A sooty cloud over an Oxfordshire town is likely to have come from a coal-fired power station, energy firm NPower has said.
Soot formed a layer on buildings and vehicles in the area
The firm, which runs Didcot A Power Station, said "all indications" showed the deposits were from the site.
In a statement, it said the soot, which formed a layer over vehicles and buildings in the area, seemed to be carbon from partially-burned coal.
Production had been reduced but there was no risk to health, NPower added.
Engineers are working to find out the precise cause of the cloud.
The firm said it hoped to have more information by Friday morning.
The statement said: "The carbon deposits are not a health risk, however, we do take our responsibilities as a neighbour seriously and we are very sorry for any concern and nuisance this has caused.
"We have logged all calls and queries we've had on the issue and we'll be contacting those people affected."