Extremely harmful chemicals have all sorts of smells

Our sense of smell likely evolved to help us avoid eating harmful stuff such as poisonous plants or spoiled foods. It's particularly useful since we can employ it prior to actually placing anything in our mouth. To smell something is to detect and interpret airborne molecules, which often arise from a nearby liquid or solid. These molecules enter our nose and activate olfactory receptors, producing signals that travel to the brain where a smell is perceived. We have hundreds of receptor types, and it's thought a particular smelly molecule will activate a specific set of receptors, each to a distinct degree, which encodes the odour our brain puts together for us to sense. Most of the molecules capable of making their way to your nose have odours. For this post, I've compiled a list of what highly toxic chemicals smell like. Y'know, just 'cause. It's definitely not exhaustive, but I've tried to cover a wide range of odours.

Chemical weapons

Many toxic chemicals with distinct smells were used as weapons during World War I:

• Bromobenzyl cyanide is a type of tear gas that smells like sour fruit. It's an oily dark brown liquid and can cause cyanide poisoning in larger doses.
• Chloracetophenone (phenacyl chloride) is another type of tear gas that can severely damage the eyes and lungs. It's a colourless to brown crystalline solid that smells like apple blossoms.
• Chlorine is a highly irritating greenish-yellow gas capable of causing severe lung damage as it reacts with body water to produce corrosive hydrochloric acid. It has a very strong odour that tends to remind people of chlorine-containing bleach.
• Chloropicrin is another irritant that causes violent coughing, tear formation, vomiting, and painful breathing. It's a colourless liquid currently used as a pesticide (e.g. to fumigate soil prior to planting seeds). Chloropicrin has a sweetish, anise odour often associated with flypaper.
• Phosgene is a colourless gas that can cause severe lung damage. It has an odour reminiscent of green corn, musty hay, or pumpkin.
• Sulfur mustard has been described as resembling used motor oil. It's an insidious poison since it doesn't usually have any immediate effects following exposure, yet 12-36 hours later exposed skin begins to blister and disintegrate. It can cause tremendous damage to the eyes and respiratory tract, and is also carcinogenic. Sulfur mustard smells faintly of mustard plants, garlic, or burnt horseradish.

Lewisite is an oily dark greenish brown liquid that was produced during World War I by both sides but never used. It causes grayish skin burns, lung irritation, and systemic arsenic poisoning (since it contains arsenic). Its effects have been described as being more severe than sulfur mustard. Lewisite smells like geraniums.

Lewisite, which will totally mess you up, smells like pretty geraniums (Source)

Nerve agents inhibit the enzyme acetylcholinesterase, resulting in excessive amounts of the neurotransmitter acetylcholine at sites where nerves end throughout the body. This results in crying, nose running, drooling, sweating, puking, diarrhea, convulsions, and generalized muscle weakness that adversely affects respiratory function and can lead to death. Nerve agents usually have an odour due to the presence of impurities. Smelly agents include:

• Tabun (GA) is a cyanide-containing liquid with an almond or fruity odour.
• Soman (GD) is a liquid that smells like camphor or rotting fruit.
• Cyclosarin (GF) is a flammable colourless liquid with a sweet yet musty odour that reminds some folks of peaches or shellac.

Hydrocarbons

Acrylonitrile (C₃H₃N) is a flammable and possibly carcinogenic liquid capable of damaging several organs of the human body. It's used to make acrylic yarns as well as composite materials found in aircraft. Burning it will produce hydrogen cyanide and nitrogen oxides, both of which are toxic in their own right. Acrylonitrile has a strong and unpleasant smell reminiscent of onion or garlic.

Carbon tetrachloride (CCl₄) is a non flammable liquid that is incredibly toxic to the liver and possibly carcinogenic. Historically it has been used in fire extinguishers, as a dry cleaning agent, and in refrigerators. Carbon tetrachloride has a strong sweet odour.

1,1-Dimethylhydrazine is a flammable colourless liquid used in rocket fuels. It's a carcinogen, is severely corrosive, and can damage the liver, lungs, and brain. 1,1-Dimethylhydrazine has an intense fishy or ammonia-like odour.

Methanethiol (CH₃SH) is a colourless, flammable, and sulfur-containing gas. It's present in very small amounts in farts, marshes, and certain plants. Higher concentrations can fatally damage the brain. Methanethiol has a putrid odour that has been likened to garlic or rotten cabbage.

Methyl isocyanate (CH₃NCO) is a flammable liquid used in the manufacture of a particular group of pesticides known as carbamates. It kills by severely damage the lungs, causing fluid accumulation, bleeding, and inflammation with scarring. Methyl isocyanate has a sharp, unpleasant odour.

Nitrobenzene is a possibly carcinogenic pale yellow liquid that can cause organ damage and methemoglobinemia, the latter of which disrupts the ability of blood to move oxygen throughout the body. It has been described as smelling like bitter almonds (a la cyanide). The scent of nitrobenzene also tends to remind people of shoe or floor polish due to its presence in these products. Due to its smell, it was historically used in perfume manufacturing under the name of essence of mirbane.

Parathion is an organophosphate insecticide derived from nitrobenzene. It inhibits acetylcholinesterase, which as previously discussed for nerve gases, is an enzyme we depend on to clear out parts of the nervous system so our muscles, brains, and various liquid-producing organs work properly. Toxic effects include convulsions, cardiac arrhythmia, and damage to the liver and kidneys. Parathion apparently smells like garlic or rotten eggs.

