Hastings' Dictionary of the Bible (1898)
DUST. " The custom is supposed to have been common among the Jews, when they had set a foot on heathen ground, to shake otf the dust, so as to carry nothing unclean or polluting into their own land. Dust thrown into the air, 2 Sam 16:13; Acts 22:23, was an expression of rage and threatening, while the very act probably increased the passionate hatred. "Dust and ashes" are coupled together as a phrase describing man's feebleness as contrasted with divine strength. Gen 18:27; Job 30:19. Dust, Rain of. Deut 28:24.
In Judaea or its immediate vicinity are plains or deserts of fine sand, which when agitated by a violent wind makes most terrific and desolating storms. Eastern travellers describe them particularly, and think them much more dreadful than storms at sea. This fact affords us a striking illustration of the nature and horrors of the plague mentioned in Ex 8:16.