From Purity and Danger: An analysis of concepts of pollution and taboo, 1966, by British anthropologist Mary Douglas, who observed that dirt is defined by its context.

Wellcome Collection’s exhibition on Dirt’s succinct introduction to dirt - 

‘Ubiquitous, yet ill-defined, dirt takes many forms, from the palpable to the symbolic. ‘Dirt’ is a term used to encompass dust, excrement, rubbish, bacteria and soil.  

It is also used as a metaphor to denote social, cultural or ethnic ‘outsiders’.

Humans, like all living organisms, are efficient generators of dirt, which may partly explain why dirt can provoke visceral fear or disgust.  

Dirt may be seen as a marker of civilisation: the variety and scale of the waste produced by our factories is the uncelebrated evidence of industrial and economic advance.