The discrepancy between Jay Gatsby's romantic dream visions and reality is a dominant theme in F. Scott Fitzgerald's American classic, The Great Gatsby. In the introduction of Gatsby by the narrator, Nick Carraway, this theme is introduced as Nick reflects upon the nature of Gatsby:
it was an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again. No--Gatsby turned out all right at the end; it is what preyed on Gatsby, what foul dust floated in the wake of his dreams that temporarily closed out my interest in the abortive sorrows and shortwinded elations of men.
This foul dust is mentioned throughout the novel and is symbolic of the corruption of everything that it encounters; it represents the moral irresponsibility of the affluent Americans of the Jazz Age, an irresponsibility that is sure to destroy the romantic illusions of Jay Gatsby. It is the "dirty truth," the reality of a corrupt age and a dream built upon Gatsby's own moral corruption from his ill-gotten wealth and the moral corruption of his "golden girl" whom he pursues. The "foul dust" symbolizes how the means corrupt the end.
In Chapter 2, Fitzgerald presents a similar symbol, the Valley of Ashes. This is a place where the waste of industries is dumped, conveniently located between the city and the "Eggs" where the wealthy live. Symbolically, it represents the wasteland of people's hopes, and desires. Along with the symbolic Valley of Ashes where the rich come to dump there the reminders of their excessive indulgence, the foul dust follows Gatsby's romantic attempts to present himself in a favorable light to Daisy and others he wishes to impress.