Safe current limit for human contact?
Here is an article titled "A review of hazards associated with exposure to low voltages" which I used as a reference when answering a medical safety question (I design embedded hardware and firmware for medical devices which go through FDA approval).
Because the body has a minimum resistance of around 550 ohms, to get enough current to do damage a minimum theoretical voltage level of around 16.5 Vpeak is required (corresponding to a current of 30 mApeak, which can induce respiratory paralysis if conducted across the chest for several minutes of contact at this low voltage). Based on the cases studied by the author, the single lowest voltage reported to cause transdermal electrocution in an adult is 25 volts.
For less than one minute of contact, currents >40 mA are required to cause ventricular fibrillation, corresponding to a theoretical voltage of 27.5 Vpeak. For less than one second exposure, >100 mApeak and 55 Vpeak are required. The author states that in all the cases he studied, there was no accidental electrocution from short-term exposures to voltages below 50 Vpeak.
As rules of thumb, I've generally thought of myself as a 70 kΩ resistor to ground that feels pain at around 1 mA, which can be driven by 70 V or so. In my experience, the pain threshold is slightly above 48 V.
I can't say that I have any good medical science to back this up, but there are a few empirically obtained data points in that I'm not dead yet.
Voltage doesn't really matter, it's a requirement to get a certain voltage to pass through the skin, but voltage doesn't have any impact on "damage".
Current is what does damage.
I've heard tons of claims as to what will kill you. In EE school is was 60mA AC and ~100mA DC across your chest that would send your heart into fibrillation.
I've seen claims that < 10mA directly through your heart could do the same. Honestly both are probably correct. I don't know what a real electrical model of the body looks like, but I don't have a hard time believing that if there were 100mA running through my body from one hand to the other that only 10% would pass through my heart directly.
I've worked on live phone lines before (~58V DC with off hook) and that didn't pass through my skin initially. A half hour of being in the 105°F degree attic and sweaty hands later, it made my finger twitch and didn't feel good. On another occasion I was working on a phone line when someone dialed it... that sucked... the ring pulse is 120V AC (current limit though) and does not feel good at all.
It only takes a couple milliamps to seriously get your attention, 10+mA will lock up muscles, this is highly frequency-dependent though.
To get back to your point... greater than 100-200mA is when you'd expect to start to see flesh burning and things like that. But obviously from the heart discussion above, localized currents that are much smaller can be deadly.
I don't really know if there is a firm rule as to what's "safe". The current debate over the use of tasers, for example, would seem to indicate there isn't much conclusive evidence.