Some thoughts on when to change air filters

So we all know that Beijing has notoriously bad air pollution, specifically of the small so-called pm2.5 variation. 

As a result, air purifiers are all the rage in this city. But I have a question: how do you know when you need to replace your air Purifier's filter? 

Here is a striking example: the air output of this taobao-bought air purifier is just as bad as untreated air. You might also be able to see that the filter inside is darkened and grey. 

As a comparison, check out the same model, but with a clean filter here:

Pretty big difference huh? Time to change filters!!