Nearby buildings are badly damaged

It is believed the heat has damaged nearby industrial buildings.

Structural engineers will be examining other premises.

As investigations continue, medical advice is being offered to people living near the Distillex factory in North Shields.

Thermal imaging cameras are being used to find any remaining "hot spots" amongst the debris.

It took 25 fire crews five hours to bring the fire under control.

Firefighters spent the night at the site damping down.

It is believed a spark ignited flammable material in a skip and the fire then ran out of control.

Fire extinguisher

Distillex managing director Adrian Jones told BBC News Online: "I believe that as they were making a transfer to the skip a spark was created as a natural result of metal contact.

"That caused the ignition of a material that was in the skip. That could be a rag with oil or solvent on it.

"The operator tried to dampen it with a fire extinguisher but was unable to and as a result the fire spread."

The smoke was seen from 20 miles

In a series of explosions, containers and drums were thrown into the air.

As the fire took hold, thick billowing smoke could be seen more than 20 miles away.

Eyewitness Bruce Walker said: "At its height, the flames were about three times higher than the building."

Most of the 500 people were allowed back on Friday night, but up to 100 spent the night at a leisure centre.

Health problems

Northumbria police say advice on health issues is being offered.

Worried people should contact their GP, their local accident and emergency department or NHS Direct on 0845 4647.

Witnesses heard explosions

Northumbria Police Deputy Chief Constable Michael Craik said it was declared a major incident from the outset.

All aircraft were diverted away from the area, Metro trains stopped running between Tynemouth and North Shields, and the Tyne Tunnel road link under the river was closed.

More than 150 police officers were deployed to counter the traffic chaos and patrol the exclusion zones where hundreds of houses were empty.

Second incident

Bill Dixon of North Tyneside Council told the BBC on Saturday: "Everything hopefully is back to normal.

The work of the fire service prevented much more damage occurring."

The Distillex factory employs 12 people and is thought to contain between 40 and 50 different types of chemicals.

The Lancashire-based firm recycles solvents used in industry.

The plant was at the centre of a similar alert in January when a cloud of toxic vaporised methylene chloride was released after chemicals overheated.