It's the outer layers of skin which contribute to most of the resistance of the body. Once you pierce below those and go into a deep, open cut, the resistance is much lower.
Voltage and current figures for the electric chair show that once the initial high-voltage jolt has broken down the resistance of the outer skin the head-to-ankle resistance can be as low as 200 ohms!
That's an extreme example obviously, but it shows why medical apparatus has to be designed extremely carefully to avoid leakage currents which would be quite dangerous if applied to open wounds.
Along with proximity sensors, light curtains, etc., another possibility to consider if an actual metallic contact circuit is required would be to have the relay switched by a simple transistor control circuit. The "energized" trigger contact could then be fed by a very-high value bias resistance which would limit the current to a few microamps.
