ma jum is a well-known environmental activist in china. his work has helped spark a sea-change in the way people think here. ma: ten years ago if you'd stop a passersby, many of them would maybe say, we need to develop first. but now if you check with the citizens, i think most of them -- most likely the answer would be, you know, we need clean air, clean water, safe food. reporter: those demands are putting the government under pressure to deliver. the streets of beijing are now crowded with tens of thousands of shared bikes, and many factories have been shut. but the smog is an ongoing problem, especially when the wind blows from the wrong direction. this video shows how the entire cityscape can disappear beneath a blanket of pollution in just 20 minutes. much of it comes from neighboring provinces, where the coal lobby is still powerful and local officials are willing to turn a blind eye. jobs are at stake, so implementing environmental laws is problematic. around 28% of co2 emissions worldwide originate in china. the u.s. is in second p