Halifax Regional Fire and Emergency says there are no major injuries after a silo roof explosion at the Burnside industrial park in Dartmouth.

The fire service says witnesses reported a loud explosion on MacDonald Avenue around 3:20 p.m. Monday.

Three people were working at Quality Concrete Limited on MacDonald Avenue at the time. They were all checked out and released.

While several witnesses reported seeing smoke, fire officials say there was no fire, and what appeared to be smoke was actually fly ash from the silo.

"Despite many people initially thinking there was a fire because they saw what appeared to be smoke, it was in fact (fly ash) from the silo," said the department.

Fly ash is used in the making of concrete and is carcinogenic, fire officials say. Halifax Regional Fired and Emergency Deputy Chief Roy Hollett said the silo held approximately 35 tonnes of fly ash in a pressurized container.

The department said pieces of the roof could be seen 300 metres away, while the top of the silo landed on the hood of a car.

Many cars and buildings in the area were covered in the fly ash, which spread across a 1,000 square-metre area, Hollett said.

While employees in nearby businesses were asked to stay inside as a precaution, fire crews sent a drone up to assess the damage and debris field.

Several tweets posted by people in the area reported smoke and dust in the air earlier Monday and said that fire units and the police were seen responding to the site of Quality Concrete in Burnside.

“We heard a huge boom,” said Easter Seals Nova Scotia president and CEO Joanne Bernard. “The building shook. At that time we thought it was smoke.”

Witness Paul Raymond, who works at Parts For Trucks, went outside and saw an employee from Quality Concrete comes across the street.

“He was scared,” Raymond said. “We made sure he was calmed down.”

Spokeswoman Tracey Tulloch of Quality Concrete's owner, Stevens Group, confirmed that there had been an incident at the plant, but she declined to give details because of an ongoing investigation by the provincial Labour Department.

"A fly ash storage silo is what we are looking at," said Tulloch. "We don't know specifically what transpired."

Hollett said streets in the area would remain closed as the fly ash was cleaned up. It needed to be cleaned up as soon as possible because it turns to mud when it gets wet.

With files from CTV Atlantic’s Amanda Debison and The Canadian Press.