Cyanide can be in the form of a colorless gas or a crystal, but either way, it's pretty deadly. It smells like bitter almonds, and exposure to cyanide can cause symptoms such as headache, nausea, rapid breathing and heart rate, and weakness in just minutes. If untreated, cyanide kills by basically depriving the body's cells of oxygen. And yes, it can be made from apple seeds, but don't worry if you eat a few. You'd need to eat about ten apple cores before there was enough cyanide in your system to have an effect. Please don't do that.
Hydrofluoric Acid is a poison used in the making of Teflon, among other things. When in liquid from, it can easily seep through the skin and into the bloodstream. It reacts with the calcium in the body and can even destroy the underlying bone. The scary part? At first contact it's completely painless, leaving more time and possibility for it to do its damage.
Arsenic is a naturally occurring crystalline metalloid and is possibly one of the most well known and common poisons used as a murder weapon in the late 19th century. However, its known use in such a capacity goes back to the mid 1700's. Arsenic poisoning can kill in a few hours to a few days. Symptoms are vomiting and diarrhea, which made it difficult to distinguish from dysentery or cholera 120 years ago.
Belladonna or Deadly Nightshade is a highly poisonous herb/flower with a very romanticized history. The poison comes from an alkaloid called atropine, and the entire plant is poisonous, with the root being the most deadly and the berries being the least. However, just two ingested berries is still enough to kill a child. Some people use Belladonna recreationally as a hallucination, and in Victorian times women often used a tincture of Belladonna to dilate their pupils and make them appear more wide and glassy eyed. Before death, Belladonna can also give you seizures, rapid pulse, and confusion. Don't play with Belladonna, kids.
Carbon monoxide is odorless, tasteless, colorless, and slightly less dense than air. Also it will poison and then kill you. Part of what makes Carbon monoxide so deadly is because it's difficult to detect; it's sometimes called the "silent killer." It inhibits your body's ability to carry oxygen where it's needed - like to cells. To keep them alive and stuff. Early carbon monoxide poisoning symptoms are similar to the flu, minus the fever - headaches, weakness/lethargy, sleeplessness, nausea, & confusion. Luckily, you can get a carbon monoxide detector at every hardware or home improvement store!
The most deadly tree in all of North America grows in Florida. Because where else would it grow, honestly? The manchineel or beach apple tree has small green fruits that look like, well apples, and they are supposedly sweet. Don't eat them. Don't touch this tree. Don't sit near it, or under it, and pray you're never standing downwind of it. The sap will make your skin blister upon contact, and if it gets in your eyes, you might be blinded. The sap is in the leaves and bark, so don't touch those either. It is supposedly what killed Ponce de Leon. Lame.
Fluorine is a pale yellow gas that is highly poisonous, corrosive, and will react with nearly anything. It takes a concentration of 0.000025% for fluorine to become lethal. It blinds and suffocates you, like mustard gas but oh, so much worse.
Compound 1080, also known as Sodium fluoroacetate, is used as a pesticide. It's also found naturally in a few plant species in Africa, Brazil, and Australia. The scary part of this odorless and tasteless deadly poison is that there is no known antidote. Strangely, bodies of those who died from ingesting this poison remain poisonous for up to a year.
The most dangerous man-made poison is called Dioxin, and it only takes 50 micrograms to be lethal to an adult human being. It's the third most toxic poison known to science, 60 times more toxic than cyanide.
Dimethylmercury (a neurotoxin) is terrifying because it can penetrate most standard protection equipment - like thick latex gloves. Which is exactly what happened to a chemist named Karen Wetterhahn in 1996. She got a single drop of the colorless liquid on her gloved hand, and that was it. Symptoms started showing up FOUR MONTHS later, and in six months, she was dead.
Aconite is also known as monkshood, wolf's bane, leopard's bane, women's bane, devil's helmet, queen of poisons, and blue rocket. It's technically a genus of over 250 herbs, and most of those are extremely poisonous. The flowers can be either blue or yellow, and while some of the plant is used for traditional medicines, it's also been used as a murder weapon within the last decade.
The toxin found in deadly mushrooms is called Amatoxin. It attacks the cells of the liver and kidneys and shuts them down in a matter of days. Sometimes it also attacks the heart and eventually the central nervous system. There are treatments, but they aren't a guarantee. It's heat stable and can't be destroyed by drying. Again, if you aren't 100% sure it's good to eat, don't eat it.
