Are air purifiers effective when windows are open?

Data shows that without an air purifier, keeping doors and windows closed in your house will keep particulate pollution levels at 50% the level of outdoor air. But for some, keeping windows open is a necessity. You might want to keep your windows open to reduce chemicals such as VOCs and formaldehyde levels down in your home or office, or maybe to reduce CO2 levels inside.

Can we have it both ways – clean air AND open windows? Or put it another way – do air purifiers work when windows are open? CK from the Smart Air team put this question to the test by running the DIY Cannon air purifier Smart Air’s 12m2 Beijing test lab six times with the window open and closed.

The Test – Air purifier vs open window

The effect on indoor air pollution levels of having your window open will be more prominent when outdoor air is hazardous, so CK chose two days when outdoor air pollution was bad (February 14th and 15th) for his tests. During these two days, Beijing’s AQI averaged 239 and 290 according to the US Embassy.


Pollution levels on day of test Outdoor air pollution levels on day of air purifier test

To keep the baseline level of pollution consistent across tests, CK opened the window to let in outdoor air (and outdoor air pollution) until it the indoor air pollution levels were the same as the outdoor level.

When the particle counts for indoor and outdoor air were equal, CK turned on the Cannon air purifier on high for 20 minutes. He ran three tests with the windows open and three tests with the window closed.

Pollution levels with Air purifier and window open Pollution levels with air purifier and window closed

Results – can you use a purifier with your window open?

With the window closed, the Cannon reduced particulate by an average of 90% after 20 minutes. With the window open, the Cannon air purifier managed a 60% reduction.

Air purifier window open vs. window closed effectiveness

Window open versus closed air purifier test

The data shows that it’s clearly better to close the windows with running your air purifier. No shocker there – we could have guessed that already! But here’s the more interesting part: even with the windows open, it’s still possible to significantly reduce indoor particulate by turning on an air purifier.

Turning on the air purifier with the window open still reduced pollution levels by 60% within 20 minutes.


Bottom Line – can I open my windows and turn on my air purifier?

Closing your windows and turning on your purifier will give you the lowest PM2.5 levels in your home. That’s obvious.

However, if you have to balance the PM2.5 threat against high levels of VOCs or CO2 indoors, it is possible to get some ventilation AND significantly reduce PM2.5 levels indoors with your window open and purifier on.

You could either choose to keep your window open at a crack whilst running your air purifier, or
you could equip yourself with a super-charged purifier such as the Smart Air Blast or Blast Mini. If you were to go for an over-powered purifier for your room (the Blast cleans 130sqm and the Mini 85aqm) then you’ll be able to get really clean air whilst also getting rid of any indoor pollutants!

Blast & Blast Mini effective air purifiers window open


The Sqair air purifier Kickstarter

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Thanks for this. Down here in Hong Kong, the pollution levels aren’t usually as dramatic as those of Beijing, but they’re certainly nothing to be satisfied with. The willful ignorance of many Hong Kongers and obsession with “ventilation”, coupled with the government’s distorted AQHI which doesn’t reflect international standards, means that people nonchalantly keep windows open when they really shouldn’t. I’m the only foreigner in my workplace and am really struggling with colleagues who insist on windows being open to let in the “fresh” air now that it’s cooling down, which is when the pollution really sets in. I’ve got… Read more »


Us foreigners in Beijing contend with the same sort of thing. On even the worst pollution days of the year, a coworker might open the window by my desk. I have had a few nasty arguments about this. I ended up wearing a dust mask while inside the office, because I just can’t be bothered getting into a conflict over it.

All this, while my company has invested hundreds of thousands of RMB in air purification systems.

At least it’s not a complete waste, I guess.

Kent Lee

Hey stick a hepa filter in the window…get fresh air and clean for everyone too!

tim macmillan

Thanks for taking the time to do this useful test. regards Tim of Australia

Thanks, Tim! I had always wondered about that question too.


Very interesting test!

It would also be interesting with different scenarios, like low pollution days, and the window only slightly ajar, which is how most people probably keep their windows. Also, a longer time frame would be interesting, to see where the levels stabilize.