Q&A: Do ACs bring in more dirty air?

tmthyliu 提问: Just built a DIY air filter, pretty excited to have clean air in the house! I was wondering though, how does this work when the air conditioner is on? Does it make a difference at all? I don’t know how ACs work but it seems they pump more air into my room (where does the air come from, outside??). Would it make sense to strap a filter on the AC unit? When you ran your tests did you have the AC on in the same room? Thanks for the amazing work!

Excellent question! I long wondered whether ACs bring in dirty outside air. I tested that question with my particle counter and the answer is no:


And a video test here:


To the two other questions:

  1. Does the fact that the AC is moving air influence purifier effectiveness? I suspect if you were blowing air around AND running your purifier, you might actually get slightly better performance because it’s taking that clean air and spreading it around the room. But that’s just a hunch.
  2. Would it make sense to strap a filter onto an AC? Yes! I wouldn’t strap a HEPA there because the AC isn’t strong enough for it, and you’d really weaken the cooling effect of the AC. But a weaker filter could help.

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So I have a wall mounted AC unit in Thailand with a compressor outside. The air is definitely not coming from inside as the only vents on the wall unit are exhaust cold air coming in to the room.

I gather I can’t put a HEPA filter on the inside, but how about strapping one to the outdoor compressor like in your home made DIY flat fan version? It wouldn’t be totally flush so would it still have any effect? Or just break the machine?

Many thx

Hi Simon. Double check the top of the AC unit, normally the air inlet (where the air is sucked in) is there, and then the air is blown out the bottom. If you have a particle counter or air quality monitor, one easy way to see if the AC is really bringing in outdoor air is to put the monitor right next to the AC. Placing HEPA filters on the AC unit inside isn’t ideal as you say – the AC fan isn’t strong enough to push air through a HEPA filter. I’d double check that the outdoor compressor really… Read more »