Anthrax is actually a bacteria called Bacillus anthracis. What makes you sick isn't so much the bacteria itself, but rather the toxin it produces inside the body. Bacillus Anthracis can get into your system via the skin, ingestion, or inhalation. The mortality rate for inhaled anthrax is as high as 75% , even with treatment.
Hemlock is a classical poison, being used as a form of execution regularly in ancient Greece, including on philosopher Socrates. There are several varieties, and in North America, water hemlock is the most common. You can die from eating it, and people still do, thinking it's an acceptable forged salad ingredient. Water Hemlock causes painful and violent convulstions, cramps, and tremors. Those who survive can have long term issues such as amnesia. Water Hemlock is considered the most deadly plant in North America. Serious note: Watch your children - even older ones - outside. If you forage, KNOW WHAT NOT TO EAT. Don't eat anything you aren't 100% sure is safe.
Strychnine is commonly used to kill small mammals and birds and is frequently a main ingredient in rat poison. It can also be deadly to humans in larger doses. It can be ingested, inhaled, or absorbed through the skin. First symptoms are painful muscle contractions, nausea, and vomiting. Muscle contractions eventually lead to asphyxiation. Death can result in as little as half an hour. It's a seriously unpleasant way to die, for human or rat.
Maitotoxin is considered by most people who know about these things to be the most potent marine toxin. It grows in a type of dinoglagellate called Gambierdiscus toxicus, and if those words confused you, just think of deadly plankton, and you get the general idea. In mice, maitotoxin has the highest toxicity for nonprotein toxins.
Mercury - as in the silvery stuff in old school glass thermometers - is a Heavy Metal that's quite toxic to humans if you inhale it or touch it. It can make your skin peel off if you touch it; if inhaled or ingested, it will eventually shut down your central nervous system and you'll die. Before that, you'll likely experience kidney failure, memory loss, brain damage, and blindness.
Polonium is an element and has been linked to the deaths of everyone from Yasser Arafat to Russian dissidents. The most common form is 250,000 more toxic than hydrogen cyanide. It's radioactive and causes damage by the release of alpha particles (they don't get along with organic tissues). Alpha particles can't pass through the skin, so polonium needs to be ingested or injected into the victim. However, if that happens, it doesn't take much. One theory states that a gram of polonium 210 could kill as many as ten million people if injected or ingested, via radiation poisoning or cancer later on. Let's all be glad it's not on Amazon Prime...
The Suicide Tree or Cerbera odollam works by disrupting the heart's natural rhythm, often causing death. A member of the same family as the Oleander, it used to be used for "trial by ordeal" in Madagascar. It's estimated that 3,000 people a year died from ingesting Cerbera poison before trial by poison was outlawed in 1861. (If you survived, you were innocent. If you died it didn't matter because you were dead.)
Botulinum toxin is caused by Clostridium Botulinum bacterium, and it's an incredibly powerful neurotoxin. It causes paralysis which can lead to death. You may know Botulinum toxin by it's commercial name - Botox. Yeah, that stuff the doctor injects into your mom's forehead to make her less wrinkly (or into your neck to help with migraines) paralyzes muscles with Botulinum Toxin.
Pufferfish is considered to be a delicacy in some countries called Fugu; it's a dish some will literally die for. Why? Because it contains Tetrodotoxin around the internal organs, and around 5 people a year die in Japan from eating Pufferfish that's not prepared correctly. Yet they persist.
Sarin Gas gives you the worst minute of your life you can imagine, but the silver lining is that it's also your last? Your chest gets tight, and then tighter, and then tighter, and then...it relaxes because you're dead. Though making Sarin was outlawed in 1995, that hasn't stopped it from being used in terrorist attacks.
The Golden Poison Dart Frog is tiny, adorable, and quite deadly. Just one frog is about the size of the end of your thumb, and contains enough neurotoxin to kill ten men! A dose that equals about two grains of salt is enough to kill adult humans. This is why some Amazonian tribes have used their venom to coat the tips of their hunting darts. Just a graze will kill you good and dead within minutes! A good rule: If it's a frog, and it's yellow, blue, green, or red, don't touch it.
Ricin, like Anthrax, is easy to lace letters with, but a bit more lethal. It's made from Castor beans, the same plant we get Castor Oil from. It's most deadly when inhaled, and a pinch will kill you very dead.
Codnamed "Purple Possum," VX is the most potent nerve gas on Earth. It's completely man made, and we can thank the United Kingdom for that. Technically it was banned in '93, and the US has supposedly dumped it's supply. Other countries are "working on it." Which we should totally trust, because governments are known for being 100% transparent about these things.