On the floor directly under the robot? Then perhaps the link I posted on your other thread will help: it's for detecting dust on a surface beneath the sensor. It should be possible to modify for your uses, depending on the kind of surface you'll be on and what amount of dust you want to detect.
It's more or less the same idea that Mike posted, but instead of detecting the dust in a tube it does it by bouncing the light off the floor and seeing how much is reflected. More light reflected means less dust.
Torben
[Edit: Note that neither I nor the author (apparently) have actually built the circuit in that link.
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