Are air purifiers effective with a window open?

Will my purifier still be effective if I have a window open? Of course, closing windows keeps pollution out. 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 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? 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 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.

Window Open Closed Purifier Pollution

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).

Filter Front Back Effectiveness Efficiency 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.

Filter Front Back Better Pollution Test

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?

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.

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 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.

 

Blast Brothers Smart Air FFU Purifier

 

 

The Sqair air purifier Kickstarter

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Em

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 »

DX

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.

Ras

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.