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Recently a user posted the question Do Mercury have a NFPA 704 classification?.

Why the present question is not a duplicate:
Although there is no accepted answer to the above question, I think it's a fairly obvious "yes", and that my question rather is regarding how there could be such a diversity of classifications arising from this single, widely used standard.

Background:
According to the US National Fire Protection Association (NFPA), NFPA 704: Standard System for the Identification of the Hazards of Materials for Emergency Response:

This standard shall address the health, flammability, instability, and related hazards that are presented by short-term, acute exposure to a material under conditions of fire, spill, or similar emergencies.

These are typically displayed in the widely recognizable "fire diamond" (generic image from Wikipedia, not for mercury):

enter image description here
Here, blue indicates health hazard, red indicates flammability, and yellow indicates chemical reactivity.

Data:
The following are some values given from different sources for the "health" rating (scale of 0-4, where 0 is the most benign and 4 the most hazardous) for elemental mercury:

The National Institute for Occupational Safety and Health (NIOSH): Health = 1
MSDS from NIST: Health = 2
MSDS from Fischer Scientific: Health = 3
MSDS from Sigma-Aldrich: Health = 4 (link from comment on first link above by MaxW)
SDS from Sigma-Aldrich: Health =2

Question:
My question is simply: why is there such disparity in the NFPA health ratings given for elemental mercury?

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I believe that the radical rage of possible hazard ratings for elemental mercury arise from different assumptions about how the mercury is handled. I can't prove this, but it makes sense when you consider the possible hazards arising from mercury use.

Properly handled liquid mercury is not particularly harmful. There are no acute risks and a competent person could drink it and handle it with little risk. In a chemical laboratory, for example, it is common to use mercury manometers to provide limited gas overpressure for supplies of nitrogen or argon. This involves bubbling gas through mercury. As long as the vapour is safely vented to a scrubber, this is not a hazard to people in the lab.

Equally, chemists can use mercury safely with few precautions as long as they clean up any spills thoroughly. The hazard primarily comes from vapour buildup which, if it is allowed to happen, will create serious long term chronic problems.

One example of potential danger is if non-experts come into contact with mercury. Some old gas-flow meters for domestic gas supplies used mercury which could be discharged from the meter in certain rare situations, leaving droplets of mercury around the floor near the meter. This can (and has) created a great deal of hazard in a domestic environment. This hazard is magnified enormously, for example, if the householder tries to clean the mercury spill by vacuuming it up. The result is a fine spray of mercury droplets inside a warm vacuum cleaner, which vastly increases the vapour emission and may spread fine droplets widely through the house. If badly ventilated, this can be a serious chronic issue.

The two situations span a wide range of hazard. But the difference is the context: in experienced hands mercury is not very hazardous; in other hands it is potentially very dangerous.

In short, the different rating may reflect different assumptions about who is using the mercury: someone who knows what they re doing or someone who does not.

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  • $\begingroup$ Thanks for the answer, and what you are saying may well be the case. Trying to think of a comparable situation (another chemical that is safe in experienced hands, dangerous in others) I looked up NFPA 704 Health values for isopropanol, acetone and ethanol. I consistently found 1,1 and 2, respectively. Given that ethanol is safe to ingest in small amounts and lethal in slightly larger amounts, it seems by your argument that there would be some variation in health rating. And since the other 2 are actually far more toxic when ingested, I would expect some sources --continued-- $\endgroup$ – airhuff Dec 6 '17 at 19:42
  • $\begingroup$ to have higher health ratings for acetone and isopropanol than for ethanol. My comparison of the health ratings for these compounds compared to mercury isn't perfect, and what you are saying may be correct. I just wish there were some more concrete answer citing specific, confirmed criteria for this disparity. $\endgroup$ – airhuff Dec 6 '17 at 19:51
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Mercury vapor built up over time causes a huge host of problems. Metallic mercury at room temp can be played with as long as it's cleaned up properly. Boiling mercury is obviously worse. I agree with NIOSH, metallic mercury has been incredibly low on my list of scary substances for some time. I'd much rather handle it bare-handed than lead, for example.

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  • $\begingroup$ While this attempts to answer the question, it does so in a terrible way. $\endgroup$ – Jan Nov 6 '17 at 10:12
  • $\begingroup$ Sorry, had a glass of Hg for breakfast and I'm feeling a bit funny. Point taken though. $\endgroup$ – JimmyShinny Nov 6 '17 at 12:42

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