I have to dig up the exact quotes from sources but ancient people did know mercury was dangerous, well at least they did in the begining of the eneolithic.
Not that it stopped them from using it as an elixir of immortality.
The Romans tried not to if it could be avoided.Just because a few people in the past knew how dangerous mercury was, doesn't mean that this information was evenly shared along with the rest of the world. Nor that others even agreed with them. Plus, dangerous or no, it was still a useful substance for other purposes. It was known as far back as the First Century BCE that lead was poisonous, but the Romans still used it to make pipes for water.
Shi Huang Di will live longer in short the Qin dynasty might last longer.
Drinking metallic mercury isn't very dangerous since very little of it gets absorbed. It was used as a laxative in the US until fairly recently. Most of the problems that come from handling metallic mercury are the result of breathing in mercury fumes, which is what gave rise to the phrase "mad as a hatter" due to the mercury used in the felting process. Cinnabar (mercury sulfide), on the other hand, is much more toxic when ingested and the risks of handling it were understood by the Romans. However, it is not water soluble and does not cause the neurological problems seen in inhalation mercury or methylmercury poisoning. Methylmercury is what poses the greatest risk to human health, but it was not present in high enough concentrations in the environment to cause problems until after the Industrial Revolution.What if instead of thinking the extremly toxic liquid metal was a good thing to drink, ancient people understood that mercury was dangerous?
His paranoia would most likely not have been due to drinking mercury or swallowing mercury compounds. If he was constantly around mercury fumes, on the other hand, they may have caused neurological problems.Also, he'd be a hell of a lot less paranoid in the later years of his reign.