*WHOOPSIE!* Apparently I didn't pay enough attention while doing the copy/paste of the riddle & the line I added ... wasn't added.
(I'll blame this one on the coffee in the keyboard this morning. )
Apologies, the riddle should read:
I am essential, and dangerous.
You can hear me, until you can't.
I may make you hot, won't slow you down.
You can't see me, until you can,
Then see me only through others.