Emergency responders were called to the Holly Industries building on Eastern Avenue just after 12:30 p.m. Thursday.
Rick Bonnette, the mayor of the Town of Halton Hills, told CBC News that five people were inside the Acton plant at the time of the explosion, but none of them were injured.
Halton Hills Deputy Fire Chief John Martin said that residents living within 100 metres of the explosion site were evacuated, but later allowed to return home.
The Ontario Ministry of Labour, the Ministry of Environment, and the provincial fire marshal have been notified.
The cause of the explosion was not immediately known.