If buying an air purifier is the first step to breathing clean air, the second step is knowing how to use it best! Here are the 4 most important things to know about how to use an air purifier—based on data, not intuition.
1. Note down the date of purchase
HEPA filters have a lifespan. And unfortunately, the best way to judge when to replace filters is by the date. When I first started testing purifiers, I thought I’d be able to judge based on the color of the HEPA. However, the data showed that the HEPA remained effective long after it had turned black. Color is an unreliable indicator of when to replace a filter.
Although there’s a bit of imprecision, at least it’s easy to do and backed by data: Note down the date you first used the purifier and replace the filter after the specified number of months.
The numbers above are calculated assuming the purifier is on 8 hours a day. You can adjust the number of months based on the number of hours you use the purifier each day.
2. Keeping your rooms well-sealed
One way to increase the effectiveness of your air purifier is to make sure the room is well sealed. The easiest way to improve your home’s seal is to use insulation tape to plug up gaps around windows and doors.
The downside of a well-sealed room
However, there is a cost. The better sealed our homes are, the more the CO2 we breathe out will build up. That’s not a big deal if there aren’t many people in the room, but if there are several people in the room, CO2 can reach levels that will make people drowsy.
Some other pollutants coming from indoors will build up too. One example is formaldehyde coming from new furniture and flooring. Smart Air’s real-world tests show that activated carbon filters will reduce formaldehyde and other VOCs.
If you want fresh air, our tests have found it still possible for purifiers to clean the air when a window is open. Of course, with a window open or a poorly sealed room, you would need to use a stronger purifier and replace the HEPA sooner.
3. Where to place your purifier
Many people place their purifiers against the wall because it’s out of the way and looks nice.
But how does that affect the purifier’s performance?
For purifiers to work best, they need room to take in the air and push it out to the room. If it’s backed up against the wall, it can’t take in as much air.
Just how far from the wall does it need to be? I decided to put our purifiers to the test.
The test method
I ran the test on the Blast, Blast Mini, and Cannon. When the purifier was flat against the wall, the airflow was only about 5%! When the purifier was 4cm from the wall, the airflow increased to 94%. At 10cm (the length of a typical computer mouse) from the wall, airflow was back up to 100%.
Bottom line: For maximum performance, you should place your purifier at least 10 cm away from the wall. However, even just 4 cm away from the wall (about the length of a pen cap) will give close to top performance.
4. You don’t have to run your purifier all day
Some purifier companies suggest running the purifier 24 hours a day, even if you’re not at home.
Smart Air’s tests in real-world homes found that purifiers like the DIY Cannon can reduce particulate by 80% in a 13.5m2 Beijing room in just 20 minutes. This means you don’t have to waste the energy or HEPA lifespan when you’re not at home.
Can I turn my purifier off at night then?
The flipside is that we have to keep purifiers on at night. Some people have the intuition that if the purifier cleans the air, they can turn it off, keep the windows closed, and the air will stay clean all night. However, we tested this by turning a purifier off in a real Beijing apartment and testing how long it took for the air to get polluted again.
The data showed that particulate levels returned to 50% of baseline the pollution level after just 15 minutes and back up to 100% after 80 minutes. Thus, we need to run our purifiers all night. (Side note: that’s also part of the reason auto modes like on the Xiaomi model frequently leave air at unsafe levels.)
Kang Wei is a chemical engineer from the National University of Singapore. He’s currently working on R&D and engineering at Smart Air.