For them to be effective, you'd be replacing them every day. Possibly twice a day.
You are drawing more in, under exercise, than would normally be drawn in in the same time by someone wearing one in a factory.
Saturation point is reached quicker & just past that is where you start drawing more of the very chemicals you are seeking to keep out, into your lungs. Was given one to try, which I was informed would indicate by a small strip getting darker, when it required replacing. Three miles into the commute it required replacing. On that basis 12 a day, minimum, would be required.
First one found. Eight hour work life.
http://www.arco.co.uk/103/content/factsheets/1SS031 5 MSA Affinity VFFP3D Fold Flat Resp.pdf