The contribution of air pollution to the decay of building stones is unimportant compared with the effect of climatic and biological forces. This is confirmed by the following arguments: 1.) Limestone, which theoretically ought to be attacked by the corrosive components of rain, does not show serious damage in Germany. 2.) Deepest decay occurs on sandstones. This decay is not due to air pollution because most sandstones do not contain components which could react with pollutants. 3.) There is no sulfate formation, except that which is apparently due to other influences. 4.) The decay of sandstone by natural weathering has been thoroughly investigated since the 19th century and there is no difference between these early observations and our own. 5.) Objects which show the greatest deterioration today were treated extensively in the past. 6.) Damage in rural areas is as serious as that noted in urban areas. 7.) Crusts of gypsum, which are thought to be reaction products of polluted air and stone, occur on intact limestones and bronzes.