A major incident was declared after a chemical leak at Grangemouth docks.
Grangemouth docks, where the chemical leak was reported
White smoke was seen coming from a freight container at the docks, which is located close to Scotland's largest oil refinery.
The vapour had the potential to cause slight skin irritation, according to a spokesman for Forth Ports Authority.
Personnel were moved out and the night shift cancelled but no-one has been hurt and there was said to be no danger to the population.
Thirteen fire crews were in attendance at one stage.
The incident is not affecting the nearby oil refinery, run by Innovene, which was sold by BP to INEOS last year.
Central Scotland Police said that the vapour initially caused a large plume above the River Forth.
"The current wind direction means the vapour is being blown towards the river where it is dissipating," a spokesman said.
"Efforts are being made to establish the identity of the vapour."
He said there had been no requirement for an evacuation and no known immediate danger to the public.
As a precautionary measure, the public were asked to keep windows and doors closed for the time being and stay inside, he added.
The incident was declared a major incident under the Central Scotland Strategic Co-ordinating Group protocols.
Scottish Green Party co-convener and Lothians MSP Robin Harper said the incident was "very worrying indeed".
"I would expect a full explanation of why this has happened and that everything possible is being done to safeguard public health," he said.