I've been wearing a surgical mask on top of a valved N99 some of the time. That seems to satisfy. Maybe I could put more serious filtration material over the

I've been wearing a surgical mask on top of a valved N99 some of the time. That seems to satisfy. Maybe I could put more serious filtration material over the valve though. I would like to find some affordable PAPR's (powered air purifying respirators). I see a number of places claiming to make them, but none seem that actually available to buy. It might be interesting to measure the amount of CO2 inside a mask after a person has been wearing it for a while. There are some differences between mask types, but I don't know if any would appear between mask brands and models. Here is a study where they tested some masks that way: https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-021-06056-0 https://bmcinfectdis.biomedcentral.com/articles/10.1186/s128... I have a CO2 sensor on order from Adafruit and while try doing some tests like this myself when I can.