An air pollution emergency has been declared in Nigeria's southern city of Port Harcourt and an asphalt plant has been shut for "belching out thick smoke", the Ministry of Environment has said.
Residents protested on Tuesday, waving hands covered with soot stains.
Businessman Charles Adolor said he and his wife have been wearing face masks to protect their health, adding:
If I am having my bath, the colour of the water, the stains on the sink are always black. Before we can use already-washed plates we have to rewash them again."
The smog first appeared in November, and officials finally brought in experts last week to find out the cause, AFP news agency reports.