A stream of liquefied petroleum gas would have made the family of seven unconscious within minutes and killed them by suffocation, scientists said.
Unlike Towngas which contains toxic carbon monoxide, the gases that make up LPG - butane with some propane - are harmless except if there is a build-up, which would prevent people breathing enough oxygen.
An average of six people a year since 1991 have committed suicide by gas poisoning, according to government figures.
'It is a simple asphyxiant. Any inert gas like nitrogen or helium would do it,' said senior lecturer in forensic medicine at Hong Kong University, Paul Dickens.
LPG gas, a by-product of refining petroleum, is odourless but gas firms add a small amount of a powerfully smelling gas, ethyl mercaptan, so people can smell leaks.
This is toxic but not present in enough quantity to cause harm.