Situation:

• apartment of 42m², ground level, recent from 2013
• humidity is 40%
• apartment near a big city, in the suburbs
• near freeways
• we have a very good air handler with 2 extraction systems: one in the kitchen one in the bathroom
• we have 2 windows which have an opening on their top parts, they blow air inside the apartment
• these openings are like these https://dekorplast.com/en/products/windows/wentylacja/

What's happening:

• in the evening it smells pollution. It smells like something being burned nearby.
• Our throats are sore in the mornings
• it smells stronger in the winter (I guess, some people heat with wood which increases pollution)

–> we need something to clean the air if we wanna live longer than 40

What I did:

• I bought some standard HEPA filters on the Internet
• I cut them
• and I put them flat against the air inlet on both my windows:
• • I removed the plastic cover
• • Then I flatten the HEPA filter against the window frame on top of the air holes
• • And then I put back the plastic cover on top of the HEPA filter
• • That way my HEPA filters stay in place and they filter 100% of the incoming air

What happened using this rigged system:

• The HEPA filters got black sooth in them after a couple days
• It doesn't smell burned in the apartment anymore. If I open the windows, it still does smell like something being burned outside.
• Unfiltered air still gets in the apartment, but much more slowly
• We don't have sore throats in the mornings

My question

–> Because I wonder if the "Air purifiers" are useful. Please read my analysis below.

My wife and I, we thought maybe we should buy an "Air purifier".
There are many on the market with many different prices. We know and that's not the issue/point here.

However…
When I think a little… on how purifiers work…

They have a fan that pulls air in, they filter it using a (often custom) HEPA filter + carbon filter, and then it goes back into the room as clean air.

WHICH MEANS

That one must let the crappy air in first, so that the purifier cleans it then.

We have to trust that the purifier is strong enough that it sucks air in faster than the crappy outside air has time to deposit inside, and faster that it could reach our lungs.
Correct?

But since these air purifiers do not plug on the air inlets nor cover them, they will never filter 100% of the outside air coming in.
Still some 'unpurified' air will somehow flow in and move outside of the purifier's reach.

Contrary to my simple rigged way, which does cover the inlet air and which hence cleans 100% of the foul air.

So my idea is to buy these custom HEPA filters from some air purifier sellers, and just stuff them under my window cover thingy. Problem solved, in my view.

But… am I correct?
Did I miss something?

Pretty much, I believe air purifiers are useless unless they are connected to the air inlets and covers them. Which isn't the case.

Am I right?