A Spokeswoman for Natural Resources Wales said that no environmental damage "of any significance" happened as a result of the blaze.
She said their biggest concern in incidents such as this is dealing with the water used to fight the blaze, adding: "It picks up a lot of pollutants from the burning material and can cause a lot of damage if it gets into streams and rivers."
"However, we're confident that the vast majority of the firewater used at Ystrad Mynach ran into the foul water drains and therefore poses little risk to the local environment."
A spokesman for South Wales Fire and Rescue said the incident was "being scaled down" with some firefighters staying at the scene.
Firecrews from Caerphilly and Aberbargoed were originally called to the fire at 8.50am this morning.
The road has now been re-opened and people evacuated have now returned to the buildings.
Firecrews are cutting away fabrication of the building next to the fire to check the fire spread.
A spokeswoman for the fire service said the fire involves a skip, two vehicles, a large caravan and tyres. Firefighters are attempting to stop the fire spreading to buildings.
A TYRE fire has broken out at an industrial estate in Ystrad Mynach.
Fire fighters were called to the blaze at Dyffryn Industrial Estate shortly before 9am this morning and have advised the public to bring washing in and keep windows closed.
The industrial estate is to the south of Coleg y Cymoedd Ystrad Mynach, formerly Ystrad Mynach College.
The A469 in Llanbradach has been closed as a result of smoke blowing into the road and there is heavy traffic in both directions between Llanbradach roundabout and the Dyffryn Industrial Estate roundabout. A diversion is in place.