Buying an air purifier (update)

After using the air purifier for a few months, I'm quite sure it can actually lower the PM2.5 level.  How do I know?  Coz I've also bought a PM2.5/10 detector (You can get a cheap one for around US$40-50).  Does it help my cats and myself to relief allergic symptoms... hmm I'm not too sure.  Sometimes it seems a little better, but the result not significant enough to convince me yet.  I did not turn it on 24 hours/day though, usually only 8-12 hours/day when I'm at home.

Anyway, one thing for sure, the HEPA filter is expensive, so I bought some cheaper filters from 3M and put them in front of it in the hope that the HEPA can last longer.  And it looks good as the 3M filter got really dirty after 3 months, and the HEPA still look relatively clean.  I actually also put some sponge, which is really cheap, in the very front to reduce large particles from getting inside.

Most economic air purifier probably has simliar struture: (1) front filter to block large particles (in my opinions, the holes are usually too large, and still a lot of dust can get in),  (2) Active carbon filter, (3) HEPA filter

My setup: Front filter -> Sponge (really cheap) -> Active carbon filter -> 3M filter (more costly, but still cheaper than HEAP replacement) -> HEPA

HEPA with 3M filter:

Sponge