A 40-year-old man in Brisbane has blown off his hand while mixing chemicals to make homemade rockets.

Key points: Police say the man was making homemade rockets at the house

Police say the man was making homemade rockets at the house He is in a critical condition in hospital with a broken leg, a missing hand and other injuries

He is in a critical condition in hospital with a broken leg, a missing hand and other injuries His four-year-old son and a 74-year-old man were also home at the time, but are unharmed

Queensland police said the man's four-year-old son and a 74-year-old man were also in the Chermside West house when the explosion blew out windows and sent a shock wave through the home.

They said all three people were lucky to be alive as the force of the explosion could have been deadly.

The man was taken to the Royal Brisbane and Women's Hospital in a critical condition with a broken leg, a missing hand and other injuries.

One of the homemade rockets set up for launch in the home's backyard. ( ABC News: George Roberts )

He was rushed into the operating theatre for emergency surgery.

The toddler's mother was at work when the chemicals blew up about 8:00am.

Police Inspector Geoff Acre said the child and the 74-year-old man were standing nearby when the man was mixing the chemicals.

"But they've gone upstairs to have breakfast and were in the kitchen when the explosion occurred," Inspector Acre said.

"So luckily they weren't in the vicinity when it actually went off — he's very, very lucky to be alive."

Inspector Acre said it was lucky there were not three fatalities.

"In all honesty, had it exploded a minute earlier, we would had three people deceased," he said.

"The explosion has blown out all the windows [on the lower level of the house], so obviously there's a lot of glass around the place there, which again is lucky the small child and the elderly guy weren't downstairs at the time that thing exploded."

Police said the man had been making rockets for a while, possibly to entertain his son.

"He's had a few successful launches in the past," Inspector Acreman said.

The backyard of the home with a homemade rocket set up. ( ABC News: George Roberts )

'Force inside that house was massive'

Queensland Ambulance Service spokesman Brad Hardy said he was "very surprised" the man had survived the explosion.

He said the man had lost his right hand and had other injuries, but none appeared to be life-threatening.

"The injuries were significant to him — it is amazing he's survived these injuries," he said.

Mr Hardy said the "force inside that house was massive".

"He was right in the middle of it … glass [was] everywhere, the doors were smashed, everything inside was destroyed," Mr Hardy said.

An elderly neighbour, who only wanted to be referred to as Bron, said the explosion had woken him.

"There was a bang, like a bomb, so I just got up from bed and looked though the window and smoke was pouring [out]," he said.

Police are trying to determine what the chemicals were being used for and why they exploded. ( ABC News: Stefan Lowe )

Holly Barrett, whose balcony had a clear view across the road to the house where the explosion happened, said there were emergency service crews everywhere.

She said her housemate had seen the man come out of the house after the explosion.

"He was bloody all over the face, all over his arms, all around his chest," Ms Barrett said.

Police and paramedics have urged people not to mix or play with chemicals, even those commonly found around the house.

Police said they were trying to determine what the chemicals were being used for and why they exploded.

Officers are investigating whether charges could be laid.