12 VDC CAN kill and has killed people.
While 12V is almost always safe, worst case situations can and have lead to death.
Mechanism may be ventricular fibrillation BUT paralysis of the respiratory muscles occurs at about 20% of the current needed to introduce fibrillation.
See discussion and references at the end of this answer.
12 VDC applied across the chest has killed volunteers despite medical experts standing by !!!
(From memory - volunteer prisoners participating in medical research).
Carry a car battery with exposed terminals on a hot day when you are sweating and press the terminals to your body (as could happen worst case when lifting the battery etc) and you may end up repeating the experiment.
Once conduction into the body starts you get a very low impedance / resistance circuit into what is essentially a large bag of dilute saline solution.
There are two mains "what kills" issues.
One is general trauma - burns etc, and that is obviously very situation and person dependant. I've had shocks from 1200 VDC, 230 VAC, 50 VDC, RF and miscellaneous other sources. No major burns. I'm still alive
Enough current for long enough to stop your natural heart rhythm and throw it into fibrllation.
At typical domestic voltage levels you are USUALLY safe if the current flows for well less than one ventricular heart valve cycle and at "low enough" current.
Earth leak circuit breakers (ELCB) also called ground fault interrupters (GFI) and other names aim to trip at currents somewhere under 10 mA and from memory (references later - rushing) in about 10 mS = well short of a heart cycle.
A shock from a circuit protected with an ELCB / GFI device will be felt but will USUALLY not be fatal.
A 9v battery on the tongue almost certainly won't kill.
A 9v battery across the chest with saline solution (or sweat) just might - probably not.
A 12V "car battery" or any high current source from a few volts up MAY kill in the very worst case. Hand to hand I havve never heard of shock occurring or being felt.
110 VDC (not AC) routinely killed Edison's linesmen.
50 VDC MAY not be felt with dry hands on a dry day. On a high humidity day brushing the back of the hand with terminal strips with 50 VDC on causes annoying minor shocks (as experienced in eg Telecom wiring frame jumper running (based on my long ago experience)
75 VAC imposed on 50 VDC gives a very nasty shock sometimes. Worst case this could kill.
High current 1200 VDC hand to body somewhere may not kill - I'm still alive.
Can 12 Volts kill?
Probable? - no.
Possible? - yes.
Data point: Note that this is a completely true and non-fabricated account. I have a friend (still alive) who built a lamp to take flounder fishing. It used a 12V SLA battery and an Aluminum pole with the light at the top. Flounder fishing involves wading through shallow salt water. In the course of fishing he discovered that an electrical fault existed - in some manner he was exposed to 12 VDC between his hand holding the pole and the water he was standing in. He was completely unable to release his grip - the current flow exceeded his "let go" threshold. regardless of how "worst case" this may have been and what various tables and standards say, it was clearly possible to reach his personal can't-release level. The literature states that respiratory paralysis can occur at currents not significantly greater than the can't release level. If he'd been by himeself (never a wise idea with such activities) he may have found himself floundering :-). Note that this was a hand to leg current path. Chest to chest worst case can be reasonably expected to be potentially higher.
The table below is from this page.
this is not a primary reference source but the figures used have been obtained from an "official" source. See above page.
Note that for 60 Hz Ac ventricular fibrillation is stated as occurring at 100 mA but paralysis of respiratory muscles occurs at 20 mA . These limits are very much user and situation dependant but give an order of magnitude indication.
With very informal equipment I measured 1500 ohms resistance across two areas on my abdomen. I decided not to measure across my chest in the vicinity of the heart. I used flat contacts with no skin penetration. At 12V, if resistance did not change with current flow (and I'd expect it to probably drop) a current of 8 mA would be produced. Measurement with skin penetrating electrodes may reasonably be expected to increase this significantly.
A superb discussion of electrical safety, current levels in various situations and consequences can be found here. The writer's competence and bona fides are above reproach*. The discussion relates to the provisions of standard IEC60990 'Measurement of touch current and protective conductor current'. This is a "for money" standard that I do not have access to, but excerpts from it are provided in the above reference and elsewhere.
- '*' P E Perkins PE.
Convenor IEC TC108/WG5,
IEC 60990 'Measurement of touch current and protective conductor current"
A careful but less than exhaustive examination of the above document and other related web material makes it very clear that
Full copy of standard ECMA287 - Safety of electronic equipment
Touch current comparison data paper - P Perkins
NIOSH - worker deaths by electrocution
Accounts of two deaths by electrocution. One at 12V. One at 24V. Note that BOTH these are unsupported heresay reports and actual cause of death may not have been electrocution.
Table 1. Estimated Effects of 60 Hz AC Currents
1 mA Barely perceptible
16 mA Maximum current an average man can grasp and "let go"
20 mA Paralysis of respiratory muscles
100 mA Ventricular fibrillation threshold
2 Amps Cardiac standstill and internal organ damage
15/20 Amps Common fuse or breaker opens circuit*
* Contact with 20 milliamps of current can be fatal.
As a frame of reference, a common household circuit breaker may be rated at 15, 20, or 30 amps.
Interestingly - this answer has 1 downvote, and surprisingly few upvotes considering the undoubted truth it tells. Maybe the downvoter and anyone who doesn't think it is a good answer would like to tell me why? The aim is to be balanced and objective and as factual as possible. If it falls short please advise.