Brisbane factory still smouldering

A Brisbane factory is still smouldering and remains a crime scene after a fire sent toxic plumes of smoke across the city's north on Sunday.

It took 50 firefighters to stop the blaze at a Virginia factory from spreading to neighbouring businesses, and authorities were forced to declare an emergency situation and set up an exclusion zone.

The declaration was revoked about 11pm but firefighters continue to monitor the smouldering site, and a crime scene has been declared as investigators work to determine what caused the fire.