▁released ▁by ▁T sing h ua ▁University ▁and ▁The ▁Asian ▁Development ▁Bank ▁in ▁January ▁ 2 0 1 3 , ▁L anz hou ▁was ▁among ▁one ▁of ▁ten ▁most ▁air ▁poll uted ▁cities ▁in ▁the ▁world . ▁Air ▁quality ▁reported ly ▁was ▁so ▁poor ▁that ▁at ▁times ▁one ▁could ▁not ▁see ▁L ans han , ▁the ▁mountain ▁rising ▁up ▁along ▁the ▁south ▁side ▁of ▁the ▁city . ▁L anz hou ▁is ▁also ▁the ▁home ▁of ▁many ▁fact ories , ▁including ▁some ▁involved ▁in ▁pet role um ▁processing , ▁and ▁suff ers ▁from ▁large ▁dust ▁storm s ▁k icked ▁up ▁from ▁the ▁G obi ▁Des ert , ▁especially ▁in ▁the ▁winter ▁and ▁spring . ▁In ▁ 2 0 1 1 , ▁using ▁Chinese ▁statistics , ▁the ▁World ▁Health ▁Organ ization ▁reported ▁that ▁L anz hou ▁had ▁the ▁worst ▁air ▁quality ▁among ▁eleven ▁western ▁Chinese ▁cities . ▁Its ▁annual ▁mean ▁PM 1 0 ▁ µ g / m ³ ▁of ▁ 1 5 0 ▁is ▁ 7 ▁times ▁the ▁safe ▁level ▁established ▁by ▁the ▁World ▁Health ▁Organ ization . ▁It ▁was ▁worse ▁than ▁Be ij ing ▁with ▁its ▁reading ▁of ▁ 1 2 1 . ▁ ▁Since ▁then , ▁authorities ▁have ▁taken ▁measures ▁to ▁improve ▁air ▁quality , ▁which ▁have ▁largely ▁been ▁successful . ▁" Th ir teen ▁poll uting ▁enter pr ises ▁with ▁excess ive ▁capacity ▁have ▁been ▁closed , ▁more ▁than ▁ 2 0 0 ▁highly ▁poll uting ▁enter pr ises ▁were ▁susp ended ▁in ▁winter , ▁and ▁ 7 8 ▁industrial ▁enter