I cannot dispute any of your facts, however, I have found some other information that might have a bearing on the immediate hazard evaluation.=A0 I would point out that even if mercury vapors penetrate the barriers mentioned as methyl mercury does, there is, nonetheless, a difference between mercury and methyl mercury.=A0=A0 Again, it is the rate at which the mercury reacts in order to behave toxically.=A0 As far as I know, this also is relatively slow, and “as it goes in, so it goes out.”=A0 Methyl mercury is clearly another story and as I recall it arises (in nature) from the action of bacteria on mercury. =A0The entry for mercury in Deichmann and Gerarde’s Toxicolgory of Drugs and Chemicals (1969) begins “The amount of liquid metal [mercury] accidentally ingested when a thermometer bulb is broken in the mouth is harmless.”=A0 (This may be in anticipation of the most common concern when consulting their work.)=A0 Nonetheless, one may deduce that the mercury (from such an acute exposure) leaves the body before causing significant harm.

According to=A0 I. Langmuir=A0 in Physical Reviews (vol. 8, # 4, 1916, pp 149 – 176), the maximum rate of pure mercury evaporation at 100 degrees C in perfect vacuum was said to be 0.0117 g cm^ -2 sec^ -1.=A0 Assuming that the standard dependence of rate on temperature (doubling for every 10 degrees C increase) applies, at 20 degrees, this would mean that the rate would be 1/256 of that rate.

D.J. Benjamin (Materials Research Bulletin, vol 19, #4, 1984, pp. 443 – 450, the abstract thereto) 17 hours exposure of mercury to moist air (which is not unlikely to have occurred before the mercury in question was brought to the school) causes oxidation that reduces the rate of evaporation by a factor of about 1000.

Based on the above, the rate of evaporation of mercury at 20 degrees C would be about 4.6 X 10^ -5 mg cm^ -2 sec^-1.=A0 Given that a square centimeter of mercury seems pretty big for the case in question, and given reasonable air exchange, I would be inclined to stick with my assessment that excessive alarm is not warranted.

Thank you for your indulgence,

Ben

The evaporation rate of mercury is fast (4 times butyl acetate)!  The vapours are heavy and will concentrate more at the floor. The vapours will penetrate the blood-brain and the blood-placenta barriers as methyl mercury do; thus a special risk for pregnant women.

 

It is correct that 5 mg Hg will not produce deadly concentrations in an average room but easily higher concentrations than many establish limit values (see table):

 

Tabel: Limit values in North america for mercury air exposures (Caravati et al., 2008; Richardson et al., 2009; a.o.).

Air concentration (mg/m3)	Explanation< /b>	Authority
10	Immediately Dangerous to Life and Health (IDLH) (30 min)	NIOSH
0,5	Limit value for short-term exposures	WHO
0,1	Permissible Exposure Limit (PEL-TWA)	OSHA
0,05	Time-weighed average concentration (TWA)	NIOSH< /span>
0,025< /span>	Threshold Limit Value (TLV)	ACGIH< /span>
0,03	Recommended concentration after commercial cleaning	ATSDR< /span>
0,001< /span>	Recommended inhalation zone limit in homes after spill	ATSDR< /span>
0,0018	Reference Exposure Level (REL), (1 hour)	CalEPA
0,0003	Reference Concentration (RfC)	USEPA< /span>
0,0002	Minimum Risk Level (MRL)	ATSDR< /span>
0,00006	Reference Exposure Level (REL), long term concentration	Health Canada
0,00003	Reference Exposure Level (REL), long term concentration	CalEPA

 

Yours truly,

Not wishing to be disputatious concerning the hazards of mercury, nonetheless, I think before worrying about the saturation vapor pressure of mercury, I suspect that one should also consider the rate of evaporation of mercury as well.  I don’t know what it is, but were it all that significant people would probably have to have been refilling McLoed gauges with some frequency.  In fact, I don’t recall that ever being necessary, nor do I recall finding significant amounts of mercury in the Nitrogen traps of vacuum systems under normal circumstances.  Given that most rooms have reasonable air exchange, I suspect that rooms seldom reach saturation.

To refer to the example of 5 mg of mercury reaching the saturation concentration of 15 mg m^-3, one would be dealing with a room with a volume of 1/3 m^3, an unusually small room.  A 20’ X 20’ X 10’ room would hold (at saturation, with no air exchange) ~ 1.6 g of mercury, a mere drop, but if a drop of mercury evaporated at a significant rate an amount that might be noticeable.

What I am suggesting is that the rate of evaporation might be more important than the saturation vapor pressure and may reduce the hazard (at least for the short term) and that one might not need to be overly alarmed.

Thank you,

Ben