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In movies I often see green used to color dangerous and/or poisonous substances colored green, like green clouds of gas considered poisonous, and green acid considered dangerous. Can anyone explain this, or explain where this came from?

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    Or the Pepto Bismol color of Klingon blood whom were considered "evil" at times. I always thought pink blood was rather an odd choice for such a hearty warrior race. – iMerchant Jun 2 '17 at 16:28
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    Related question on Chemistry.SE: Why are acids usually depicted in fiction as green? – Loong Jun 2 '17 at 18:11
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    Because the people invoved don't like Broccoli? (Unless they are doin a James Bond movie, that is) – Hagen von Eitzen Jun 2 '17 at 18:20
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    Ha, I made a reference to this question in a world building question a couple days ago. Nice to see the answers (I didn't know arsenic was green), so thanks for asking! – Draco18s Jun 2 '17 at 18:21
  • This is a really great question (with great answers), but at the same time I do not think it can be on-topic, since it does not ask about a specifically SFF trope. I am thus voting to close. – Adamant Jun 2 '17 at 18:21
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Because it was forever linked to poison by Radium and Green pigments in the 1800's. Paris Green and Scheeles Green in particular helped cement this association early on.

Radium Poisoning

enter image description here

It was clear from the beginning large doses of Radium was dangerous, the Curies made it clear early on, but it was commonly held that in small doses Radium was beneficial.

Those who painted radium watches suffered from drastic Radium poisoning, and would glow green. The radium girls destroyed the reputation of Radium, giving us the classic radioactive green glow association

What Made Green Pigments So Toxic?

Both were arsenic compounds that produced popular shades of green, known for their brilliance. They were excessively used

Paris green (copper(II) acetate triarsenite or copper(II) acetoarsenite) is an inorganic compound. It is a highly toxic emerald-green crystalline powder3 that has been used as a rodenticide and insecticide,[4] and also as a pigment, despite its toxicity.

https://en.wikipedia.org/wiki/Paris_green

So much so that Paris Green was later used as an insecticide. It was used to kill mosquitos, and in the Paris sewers to kill rats.

https://en.wikipedia.org/wiki/Paris_green#Insecticide

How Did They Respond in the 19th Century?

Green gained an ominous reputation, with tales of women in green dresses collapsing, newspaper printers being overcome by fumes, and children wasting away in bright green rooms

enter image description here

https://en.wikipedia.org/wiki/Scheele%27s_Green#Toxicity

Famous Deaths

It's widely implied that these green pigments contributed to Napoleons death due to the lavish green suite he slept in.

There are also reports it caused health issues with famous artists who used the pigment, such as blinding Monet

Other Dangerous Colours

There are other colours with similar associations, I would recommend this Ted Ed video

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    I was reading your answer, and was going to link to that Ted Ed video if you didn't. Very nice answer! :) – Shokhet Jun 2 '17 at 17:25
  • Thanks @Shokhet ! If you know any more references I can add let me know :) – Tom J Nowell Jun 2 '17 at 17:26
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    Well, there is always absinthe. – Mr Lister Jun 2 '17 at 18:27
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This comes from Real Life; see Why is green the color of poison? for example.

Have you ever wondered why poison is so often associated with the color green? Take movies or video games, for example: even the vials containing poison would frequently be made of a green glass. Sure, there are poisonous plants and all, but look no further than the 18th century, when some toxic green pigments forever ruined the reputation of the color.

...

The beloved pigments of Cézanne and Monet, who excessively used them in paintings, Scheele’s and Paris green were so toxic that they were routinely used to kill the rats. So toxic, in fact, that Monet’s blindness, Cézanne’s diabetes, and even Napoleon’s death are still blamed on them.

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Chlorine gas, used as a chemical weapon in WWI, killing thousands of people, is green. Since then, green gas is associated with poison in popular media, including sci-fi and fantasy.

quote from Wikipedia on Chlorine discovery

Scheele observed several of the properties of chlorine: the bleaching effect on litmus, the deadly effect on insects, the yellow-green color, and the smell similar to aqua regia.

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    Do you have a source showing that this is not simply a coincidence? – amflare Jun 2 '17 at 16:41
  • @armflare it is hard to provide a concrete source to such a widely used concept. I remember reading a historical novel on WWI (do not remember the title) and it was stated that survivors of that particular gas attack only remembered green gas and dead bodies of their fallen comrades. Also, since school chemistry classes, everyone should remember that green gas is a deadly poison. It became common knowledge so far. – TimSparrow Jun 2 '17 at 16:46
  • This strikes me as unlikely since so few people would see chlorine gas (only the particular soldiers and scientists that encountered it). However many people would've seen green wallpaper (mentioned in Tom Nowell's answer) which could let it enter people's heads easier as 'toxic color'. – Jam Jun 2 '17 at 19:09

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