我发表DIY净化器方法之后，经常有人问我：HEPA能用多长时间？大的过滤公司有简单的答案（比如6个月），我也希望能有简单的答案。
但是除非真的去测试，不然我也不知道应该怎么回答。而且，如果BlueAir说能在瑞士用6个月，你觉得能在北京用6个月吗？我们需要在中国做的测试。
方法
所以聪明空气的Gus每天坚持睡前在他的12.3m2卧室开机，每天用激光测试仪记录减少颗粒的百分比。这样可以测试HEPA能在真实的北京空气状况下用多久。
Gus每天睡觉的时候开一台DIY 1.0。为了算效果，我算了DIY减少了百分之多少0.5微米及以上的颗粒。
比如说，第54天是这样：
为了让趋势更明显，我算了每10天的平均效果。
结果
在第100天左右，效果降低4%了。在第140天左右，效果又降低5-10%了。不过，为了科学，Gus牺牲自己的肺而坚持到200天。
HEPA的失效一直持续。在第200天，效果降到50%以下。
我还想保护Gus的肺（尤其是因为冬天的雾霾已经开始了），所以我让他停止测试。
结论
在北京空气现状下，聪明空气的HEPA全效持续了100天（每天大概8个小时，总共729个小时）。到第140天，效果才降低4%，所以我自己会选择140天换HEPA（总共1,028小时）。
建议：我建议每140天换HEPA，每天开机8个小时，总共1,000个小时。如果你用得更多，可以按你每天开机几个小时算寿命。
提醒：这个测试是用DIY 1.0，结果不一定能用在大炮身上，因为大炮的风量更大。这个测试是在北京做的，但是在空气稍微好一点的地方（比如广州和香港），寿命应该会长一点。
公开数据
跟以前一样，我公布了原始数据，全部的200天！
Great info about Air Purifiers, keep it up. The very informative and impressive post you have written is quite interesting. Keep sharing such valuable information.
Luckily, I live in Switzerland where the average outdoors PM2.5 is below 5ug/m3 most of the year. Still, I use an air purifier, mainly for pollens, and the occasional haze where PM2.5 jumps over even 100ug/m3.
Do you think I still have to change the filter every 6 months (manufacturer recommendation), or I can stretch longer? In other words, is it the captured particle quantity that degrades a filter or the stream of air passing thorugh it?
I’m ready to do a longevity test (with a cheap SDS011-based mobile PM2.5 counter), but I only got my air purifier a month ago so it will take time.
Good question Szak! What typically degrades the purifier performance is the amount of particles that get clogged and captured in the purifier. If your air is cleaner than say Beijing air, then it’s likely that your HEPA filter may last longer. This is why the ‘manufacturer recommendations’ aren’t very helpful – they don’t tell us whether that’s for Swiss air or Beijing air. You could also try asking the manufacturer to clarify this, or best yet (as you’ve said) – get a PM2.5 counter. Once you see that the numbers are no longer dropping to what you’d expect, it’s probably time to change your filter.
Thank you. Meanwhile I found your article about the Sqair hepa filter lab test which even has the mathematical formula, and indeed it’s the mass of the filtered pollutants that seems to matter. I’m learning extremely lot from your website, thank you for sharing all this data. And if one day Sqair will be available to buy in Europe (through Aliexpress maybe?) for a reasonable shipping cost, I’ll definitely buy one!