Fire crews from Llantwit Major, Cowbridge and Bridgend are dealing with a fire at the Vale Business Park in Llandow.
South Wales Fire and Rescue Service has confirmed that there is no asbestos within the building, but they are asking people to keep their windows and doors closed.
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The pit closed in 1996 with the loss of hundreds of jobs.
Pollen levels will remain moderate tomorrow and Tuesday, but will be lower on Wednesday.