The fact that people need air filters? Or something about the way these people are doing it?
I really like their website. They're open and honest about not being experts, and about how they do things, and how they want corrections and additional information. They sell you a kit if you want it, but they make no secret about it being really easy to do by yourself.
It's a classic case of negative externalities caused by stupid human decisions and a complete disregard for the commons. People have been shitting in the commons so long in Beijing that the particulates in the air are crazy high. Everyone feels like this have to have a car instead of pushing hard for effective public transportation. Since everyone has to have a car, it hurts everyone on buses because the traffic is so bad the busses can't get anywhere on time anymore[0]. Then everyone has a building that is heated by coal, instead of making an effort to upgrade the buildings to some form of heat that isn't going to dump thousands to millions of pounds of particulates into the atmosphere.
Instead of solving the problem at the root, everyone takes an every man for himself approach to the problem like the one described by the OP. The government should provide some sort of tax incentive to every building willing to upgrade their heating to something cleaner and should tax the hell out of cars in the city and put that money towards public transportation
FWIW, I lived in Beijing back in 2005/2006 and had a solid smoker's cough after a year. It's truly disgusting how everyone is effectively poisoning each other there.
[0] Furthermore, the design of the onramps and offramps on the ring roads are such that they cause a cascading locking condition once the level of traffic hits a certain point. When the line to exit a ring road gets long enough, it blocks the on ramp. This causes locking to cascade through the whole city.
Beijing is the victim of geography where pollution is often trapped in an inversion; a problem like Salt Lake City in the states. Beijing has much fewer cars per person than say Seattle, and the problems arise from too many people more than too many cars, coupled with shitty cheap oil refined poorly leads to a huge problem in aggregate. On the other hand, the subway system has been built up nicely, and is expanding very quickly, though it is always wicked full and not very comfortable by western standards. Beijing's traffic problems simply boil down to too many people and cars and not enough road with nowhere really to build more.
Old coal boilers are another problem, and a lot of pollution blows in from the Hebei country side and gets stuck in Beijing without an outlet. All buildings in Beijing are heated by the city, so it's not even the building owner's option to be more efficient. Beijing is slowly switching over to natural gas for heating but the transition will take a couple years to finish.
The AQI right now is down to 50 where it was 340 only a few hours ago. I can only imagine we got some wind to blow out the bad air all of a sudden.
The fact that people need air filters? Or something about the way these people are doing it?
I really like their website. They're open and honest about not being experts, and about how they do things, and how they want corrections and additional information. They sell you a kit if you want it, but they make no secret about it being really easy to do by yourself.