Could air purifiers be a gimmick?

When I came home this evening, the PM25 was around 8 which is decent. During the daytime when windows are open and the house is ventilated, it goes up to about 20 for some bizarre reason.

Anyway, I was just heading to bed when I noticed the indicator was orange. That was the first time I've seen it in that colour and the PM25 indicator was near 40!

I have absolutely no idea what could have caused this drastic jump 🤨