Emily Roe for Natural Resources Wales said:
"Our officers attended the scene of the fire and can reassure local people that no environmental damage of any significance has happened.
"Our biggest concern in incidents such as this is dealing with the water that's used to fight the blaze. 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."