Cyanide poisoning: sources, symptoms and why it is an emergency

Cyanide poisoning occurs when cyanide, as hydrogen cyanide gas or as a cyanide salt, enters the body. The UK Health Security Agency (UKHSA) explains that hydrogen cyanide is rapidly absorbed and distributed following inhalation, ingestion or skin contact, and that the cyanide ion blocks the way cells use oxygen.

Tissues with high energy demand, such as the brain and heart, are most affected. UKHSA says tissues with high metabolic demand such as the central nervous system are key targets, and that lactic acidosis is a key feature that correlates with the severity of intoxication.

This page draws on UKHSA's chemical hazards compendium, which is written for incident response. The information is explanatory and does not replace emergency advice.

UKHSA says people may be exposed to hydrogen cyanide released as a combustion product during house fires, and that cyanide intoxication contributes to morbidity and mortality from smoke inhalation along with carbon monoxide. Exposure may also occur in industrial settings or from accidents during storage or transportation.

For the general population, UKHSA says cigarette smoke is considered the greatest source of exposure to hydrogen cyanide, excluding people exposed to high levels of cyanogenic glycosides in food. Some edible plants contain low concentrations in the form of cyanogenic glycosides, notably the kernels of wild (bitter) almonds, apricots and black cherries, bamboo shoots, lima beans and cassava, which can release hydrogen cyanide when ground or chewed.

Sodium and potassium cyanide are white crystal-like solids with a faint almond odour, used in industry for metal cleaning, plating and extraction and in photography. UKHSA says exposure to these salts is most likely to occur in the workplace where they are produced or used, and that it is very unlikely that the general population will be exposed to a level high enough to cause adverse health effects.

UKHSA says early features of cyanide toxicity include non-specific central nervous system symptoms, muscular and neurological effects, rapid breathing (tachypnoea) and a fast heart rate (tachycardia). For cyanide salts it lists headache, nausea, dizziness, nervousness, confusion, changes in heart rate and drowsiness.

• Headache, dizziness, nausea and confusion at lower exposure.

• Changes in heart rate and drowsiness.

• Vomiting and low blood pressure in worse cases.

• Seizures and rapid loss of consciousness.

• Collapse with cardiorespiratory depression and fluid on the lungs (pulmonary oedema).

• In the most serious cases, coma, heart and breathing problems and death.

UKHSA says hydrogen cyanide is highly toxic, with rapid onset of symptoms following acute exposure, and that it may be fatal following exposure by all routes. Because symptoms from salts can be delayed by 2 to 4 hours if ingested on a full stomach, feeling well soon after exposure does not exclude serious poisoning.

If someone has been exposed to smoke from a fire, has swallowed or been in contact with cyanide salts, or has been exposed in an industrial incident, call 999 and get them to fresh air if it is safe to do so without putting yourself at risk. Do not wait for symptoms to appear, and tell the ambulance service what the substance was.

UKHSA says if you have any health concerns regarding exposure to sodium or potassium cyanide you should seek medical attention.

UKHSA says hydrogen cyanide is rapidly absorbed and widely distributed, with most blood cyanide held in red blood cells. The body metabolises it mainly by converting it to thiocyanate, which is excreted in the urine, with about 80% metabolised by this route. The rate of natural detoxification in humans is slow, which is why large exposures overwhelm the body.

Ingested cyanide salts are rapidly and completely converted to hydrogen cyanide in the stomach, according to UKHSA, which is why the toxicology of the salts is similar to that of the gas.

UKHSA says that after a single, brief exposure to a low concentration from which a person recovers quickly, no long-term health effects are anticipated. Survivors of larger exposures may suffer long-term central nervous system damage, with observed effects including intellectual deterioration, confusion, personality changes, memory deficits and Parkinsonism.

Chronic exposure to low levels has been linked to non-specific neurological symptoms, thyroid effects and optic neuropathy. UKHSA says hydrogen cyanide has not been classified as a human carcinogen, and that exposure to sodium or potassium cyanide has not been linked to cancer.

Treatment of cyanide poisoning is hospital-based emergency care, with supportive treatment of breathing and circulation and specialist antidotes where indicated, guided by poison information services. The UKHSA documents used for this page concentrate on health effects and toxicology rather than clinical treatment protocols, so those details are not reproduced here.

People with a fire-related exposure are also assessed for carbon monoxide poisoning and burns. Hospital teams use the history of exposure, symptoms and blood tests, including lactate, as part of assessment, and follow-up looks for neurological effects after severe exposure.

Preventing house fires and having working smoke alarms reduces exposure to combustion products including hydrogen cyanide. UKHSA identifies fires as a source of exposure, and workplaces where cyanide salts are used are covered by enforced safe limits to protect employees.

Avoid chewing or crushing the kernels of fruit stones such as apricots and bitter almonds in quantity, as UKHSA notes they contain cyanogenic glycosides that can release hydrogen cyanide when ground or chewed.

Suspected cyanide exposure is dealt with by the ambulance service and emergency departments, with advice from national poisons information services and UKHSA's chemical incident teams if there is a wider incident. There is no routine pathway outside emergency care.

After discharge, people who had a severe exposure may be followed up by a GP and specialist services for neurological or psychological effects, whether the initial care was provided in an NHS or independent hospital.