The soot, made up of a mixture of coal dust and oil residue, blanketed cars, rooftops and rice paddies in the town of Sanyi.
County authorities said the dust came from a chemical plant owned by Oriental Silicas Corp. The plant was allegedly adding palm shell to wood to create more heat for its chemical processing. The extra heat ruptured the filter bag for the soot, raining black ash over the town.
Miaoli's environmental protection agency announced that it would fine the company up to 1 million Taiwan dollars (34,000 US dollars), and require the company to clean up the soot within three days.
The plant's owner has apologised and promised to pay for the clean-up, as well as any damage to property and crops.