This is the gas people sometimes use instead of helium...probably not a good idea to use around birthday candles... According to the original post:
Dangers of party balloons shown off in fireball demonstration at Australian school... A science teacher sets a fireball off at a school in Sydney to warn others of the highly reactive replacement 'town gas' for helium in party balloons. The teacher, Jacob Strickling, explains that "town gas is very reactive and ignites in a big orange fireball. This reactivity makes it very dangerous to breathe in," unlike the helium gas normally used to make voices go high pitched and squeaky.