This is GRUB’s final warning before you dig too deep in the OS list. Never hold ⬇️ for more than 45 minutes. If you do, make sure you have punch tape with a bootloader available or you’ll have to manually enter machine code instructions to get your computer back up.
Some of them use bismuth, which is as weakly radioactive as it gets, but why? It’s still a heavy metal and might be poisonous if parts of it shed off.