With a window open, air purifiers still effective?

Will my purifier still be effective if I have a window open? New test data from Smart Air finds that purifiers can still meaningfully reduce particulate pollution in a room with a window cracked.

Obvious Town: Purifiers Most Effective With Windows Closed

No one doubts that purifiers are most effective with the windows closed. That’s obvious. Real-world tests show that–even without an air purifier–simply keeping doors windows closed keeps particulate pollution levels at 50% the level of outdoor air.

But the more practical question is, if I have to keep a window open for some other reason, will running a purifier still have a meaningful effect? For some people, keeping windows open is a necessity. You might want to keep your windows open to reduce chemicals such as VOCs and formaldehyde in your home or office. Others might want to keep a window ajar to ventilate out CO2.


The Test: Air Purifier vs. Open Window


Can we have it both ways – clean air AND open windows? 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.

DIY Purifier Cannon DIY Filter

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 particulate pollution was the same as outdoors. Then, CK turned the Cannon air purifier on high for 20 minutes. He ran three tests with the windows open and three tests with the window closed.


Air purifier Window Open Closed Pollution Test

Results: Can You Use a Purifier with The Window Open?

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

Window open closed air purifier still effective

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.


Window open closed air purifier effective pollution

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


Bottom Line: Can I Turn on My Purifier and Open a Window?

Even with a window open, air purifiers can still meaningfully reduce particulate air pollution in the home–by 60% in 20 minutes in our tests. Obviously closing the windows is the most effective, but if you have to balance outdoor pollution against VOCs or CO2 indoors, it is possible to get some ventilation AND significantly reduce pollution.

Power Is Key

The best way to do this is to use a purifier that’s rated for a larger area than your room size. We do this in the Smart Air office by using the super-charged Smart Air Blast, which cleans up to 130m2. Here it is kicking butt in Beijing while the window is open.

Smart Air Blast Window Open Test Beijing


At home, I use the Blast Mini, which cleans up to 85m2, which is far larger than my studio apartment.


Blast Brothers Smart Air FFU Purifier


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“Simply keeping doors windows closed keeps particulate pollution levels at 50% the level of outdoor air” – because your lungs purified it.

Great thought! I also thought our lungs and bodies might actually act as air purifiers. However, we typically only breathe in 6L (or 0.006 cubic meters) of air a minute. In a normal 20sqm or 200sqft room, there’s about 48 cubic meters of air, so if our lungs were really filtering the air, it’d take us a whopping 133 hours just to breathe through all the air in the room! The main reason for the reduction is that the air pollution particles – being heavier than air – drop to the floor.

Vicki Levine

I was exposed to toxic levels of Formaldehyde when the person who owned the apartment before me put down floors that were bought from lumber liquidators. During that time they were importing floors from China that Were contaminated with up to 100 times the California legal limit of Formaldehyde. I replace the floors and bought an AirpuraAir purifier I took all of my lamps with their lampshades out of my room and even my stereo speaker. Is there anything else I should be doing? I am tired of blowing my nose into seventh generation tissues which supposedly have no Formaldehyde,… Read more »

Liz - Smart Air

Hi Vicki, our suggestion for formaldehyde problem is to use an air purifier powerful enough with an activated carbon filter while open your window whenever you can. Here are some articles on formaldehyde and what we can do about it:


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.