Smoke from a huge blaze at an industrial estate contained a lot of chemicals that have yet to be identified, firefighters said.
The cloud of smoke from the Langar Airfield industrial estate, in Langar near Bingham, was seen over large parts of Nottinghamshire and Leicestershire.
Residents were advised to keep windows and doors shut to keep out fumes.
Fifteen crews worked to extinguish the fire at its peak and many remained through the night to dampen it down.
“The smoke from the fire contains lots of different chemicals that cannot immediately be identified,” Nottinghamshire Fire and Rescue Service said.
The BBC understands the blaze broke out at a waste transfer site but it is not yet clear what material was in the rubbish.
However, the fire service said the “risk of asbestos has been downgraded”.
Station manager Jonathan Holford told BBC Radio Nottingham that while the “acrid black smoke” had changed in colour to white, it still posed a risk to residents and he urged people to keep their windows and doors shut.
The fire service was first called to the scene just before 14:45 BST on Thursday.
Crews from across the county and one from Melton Mowbray in neighbouring Leicestershire were deployed.
Lois Ward, a student living in the nearby village Barnstone, said she noticed the smoke at about 15:00 from her garden, and shortly afterwards could hear “popping noises” coming from the direction of the fire.
“For a while you could hear constant little explosions going on, which was very unnerving,” she said.