Bacterial action in soil produces
huge quantities of ammonia, most of which is converted to the
sulphate or nitrate in the atmosphere, and is deposited on land.
In areas of intensive farming, local atmospheric levels may be
high through the use of manure.
Ammonia is also produced by the human digestive system. It is
distributed round the body and mostly forms urea, excreted in
urine. Ammonia from external sources is generally very low, and
is readily absorbed into the body "pool".