Mary Douglas's "Purity and Danger: An Analysis of Concepts of Pollution and Taboo" explores the cultural notion of dirt and its symbolic meanings. She follows Durkheim in defining dirt as that which is out of its place (ketchup in fine in the bottle or on the plate, but not on my shirt). What Douglas does is to tie this distinction to the distinction between the scared and the profane (a long held interest of structuralist anthropology). Uncleanliness, she holds, is a cultural matter determined by actual and symbolic power structures.
"Purity and Danger" is an inquiry into different notions of dirt in different cultures, demonstrating the contingent and socially determined nature of what's clean and what's not. For example, Douglas studies the Jewish Kosher laws, arguing that they separate the easy categorized and thus understood from the threatening undetermined (Douglas later retracted this understanding of Jewish laws, see Douglas's "Purity and Danger" and Power Structures).
Douglas further argues (especially in chapter 2 of the book) is that, unlike previous notions in anthropology, the distinction between the scared and the profane did not disappear in modern times but rather manifested itself in other 'secular' terms of clean and unclean and our perception of what constitutes contamination. Douglas also holds that these notions bear an analogous form of the specific social order of a group. What makes for "dirt" is that which is considered anomalous and transgressive of normal bounds. This makes the symbolyic meanings of contamination socially dependant and thus relative.
Here you can find summaries of the first two chapters of Mary Douglas's "Purity and Danger:
Chapter 1: Ritual Uncleanness
chapter 2: Secular Defilement