now i know that lava alone will kill you. even with full iron armor. i had all iron armor but chainmail boots, not a huge difference. either way, i fell from like y-74 to y-31, which quite obviously is death, except maybe with feather falling. in my case, i fell in lava (yes this is in the nether). but i came prepared, knowing i had bad luck with lava. so i drank the potion before my 6 seconds were up (in terms of health), but i went right through the lava as if it didnt exist. i tested it with the same gear in nother world and was fine.