The Ontario fire marshal's office is looking into what caused a massive fire at an Oshawa asphalt plant.

Firefighters are standing by at the McAsphalt Industries Ltd. facility to keep an eye on hot spots until the investigation starts Monday.

The blaze broke out around 5:30 am Saturday & burned for more than 12 hours in a building housing rubber & plastic products.

By midday, officials had shut down area businesses & told residents to stay inside with the windows shut as heavy smoke filled the air.

A spokeswoman for the Ministry of the Environment says air quality readings suggest there's no cause for concern.

2 booms have also been placed in the harbour to prevent possible water contamination.

(The Canadian Press)