And 12v is more then lethal, hell a 9v will kill you if you had both hands cut and made a circuit!
I basically have to disagree.
Touchy subject and not trying to start a thread response that can spawn mounds of debate but...
12v and 9v are not going to kill anyone from electrical potential alone... maybe a burn or fallout from a high current short.
Generally, 30v is the minimum voltage required and regarded to draw enough current in most favorable conditions to cause death.
Is it possible to be killed with a lower voltage? yes, but it's beyond rare and requires ridiculous conditions to achieve.
Everyone is different however and so are the many conditions available when shocked... safety should always be your buddy.
But generally, I was always taught (in both Fl and Texas) that 30v and below are the minimum safe working voltages.
Ultimately it's the current your body can draw via 'how conductive you may be' to pull that causes the level of damage or from Fibrillation. (irregular heartbeat from current) so, open wounds and water are of course factor's.
10ma (0.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal.
you can actually measure your own resistance and use Ohm's law to calculate if what your working with can kill or injure you.
In my lifetime the 30v rule was usually followed with a generic comment: "30v+ is the point in which you begin to take extra precautions for safety sake as death is possible unless working in a high current environment which safety protocols need be enabled regardless of voltage potential.
A flying glob of molten metal from a 500A @ 1vdc copper bar short just might kill you too.
I remember from both College and Vocational courses (and as a certified Electrician in Texas) followed the 30v rule.
just my experience.