What is Cyanide?

Cyanide refers to a group of chemical compounds that contain the cyano group (-C≡N)—a carbon atom triple-bonded to a nitrogen atom. These compounds are highly reactive and readily form salts with various metal cations. The toxicity and solubility of these salts depend on the specific metal ion and the pH of the environment.

Some of the most common and toxic cyanide salts include sodium cyanide, potassium cyanide, and calcium cyanide, which are highly soluble in water and thus easily release free cyanide ions. Hydrogen cyanide (HCN) is a colourless gas with a faint, bitter almond-like odour. Sodium and potassium cyanide appear as white solids and emit the same distinct smell when damp.

What is Cyanide used for?

Cyanide salts play a key role in several industrial processes, including:

• Electroplating
• Metallurgy
• Organic chemical synthesis (e.g., acrylonitrile, methyl methacrylate, adiponitrile)
• Gold and silver extraction from ores
• Leather tanning
• Plastic and fibre production

Cyanide is also used in the manufacture of fumigants, insecticides, rodenticides, and in photographic development.

Sources of Emission & Routes of Exposure

Source of Emission

• Industrial sources:

Cyanide is released from gold/silver mining operations, chemical processing plants, metallurgical and steel industries, petroleum refineries, and metal finishing facilities.

• Diffuse sources:

Pesticide application, waste burning, and the combustion of tobacco, certain plastics, wool, silk, and paper can all emit cyanide. Other sources include tanneries, photographic processing, and metal cleaning facilities.

• Natural sources:

Some plants naturally produce chemicals like amygdalin, which can release cyanide during digestion. This is found in the pits of apricots, peaches, cherries, apples, pears, and sweet almonds. Certain blue-green algae also produce cyanide during nitrate metabolism.

• Transport Sources:

Motor vehicle exhaust is a major mobile source of airborne cyanide.

• Consumer products:

Cyanide can be found in pesticides, rodenticides, metal polishes, photographic solutions, and even lubricants and household cleaners.

Routes of Exposure

• Inhalation:

This is the primary route for occupational exposure, though it's a minor pathway for the general population.

• Oral (Ingestion):
• The main route of exposure for the general population. Most cyanide exposure comes from food, not air or water.
• Dermal (Skin Contact):

Considered a minor exposure route, though skin absorption can still be dangerous with certain compounds.

In the Environment

Cyanide can be released into the air, water, and soil through both natural and industrial activities.

• In the air, cyanide is mainly found as hydrogen cyanide gas or fine particles. Its atmospheric half-life is about 1 to 3 years.
• In water, cyanide usually forms hydrogen cyanide and evaporates, and it does not accumulate in fish.
• In soil, cyanide is fairly mobile. It may evaporate as hydrogen cyanide or be transformed by microbes into less harmful substances. However, at high concentrations, it can kill these microbes, allowing cyanide to leach into groundwater.

Health Effects

Acute Effects

Cyanide is extremely toxic. Inhaling hydrogen cyanide at concentrations of 100 mg/m³ or more can be lethal. Even lower levels (6–49 mg/m³) can cause:

• Weakness
• Headache
• Nausea
• Rapid breathing
• Eye and skin irritation

Animal studies in rats and mice have confirmed hydrogen cyanide’s high acute toxicity via inhalation.

Chronic Effects

Long-term exposure to cyanide—especially via inhalation—can cause:

• Central nervous system effects (headache, tremors, dizziness, numbness, blurred vision)
• Cardiovascular and respiratory problems
• Thyroid enlargement
• Eye and skin irritation

Animal studies have shown similar effects, including myelin degeneration.

• EPA Reference Concentration (RfC) for hydrogen cyanide: 0.003 mg/m³
• EPA Reference Dose (RfD) for cyanide: 0.02 mg/kg/day

Reproductive/Developmental Effects

No human data is available regarding reproductive effects of inhaled cyanide. However, animal studies suggest that oral exposure (e.g., from cassava) may cause foetal malformations, low birth weights, and maternal toxicity.

Cancer Risk

There is no conclusive evidence linking cyanide to cancer in humans or animals through inhalation or ingestion.

The EPA classifies cyanide as Group D: Not Classifiable as to Human Carcinogenicity.

Safety

First Aid Measures

Eye Contact:

Remove any contact lenses if present. Immediately flush the eyes with plenty of water for at least 15 minutes. Cold water may be used. Seek medical attention immediately.

Skin Contact:

Immediately wash the affected area with plenty of water for at least 15 minutes while removing contaminated clothing and shoes. Apply an emollient to soothe irritation. Cold water may also be used. Wash clothing before reuse and clean shoes thoroughly. Seek immediate medical help.

Serious Skin Contact:

Wash the affected area with disinfectant soap, then apply an antibacterial cream. Seek immediate medical attention.

Inhalation:

If inhaled, move the person to fresh air. If the person is not breathing, begin artificial respiration. If breathing is difficult, administer oxygen. Get medical attention immediately.

Serious Inhalation:

Evacuate the person to a safe area. Loosen any tight clothing (collar, tie, belt, waistband). If breathing is difficult, administer oxygen. If the person is not breathing, perform mouth-to-mouth resuscitation.

WARNING: Mouth-to-mouth resuscitation can be hazardous to the rescuer when the inhaled material is toxic, infectious, or corrosive. Seek immediate medical attention.

Ingestion:

Do not induce vomiting unless instructed by medical personnel. Never give anything by mouth to an unconscious person. Loosen tight clothing. Seek medical attention immediately.

Fire Hazards

Cyanides pose serious fire hazards, especially when in contact with:

• Acids or acid fumes
• Water or steam

These reactions can produce toxic and flammable hydrogen cyanide gas (HCN) and sodium oxide. When heated to decomposition, cyanides emit toxic fumes of hydrogen cyanide and oxides of nitrogen.

Mixing metal cyanides with metal chlorates, perchlorates, or nitrates can lead to violent explosions, especially in fusion mixtures.

Engineering Controls and Personal Protection

Engineering Controls

To limit exposure:

• Use enclosed systems, local exhaust, or other ventilation solutions to maintain airborne concentrations below the recommended exposure limits.
• If operations produce dust, fume, or mist, additional ventilation is required.

Personal Protective Equipment

When handling cyanide compounds, the following PPE is recommended:

• Splash goggles
• Synthetic apron
• Approved vapour/dust respirator
• Protective gloves

In Case of a Large Spill:

• Splash goggles
• Full protective suit
• Vapour and dust respirator
• Boots
• Gloves
• Self-contained breathing apparatus (SCBA) is recommended to avoid inhalation.

Note: These suggestions may not offer complete protection. Always consult a chemical safety specialist before handling cyanide.

Regulation

United States

• EPA regulates the levels of cyanide that are allowable in drinking water. The highest level of cyanide allowed in drinking water is 0.2 parts cyanide per 1 million parts of water (0.2 ppm). 
• The Occupational Safety and Health Administration (OSHA) has set a limit for hydrogen cyanide and most cyanide salts of 10 parts cyanide per 1 million parts of air (10 ppm) in the workplace.
• NIOSH has set a 15-minute short-term exposure limit of 5mg/m3 for hydrogen, calcium, potassium and sodium cyanide.

Australia

• Worksafe Australia: For Hydrogen cyanide, eight hour time weighted average (TWA) exposure limit: 11 5mg/m3. For other cyanide compounds, the TWA exposure limit is 5mg/m3. 
• Worksafe Australia reports hydrogen cyanide, potassium cyanide, and sodium cyanide are all very toxic by inhalation.