The T.D. unit is a standard measurement for smell intensity. The acronym was chosen to be T.D. as its not commonly known as
anything else. The measure has typical values from 1 to 5, 1 being a mild stink and 5 a ghastly stench. On the odd occasion a 6 can be recorded but this is often
associated with
asphyxiation, leading ultimatley to death.