What is "foul dust," why is important and what does it symbolize for Fitzgerald?F. Scott Fitzgerald's The Great Gatsby

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.