The increase of agricultural intensity over the last century in rural Denmark has meant that ammonia has been regarded as a significant environmental problem. The deterioration of murals in rural churches is also a matter of concern and focused attention on the potential for ammonia to accelerate damage. Ammonia concentrations measured over 12. months inside and outside nine churches often show a spring maximum outdoors, hinting at the importance of farming activities. The ammonia concentrations are on average some three times greater indoors than outdoors and mass balance calculations suggest that this arises from the decomposition of ammonium nitrate aerosols. The emissions may result from reactions of aerosols deposited at the alkaline walls, which also leads to calcium nitrate becoming the major soluble salt at the very surface layer. The quantities remain small enough, that they probably do not participate in salt damage to the murals.