How Does Cyanide Kill? Chemistry of Cyanide Poisoning

How Cyanide Works and How Poisoning Is Treated

You would have to eat a lot of apples with their seeds at one time to suffer cyanide poisoning.
You would have to eat a lot of apples with their seeds at one time to suffer cyanide poisoning. Duncan Smith / Getty Images

Murder mysteries and spy novels often feature cyanide as a fast-acting poison, but you can be exposed to this toxin from everyday chemicals and even common foods. Have you ever wondered how cyanide poisons and kills people, how much it takes before it's toxic, and whether there is a cure? Here's what you need to know.

What Is Cyanide?

The term "cyanide" refers to any chemical containing a carbon-nitrogen (CN) bond.

Many substances contain cyanide, but not all of them are deadly poisons. Sodium cyanide (NaCN), potassium cyanide (KCN), hydrogen cyanide (HCN), and cyanogen chloride (CNCl) are lethal, but thousands of compounds called nitriles contain the cyanide group yet aren't as toxic. In fact, you can find cyanide in nitriles used as pharmaceuticals, such as citalopram (celexa) and cimetidine (Tagamet). Nitriles aren't as dangerous because they don't readily release the CN- ion, which is the group that acts as a metabolic poison.

How Cyanide Poisons

In a nutshell, cyanide prevents cells from using oxygen to make energy molecules.

The cyanide ion, CN-, binds to the iron atom in cytochrome C oxidase in the mitochondria of cells. It acts as an irreversible enzyme inhibitor, preventing cytochrome C oxidase from doing its job, which is to transport electrons to oxygen in the electron transport chain of aerobic cellular respiration.

Without the ability to use oxygen, mitochondria can't produce the energy carrier adenosine triphosphate (ATP). Tissues that require this form of energy, such as heart muscle cells and nerve cells, quickly expend all their energy and start to die. When a large enough number of critical cells die, you die.

Exposure To Cyanide

Cyanide can be used as a poison or chemical warfare agent, but most people are exposed to it unintentionally. Some ways to be exposed to cyanide include:

  • eating cassava, lima beans, yucca, bamboo shoots, sorghum, or almonds
  • eating apple seeds, cherry stones, apricot pits, or peach pits
  • smoking cigarettes
  • burning plastic
  • burning coal
  • inhaling smoke from a house fire
  • ingesting acetonitrile-based products are used to remove artificial nails
  • drinking water, eating food, touching soil, or inhaling air that has been contaminated
  • exposure to rodenticide or other cyanide-containing pesticide

Cyanide in fruits and vegetables is in the form of cyanogenic glycosides (cyanoglycosides). Sugars attach to these compounds through the process of glycosylation, forming free hydrogen cyanide.

Many industrial processes involve compounds that contain cyanide or can react with water or air to produce it. Paper, textile, photochemical, plastics, mining, and metallurgy industries all may deal with cyanide. Some people report an odor of bitter almonds associated with cyanide, but not all toxic compounds produce the scent and not all people can smell it. Cyanide gas is less dense than air, so it will rise.

Symptoms of Cyanide Poisoning

Inhaling a high dose of cyanide gas rapidly causes unconsciousness and often death. Lower doses may be survivable, especially if immediate aid is provided. The symptoms of cyanide poisoning are similar to those displayed by other conditions or exposure to any of a number of chemicals, so don't assume cyanide is the cause. Do remove yourself from the cause of exposure and seek immediate medical attention!

Immediate Symptoms

  • headache
  • dizziness
  • weakness
  • confusion
  • fatigue
  • lack of coordination

Symptoms from Larger Doses or Longer Exposure

  • low blood pressure
  • unconsciousness
  • convulsions
  • slow heart rate
  • lung damage
  • respiratory failure
  • coma

Death from poisoning usually results from respiratory failure or heart failure. A person exposed to cyanide may have cherry-red skin from high oxygen levels or a dark or blue coloring, from Prussian blue (iron binding to the cyanide ion).

Also, skin and body fluids may give off an odor of almonds.

How Much Cyanide Is Lethal?

How much cyanide is too much depends on the route of exposure, the dose, and duration of exposure. Inhaled cyanide presents a greater risk than ingested cyanide. Skin contact is not as much of a concern (unless it has been mixed with DMSO), except touching the compound could lead to accidentally swallowing some of it. As a rough estimate, since lethal dose depends on the exact compound and several other factors, about half a gram of ingested cyanide will kill a 160-lb adult.

Unconsciousness, followed by death, could occur within several seconds of inhaling a high dose of cyanide, but lower doses and ingested cyanide may allow a few hours to a couple of days for treatment. Emergency medical attention is critical.

Is there a Treatment for Cyanide Poisoning?

Because it's a relatively common toxin in the environment, the body can detoxify a small amount of cyanide. For example, you can eat the seeds of an apple or withstand cyanide from cigarette smoke without dying.

When cyanide is used as a poison or a chemical weapon, treatment depends on the dose. A high dose of inhaled cyanide is lethal too quickly for any treatment to take effect. Initial first aid for inhaled cyanide is getting the victim to fresh air. Ingested cyanide or lower doses of inhaled cyanide may be countered by administering antidotes that detoxify cyanide or bind to it. For example, natural vitamin B12, hydroxocobalamin, reacts with cyanide to form cyanocobalamin, which is excreted in urine.

Inhalation of amyl nitrite may aid breathing in victims of cyanide and also carbon monoxide poisoning, although few first aid kits contain these ampules anymore.

Depending on the conditions, complete recovery may be possible, although paralysis, liver damage, kidney damage, and hypothyroidism are possible.