It's easy to think about air pollution as an outdoor problem, but an alarming number of harmful airborne particles can be found floating around your home or office. In Beijing, public awareness about the risk of indoor air pollution has been steadily growing as people seek to turn their homes into clean-air sanctuaries. By 2012, China's air-purifier market had 56 domestic and international brands. By the end of this year, that number is predicted to reach more than 200.