What would cause vapour release from elemental mercury? Would it be pressure, friction... could one stop the vapour release through cooling the mercury?
There will always be some molecules with enough kinetic energy to escape (evaporate) from the surface of a solid or liquid. For each substance and at each temperature there is a specific "vapor pressure" that when reached, the solid or liquid and the vapor are said to be in equilibrium. Again, there are always molecules escaping from a solid or liquid, but once there are enough of the these molecules in the vapor above the solid or liquid, and the equilibrium vapor pressure has been attained, then the number of molecules returning from the vapor to the solid or liquid will equal the number of molecules escaping from the solid or liquid.
The problem with mercury vapor in an open environment like the mouth is that the air in the mouth is continually exchanged with fresh air. Therefor, whatever concentration of mercury vapor has built up will, to some degree, be decreased by air exchange. Hence mercury will, more or less, continuously evaporate in such an environment. Cooling will reduce the equilibrium vapor pressure and consequently reduce the rate of mercury evaporation.
All solids have a certain vapor pressure, and if this vapor pressure exceeds the atmospheric pressure, then evaporation of the solid occurs. Some of the solid turns into gas. More specifically, it's the uppermost layer of the solid that evaporates.
What would cause vapour release from elemental mercury? Would it be pressure, friction...
Friction could definitely increase the kinetic energy of the mercury particles and increase the solid's vapor pressure. An increase in pressure could also increase the temperature and therefore kinetic energy of the solid's constituent mercury atoms.
could one stop the vapour release through cooling the mercury?
Yes, that would decrease the kinetic energy of the particles and lower the solid's vapor pressure. Here's a chart relating temperature (in Kelvins) to vapor pressure. At physiological temperatures, we're still talking about an extremely low vapor pressure.