In general, saltpetre in its pure form is not classified as dangerous. Saltpetre does not spread through the air, does not release anything into the environment, behaves in a completely odourless way and has nothing to do with mould or dry rot, as is unfortunately too often wrongly propagated.
However, saltpetre does not flourish without reason. Because it is an oxidative organic substance that reacts with the lime in the masonry, it is clear that the influence must come from the soil surrounding the masonry.