Do Air Purifiers Work with the Window Open?

Will my air purifier still work with a window open? Recent test data from Smart Air, air purifiers can still significantly reduce PM2.5 in a room even when a window is slightly cracked. Here’s what you should know:

The Obvious: Air 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 work? Will it 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 open to ventilate out CO2. In addition, new studies show COVID-19 spread is reduced drastically when both filtration and ventilation are used together.

VOCs and formaldehyde

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 Cannon air purifier

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 an Air Purifier with the Window Open?

With the window closed, the Cannon air purifier 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.

How to Use Air Purifier with Window Open

The best way to use an air purifier with the window open 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

Bottom Line: Can I Turn on My Purifier and Open a Window? Will the Air Purifier Still Work?

Even with a window open, air purifiers can work decently and 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.

Smart Air

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