Famous buildings all over the world are endangered by atm. pollution. The aggressive components are mainly of an acidic nature. The weathering is attributed to chem. and phys. processes. However, reports about the involvement of microorganisms are either not available or their involvement is denied. Nevertheless in recent yeast is has become evident that lithoautotrophic nitrifying bacteria were involved. Due to the excretion of their metabolic end-product, nitric acid, concrete is degraded. The natural process could be modelled by means of simulation expts.