Acids

Hydrogen cyanide (HCN; prussic acid) is a colourless liquid or gas (boiling point ~26 °C) used as a chemical weapon during WWI. It kills by preventing cells from using a particular enzyme to access the energy contained within organic nutrients (cellular respiration), leading to the death of tissues and organs throughout the body. It is well known for smelling faintly like almonds. Notably, cyanide smells specifically like bitter almonds, the seeds of a particular variety of almond tree that possess a "unique musty odor like dirty tennis shoes when compared with the common, or sweet, almond" (Erickson et al., 2007).

Hydrogen sulfide (H₂S; hydrosulfuric acid) is a flammable gas that can damage the eyes, lungs, and brain. Like cyanide, it kills cells by inhibiting enzymes they need to respire. It was briefly employed as a chemical weapon during WWI. Hydrogen selenide (H₂Se) is a chemically similar flammable gas that causes damage to the eyes, lungs, and liver. Hydrogen sulfide smells like rotten eggs, while hydrogen selenide possess the bouquet of decayed horseradish.

Hydrofluoric acid (HF) is a colourless liquid that is very good at destroying lungs and eyes. If absorbed into the skin, it can cause severe burns and disrupt calcium-related body processes including the somewhat essential contraction of heart muscles. Hydrofluoric acid has a strong, irritating odour that apparently reminds people of chlorine or vinegar.

Plants

Water hemlock (genus Cicuta) is a relatively tall flowering plant found in wetlands and near open waters in North America and Europe. It contains a linear hydrocarbon called cicutoxin, which causes potentially fatal seizures by blocking the action of the inhibitory neurotransmitter GABA in the brain and spinal cord. Accidental ingestion of the plant can occur due to its close resemblance to several edible plants including wild celery, wild carrot, and wild parsnip. Specifically, water hemlock roots have been described as looking like parsnips and having a parsnip, carrot, or celery-like odour.

Poison hemlock (Conium maculatum) belongs to the same family as water hemlock and similarly causes poisoning due to it being mistaken for edible plants. Less commonly, people have been poisoned when they ate wild birds that in turn had eaten buds from hemlock plants. One of the molecules responsible for the adverse effects of poison hemlock is coniine, an alkaloid that acts at the neuromuscular junction to block communication between the nervous system and muscles. This can result in paralysis, with death usually occurring by respiratory failure. Poison hemlock has been described as smelling "like a family of mice."

These cuties apparently smell like poison hemlock (Source)

Other

Nickel carbonyl (Ni(CO)₄) is a liquid produced by reacting nickel with carbon monoxide (CO). It's used by chemists to make fancy compounds that contain nickel-carbon bonds. Following inhalation, it breaks back down into CO and nickel within the lungs. The CO prevents red blood cells from effectively transporting oxygen throughout the body, which can be lethal. The nickel can cause organ damage (lungs, heart, liver, kidneys), which also can kill you. Nickel carbonyl has "a mild, non-penetrating odor, often described as sooty or musty" (Sunderman, 1979).

Osmium tetroxide (OsO₄) is a solid used by chemists and biologists to do stuff like synthesize fancy molecules and acquire images of tiny bits of living things. Its fumes that can cause life-threatening lung damage if inhaled, as well as blindness if they enter the cornea of the eye. Osmium tetroxide has been described as having an "unpleasant, irritating smell resembling simultaneously the smell of chlorine and that of rotten radishes" (Venetskii, 1974). This smell is so nasty the dude who discovered osmium decided to name it using the Greek word osme meaning "a smell" when he produced the tetroxide while investigating the new metal.

Arsine (AsH₃) is a flammable gas inadvertently generated during metal smelting and coal processing. It is produced whenever hydrogen is given the opportunity to react with arsenic, or solid metal arsenides are split apart by water or acids. Arsine is used to manufacture electronics and synthesize arsenic-containing organic molecules. Exposure can result in hemolytic anemia and damage to the kidneys, brain, and/or heart. It was investigated for possible use as a chemical weapon during World War II, but this did not come to pass due to its flammability and reduced effectiveness compared to other available weapons. Arsine, like many arsenic-based compounds, smells mildly of garlic.

Phosphine (PH₃) is a flammable gas that can severely damage the lungs following inhalation (and conversion to phosphoric acid). Once inside the body it also can inhibit the ability of cells to make energy using oxygen (i.e. aerobic respiration), which can lead to really bad things like heart damage. Similar to arsine, phosphine is used to manufacture electronics and synthesize phosphorus-containing organic molecules. It is produced when solid metal phosphides (e.g. aluminum phosphide, a semiconductor and pesticide) react with water or acids. Although it is odourless, phosphine tends to occur with other structurally related molecules (impurities) such as diphosphane (P₂H₄). These smell powerfully of garlic or decaying fish, and their presence can cause phosphine to spontaneously ignite at relatively low temperatures.

Diborane (B₂H₆) is a colourless gas that is highly flammable and can ignite in moist air. It's highly irritating and can damage the respiratory tract and nervous system. Diborane has a sickly sweet smell.

Chlorine trifluoride (ClF₃) is a colourless gas that is super corrosive (it can eat through glass and set Teflon on fire). It will very happily burn through the human body, producing both hydrofluoric acid and hydrochloric acid, which are also very harmful. Chlorine trifluoride has a strong odour that has been described as being both sweet and similar to chlorine or mustard.


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http://www.cdc.gov/niosh/npg/

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http://www.slate.com/blogs/the_vault/2013/05/24/chemical_weapons_wwii_posters_taught_soldiers_to_identify_gasses_by_smell.html