Ammonia is a compound featuring a nitrogen molecule attached to three hydrogen molecules (NH₃). It is a colorless gas with a characteristic pungent smell similar to human urine, as urine contains a large amount of ammonia in it.

Ammonia (NH₃) is mainly generated from agricultural sources, with hot-spots found particularly near intensive pig and poultry farms. Ammonia pollution causes both acidification of soils and a decrease in the richness of plant species.