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\bibitem{Leysen_etal1989} {\textquotedblleft}\textit{Air-pollution-induced chemical decay of a sandy-limestone cathedral in Belgium}{\textquotedblright}. Leysen L, Roekens E, Van Grieken R, The science of the total environment \textbf{78}, 263 (1989).   http://doi.org/10.1016/0048-9697(89)90038-7

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