The blast blew out windows of the flat in Highholm Street, Port Glasgow.
A man and a woman were injured and taken to Inverclyde Royal Hospital, Greenock, although their injuries are not thought to be life threatening.
The building was evacuated, with 11 other people removed.
The explosion happened about 2.25pm yesterday.
A police spokeswoman said alternative accommodation was being sought for those removed from the building.
She added: "Inquiries are ongoing into the cause of the explosion."
Although it could not be confirmed that gas was the cause of the explosion, Scotia Gas Networks - which operates and maintains the network in Scotland - was called to the site.
Claire Buchanan, spokeswoman for Scotia Gas Networks, said: "We arrived quickly, made safe, and assisted the emergency services with their investigations. We have now left the site.
"Until these investigations have been completed, it is too early to speculate as to what has happened."