I\u2019m not an expert in air pollution. I was just a poor grad student living in Beijing, and I started making DIY air purifiers<\/a> to cope with one January airpocalypse.<\/p>\n\n\n I published instructions on how to make these simple purifiers<\/a>, as well as tons of test data<\/a> showing it actually works (1<\/a>, 2<\/a>, 3<\/a>, 4<\/a>). Soon I found I wasn\u2019t alone. Other air-breathers in Beijing were getting so desperate with the air pollution that they started emailing me to ask if I\u2019d ship these purifiers to them. While I was still in my PhD program, I started a social enterprise called Smart Air<\/a> to ship these no-nonsense purifiers<\/a> to fellow air-breathers-in-need.<\/p>\n\n\n Soon enough, people started asking me how long the filters last. At the time, I was jealous of the big purifier companies. I wished I could give a simple answer like they give. \u201cSix months!\u201d<\/p>\n\n\n But numbers that are so perfect and round like that make me suspicious. Is reality that clean and tidy? Plus, is that six months in Sweden or China? I think it\u2019s important to base recommendations like this on actual data, so I asked people to hang tight while I put it to the test.<\/p>\n\n\n\n My first thought was that we could use the color of the HEPA to know when to change it. We could even give people a handy color strip that they could compare to their HEPA and know when to change it, kind of like those color strips for testing water.<\/p>\n\n\n Of course, to create the test strip, I needed to know the actual answer, so I needed real effectiveness data over time. To get that, my friend and co-founder Gus did a longevity test. He turned on his Original DIY<\/a> every day in his 12.3m2 Beijing bedroom for 200 days and used a laser particle counter<\/a> to track how effective it was.<\/p>\n\n\n It was important that we were testing it in real Beijing air.<\/p>\n\n\n Here\u2019s what a relatively normal day looked like:<\/p>\n\n\n After 200 days, here\u2019s what we found. To smooth out day-to-day variability, I averaged the effectiveness over 10-day periods.<\/p>\n\n\n HEPAs don\u2019t die like light bulbs. They don\u2019t die at a single point in time. So when it needs to be replaced depends on your tolerance. After performance drops 20%, that\u2019s when I would want to change it. That happened around day 160.<\/p>\n\n\n\n OK, 160 days. Below are pictures of the HEPA aging over the course of the test. Can you spot the picture where it crosses 160 days?<\/p>\n\n\n If your eyes are like mine, then we both have no clue! Here\u2019s the answer:<\/p>\n\n\n What those pictures say to me is this:<\/p>\n\n\n\n HEPAs get black really quickly, yet they remain effective for a long after. After they get black, it\u2019s exceedingly hard to tell the difference between 100-day black and 200-day black. So here\u2019s how that\u2019s useful:<\/p>\n\n\n\n 1. If your HEPA is NOT <\/strong>thoroughly black, it probably has weeks or months of life left in it.<\/p>\n\n\n\n 2. If your HEPA is thoroughly black, it might still have months left, or it might be time to replace it.<\/p>\n\n\n\n Here\u2019s another bright idea I had: I\u2019ll use my particle counter to test the HEPA!<\/p>\n\n\n Sure, I could do room tests to figure it out, but those take a mess of time. Instead, I can just hold the particle counter up to the HEPA.<\/p>\n\n\n If it\u2019s capturing 99% of particles like it should<\/a>, then I\u2019ll keep it. If it\u2019s hardly capturing a lot less, then I\u2019ll replace it. Simple!<\/p>\n\n\n\n Here\u2019s why my intuition was completely wrong. Imagine a strainer. It\u2019s pretty good at capturing particles.<\/p>\n\n\n Now imagine a dirty strainer, with a bunch of particles stuck in it. Now is it better or worse at capturing particles?<\/p>\n\n\n It\u2019s better! It\u2019s just that it\u2019s now a lot worse at letting water through.<\/p>\n\n\n\n The same thing is true of HEPA filters. They get better <\/strong>at capturing particles over time, but worse at letting air through. So using a particle counter to test the air coming directly out of the filter won\u2019t help us at all.<\/p>\n\n\n\n Since air flow is the thing that declines over time, we can test for that! We don\u2019t want a particle counter, we want one of these:<\/p>\n\n\n Indeed that\u2019s what we did at Smart Air with our Blast Air Purifiers<\/a>:<\/p>\n\n\n We tracked air flow over time<\/a>:<\/p>\n\n\n P.S. <\/strong>Make sure to test the filter when it\u2019s new too! Otherwise, you won\u2019t have anything to compare it to.<\/p>\n\n\n\n Open Data<\/strong><\/p>\n\n\n\n<\/figure><\/div>\n\n\n
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Method 1: I\u2019ll Use My Eyeballs!<\/h3>\n\n\n\n
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Is Color a Reliable Indicator of Effectiveness?<\/h4>\n\n\n\n
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Method 2: I\u2019ll Test it with an Air Quality Monitor!<\/h3>\n\n\n\n
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Method 3: The Most Practical Method<\/h3>\n\n\n\n
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