This may reflect Leviticus 14:41, the only other biblical reference to the outpouring of dust. There it was a matter of the laws of impurity. The lives of Zephaniah’s audience, infected by sin, are unclean and will be treated as the unclean is treated.
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17 I will bring distress on mankind, so that they shall walk like the blind, because they have sinned against the LORD; their blood shall be poured out like dust, and their flesh like dung.