Are Purifiers Useless If Windows are Opened?

Tests show that keeping the doors and windows closed will keep particulate levels at 50% less than outdoor air on average. But some people really want to keep their windows open. For some people, it’s an old wive’s tale that windows must be kept open. For others, it’s a way to reduce chemicals coming from new furniture or recent remodeling.

Can we have it both ways? Or air purifiers useless with when our windows are open? CK from the Smart Air team put this question to the test by running the DIY Cannon air purifier in the 12m2 Smart Air testing room six times with the window open and closed.

The Test

The effect of having your window open will be more prominent when outdoor air is hazardous, so CK chose two days when outdoor air 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.


To keep the baseline level of pollution consistent across tests, CK opened the window to let in outdoor air until it reached the outdoor level.

When the particle count reached the same level as outdoors, CK ran the Cannon on high for 20 minutes. He ran three tests with the windows open and three tests with the window closed.

Air purifier window open air purifier window closed


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 just a 60% reduction in particle count.

Air purifier window open vs. window closed

Window open versus closed purifier test

The data shows that it’s clearly better to close the windows with running your air purifier. No shocker there! 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.


Bottom Line

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.

One suggestion is you equip yourself with a super-charged purifier such as the Smart Air Blast or Blast Mini. Having your windows open whilst running a purifier like this can give you a good balance between low PM2.5 levels and low VOC or CO2 levels!

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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.

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.