“…The outdoor sources come from heating fuel, industry, traffic, etc. [ 34 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 ], due to the rapid industrialization, high energy consumption and large proportion of coal (60â70%) in the structure of energy sources in China. It has been acknowledged that there is close correlation between indoor and outdoor PM 2.5 concentration levels, and outdoor PM 2.5 is the main source of indoor PM 2.5 pollution [ 75 , 76 , 77 ].…”