DIY air filter for polluted cities

Smart air filters has a similar idea: http://smartairfilters.com/en

Both projects use a Dylos particle counter, which is a great tool.

I built a very similar thing in the US. I don't know where these fans or hepa filters are sold, but I buy a standard 20" boxfan at walmart ($16ish USD), and a 3M Filtrete AC filter in 20x20x1" size (also ~$15USD). These come in different ratings, so I buy the "allergen reducer" rating or higher.

Duct tape to the back of the box fan, done.

Fan like this: http://www.lowes.com/pd_625625-1225-HBF2010A-WM_0__

Filter like this: http://www.walmart.com/ip/Filtrete-Advanced-Allergen-Reducti...

They even have a filter embedded with carbon if you're a smoker.

I would add activated carbon filter to remove even more stuff