Items which may be carried as passenger baggage are only those defined in DGR 2.3. Helium inflated toy balloons or toy balloons inflated by air are not listed in DGR 2.3. They do not fall under the definition of passenger baggage done by ICAO. They should be carried uninflated and placed in the suitcases. Due also to the reason explained below, passengers must be advised to refrain from carrying inflated toy balloons into the cabin.
A tourist who visited Tokyo Disneyland purchased a helium inflated souvenir balloon and he was asked by the sales clerk whether he will be flying back. This sales clerk must have properly received guidance from the company about the difference in pressure on the ground and in the air.
Balloons are inflated in the normal environment of 101.3kPa at sea level. Even with domestic flights, airplanes climb up to about 7,000 meters as cruising altitude. Pressure there is about 40.5kPa. The aircraft hull will not withstand the pressure differential of 60kPa (60%) so the cabin pressure is decreased to about 75.0kPa to maintain comfortable highland environment for passengers to enjoy the flight. The inside pressure of the Disneyland balloon is 101.3kPa and the cabin pressure is 75.0kPa. The balloon cannot withstand this pressure difference of 26% and will burst.
It is therefore for the benefit of the passenger to keep the souvenir uninflated in his suitcase.