I was thinking the same thing. He has graphs for how quickly the pm2.5 levels reduce compared with a commercial machine, but not compared with running the fan a

I was thinking the same thing. He has graphs for how quickly the pm2.5 levels reduce compared with a commercial machine, but not compared with running the fan alone, without filters. I believe that it is quite possible the majority of the air is not going through the filters. It seems that much of the air might spill out down the insides of the filters, rather than being forced through. In that case, the graph could be equally explained by the fan causing outside air to be sucked in.