This period also saw the introduction of non-flammable helium as a lifting gas by the United States, while the more dangerous hydrogen continued to be used since the United States had the only sources of the gas at that time, and would not export it.
The United States began using non-flammable helium for lifting purposes during this period, while the more dangerous hydrogen remained in use because the U.S. was the only source of the gas and did not export it.