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I was watching this clip from the 2017 film The Mummy in which the characters stand next to a pool of mercury which was being used as part of an elaborate prison.

Several comments under this video imagine that standing next to a pool of mercury would be very dangerous, but I can't see any reason to think that a significant amount of vapour would be coming from the pool so it seems like the mercury just being there in liquid form would be quite harmless. But maybe I shouldn't go robbing any graves any time soon.

The automatic warning dialogue told me the question appears quite subjective. So, let's say you're standing next to the pool for no more than ten minutes and we'll define harm as requiring medical intervention or (if chronic) as having a demonstrable, long-term health effect, such as erethism. I have no idea how large the pool is in the video, but for the purposes of the question, we'll say it's a circular pool of 10 metres in diameter.

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    $\begingroup$ The expression "mad as a hatter" came from real exposure problems. But would you die instantly, no. Would you necessarily die from the mercury eventually, no. However the salient fact is that mercury certainly won't do you any good, so it is prudent to limit your exposure. $\endgroup$ – MaxW Aug 23 '18 at 4:42
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    $\begingroup$ The data on this is...frustrating to find. There's wildly different toxicity thresholds for organic and inorganic mercury, and it seems very few actual toxicity tests. I found a CDC reference that points out that over 80% of mercury in vapor can be absorbed by the lungs, and that in a room at 25C, the saturation for mercury vapor is about 18mg/m^3. Next step would be to try to find a source on toxicity (this book seems like a candidate, but paywalled) to see if this could seriously injure you in 10 minutes. $\endgroup$ – chipbuster Aug 23 '18 at 7:23
  • $\begingroup$ I won't comment on health but that sarcophagus should have amalgamated. $\endgroup$ – A.K. Aug 24 '18 at 2:27
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It depends. But there is little short term risk.

Mercury metal isn't very toxic. You can even ingest it without significant short or long term effects. But mercury vapour is nasty and you don't want to be continuously exposed to any significant amount of that (see this answer for more detail). And some mercury compounds are pretty nasty too.

Of course mercury metal emits mercury vapour so there will always be some in any room with exposed mercury. But the reason why this isn't an immediate worry is that it takes time to absorb enough vapour for it to become notably toxic. Chemists are used to handling liquid mercury in the open lab but are a lot less comfortable if it has been spilled and not cleaned up. This didn't used to be so which is one of the reasons we know the dangers: chemists who were exposed to vapour over years did suffer toxic effects. Mercury miners suffered similar long term toxic effects from the vapour. In modern labs we make sure the vapour doesn't build up by good ventilation and making sure any spills are cleaned up quickly so exposed mercury doesn't hang around emitting vapour.

So would you be in danger standing next to a pool of mercury? No, if you didn't do it for long. Or, if the ventilation in the room were good and the vapour wasn't allowed to build up. Even then, though, modern safety standards wouldn't be comfortable having pools of mercury exposed to the atmosphere: you wouldn't want to regularly work in that room with the pool.

In short, standing next to a pool of mercury even in a badly ventilated room wouldn't cause immediate harm; working next to that pool for months or years would certainly cause you harm.

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Just an addendum to Matt`s answer.

The main problem in the research lab was not the short term exposure, but it accidental spillage. Even a small spill is quite time consuming to clean and it is easy that small amounts of mercury acumulates leading to unsuspected long term exposure. In Physical Chemistry labs. mercury was commonly used to seal vacuum systems, measure pressure or in mercury diffusion pumps. Fortunately, the technology has render these techniques obsolete.

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