask (which I estimated by measuring the volume of air I exhale compared to the volume inside the N95 mask) , then we can estimate the CO2 level he’s breathing in is at (90%*600+10%*38,000)=4,340 ppm which is in the danger region for headaches, sleepiness, loss of attention, increased heart rate, and slight nausea."
This is a flawed analysis. You are assuming that you breathe as deeply with the mask on as with the mask off. With the mask off you can exhale and inhale deeply, i.e. with a greater volume of gases. With the mask on the tendency is to shallow breathe, so the volume of air from outside the mask will be reduced relative to that behind the mask. Conclusion: you may be suffering more hypercapnia than you think.