[ramonyc]

There's some interesting research going on using hydrogen instead of helium for radiosonde launches. The arctic ARM site has been using it for a few years:

https://phys.org/news/2023-05-helium-hydrogen-weather-balloo...

Overheard that the hydrogen dangers aren't as bad when something less compressive is used for the balloon, so a non-flex fabric instead of latex.

[Robotbeat]

Hydrogen is probably just fine for uncrewed balloons. Like everyone has a gas furnace or grill; flammability hazards can be managed.