What is the most toxic element to humans?

September 22, 2025 •

4 min read

By mass, polonium-210 is estimated to be billions of times more toxic than hydrogen cyanide, delivering a lethal dose with a mere microgram. Its highly radioactive nature makes it a contender for the title of what is the most toxic element to humans, and the reasons for its lethality are rooted in its unique radioactive properties rather than its chemistry.

Quick Summary

The most toxic element to humans is widely considered to be polonium-210, not for its chemical properties, but because it is a highly radioactive element that emits damaging alpha particles internally, causing rapid and widespread cellular death, organ failure, and severe radiation sickness if ingested or inhaled, even in minuscule amounts.

Key Points

Polonium-210's Lethality: Polonium-210 is considered the most toxic element due to its extreme radioactivity, not its chemical properties, with a microgram being a potentially lethal dose.
Internal Alpha Radiation: Its alpha particles are harmless externally but are highly damaging once ingested or inhaled, depositing high energy that destroys cells and damages DNA.
Widespread Organ Damage: Once in the bloodstream, polonium-210 concentrates in organs like the spleen, liver, and kidneys, causing mass cellular death and organ failure.
Cause of Acute Radiation Sickness: High doses lead to acute radiation sickness, with initial symptoms like nausea and vomiting progressing to organ failure and death.
Primary Source of Low-Level Exposure: Tobacco smoke is a significant source of low-level polonium-210 exposure, which increases the risk of lung cancer over time.
Difficult to Detect Internally: While alpha particles don't penetrate skin, special tests on blood and urine are required to detect polonium poisoning, which can initially mimic other illnesses.
Prevention is Key: The best prevention is to avoid ingestion and inhalation, particularly for individuals working with radioactive materials or those who smoke.

The Undisputed Contender: Polonium-210

While many elements are poisonous, such as arsenic and mercury, the consensus among scientists points to polonium-210 (Po-210) as the most toxic. Discovered in 1898 by Marie and Pierre Curie, Po-210 is a rare, highly radioactive metal that emits alpha particles. The danger it poses is not from its chemical makeup but from the immense energy released by this radiation once it enters the body. For context, a microgram of Po-210 can be fatal if ingested, making it far more potent than chemical poisons.

How Alpha Particles Devastate from Within

Alpha particles are essentially helium nuclei, composed of two protons and two neutrons. They are relatively large and heavy for radiation particles. This means they cannot penetrate solid objects like a sheet of paper or the dead outer layer of human skin. This is why polonium is not considered an external radiation hazard. However, once inside the body—whether through ingestion, inhalation, or a wound—alpha particles become a devastating internal threat. Inside, they travel short distances, depositing all their immense energy into a very small area of soft tissue. This concentrated energy delivery shreds biomolecules, including DNA, and kills cells indiscriminately.

The Deadly Path of Internal Exposure

After being ingested or inhaled, polonium enters the bloodstream and is transported throughout the body. It tends to accumulate in specific organs and tissues, where it continuously bombards cells with alpha radiation.

Distribution in the body: Polonium-210 travels through the blood and collects in organs like the spleen, kidneys, liver, and bone marrow.
Cellular and organ damage: The localized, high-energy assault by alpha particles causes mass cellular death. This leads to organ dysfunction and eventual failure, often presenting as severe radiation sickness.
Acute symptoms: Initial symptoms of a high-dose polonium poisoning can mimic a stomach virus, including nausea, vomiting, and diarrhea. Other signs of acute radiation poisoning follow, such as hair loss, severe fatigue, and damage to the immune system.
Famous case: The poisoning of Alexander Litvinenko in 2006 is a grim and well-documented example of polonium-210's lethal effects. His symptoms and subsequent death provided clear evidence of internal radiation poisoning.

Polonium-210 vs. Other Potent Toxins

To understand why polonium stands alone in its class of toxicity, it's useful to compare its effects to other well-known deadly elements. While chemical toxins interfere with specific biological processes, polonium's radioactive nature causes universal, mechanical destruction at the cellular level.


Feature	Polonium-210	Arsenic	Mercury
Toxicity Mechanism	Internal alpha radiation causes massive cellular damage and death.	Chemically interferes with enzymes and cellular respiration, leading to organ failure.	Disrupts the nervous system and kidneys, causing neurological and organ damage.
Mode of Action	Widespread cellular destruction from high-energy radiation concentrated in specific tissues.	Targets multiple cellular functions, especially in organs with high metabolic activity.	Primarily affects the nervous system, kidneys, and liver.
Primary Exposure Route	Internal ingestion or inhalation is required for toxicity.	Ingestion (water, food) and inhalation of inorganic forms are the primary risks.	Inhalation of vapor and ingestion of organic compounds are primary risks.
Relative Potency (by mass)	Extremely high; a microgram can be lethal.	High; depends on chemical form and exposure duration.	High; depends on chemical form (e.g., methylmercury is more potent).
Detectability (Post-Mortem)	Specific gamma-ray signature in urine samples.	Can be detected in hair, nails, and other tissues.	Can be detected in hair, blood, and other tissues.

Sources of Polonium Exposure

While a lethal dose of Po-210 is rare and typically requires sophisticated manufacturing, low-level exposure is more common than people realize.

Natural Decay Chain: Po-210 is a product of the natural radioactive decay of uranium-238 and radon-222, meaning tiny amounts exist in the earth's crust, soil, and even the atmosphere.
Tobacco Smoke: Tobacco plants can absorb polonium from the soil and phosphate fertilizers used to grow them. When smoked, the Po-210 is aerosolized and can be inhaled into the lungs, delivering a long-term radiation dose.
Industrial Sources: Polonium is used in static eliminators and other industrial equipment. These devices are regulated and designed for safety, but they highlight polonium's industrial uses.
Contaminated Food and Water: Low levels can sometimes enter the food and water supply, particularly in certain environments.

Prevention of Exposure

Preventing exposure depends heavily on the context. For the average person, avoiding tobacco smoke is the most effective measure against long-term, low-level exposure. In industrial or laboratory settings, strict protocols are necessary to prevent the ingestion or inhalation of radioactive materials. This includes using proper shielding, wearing personal protective equipment (PPE), and utilizing specialized containment systems like fume hoods.

Conclusion

While the toxicity of many chemical elements is a serious health concern, polonium-210 stands out as the most toxic due to its highly concentrated, internal radiation-based lethality. Unlike chemical poisons, which might be detected and treated, polonium's method of inflicting damage makes it uniquely devastating, delivering a fatal blow to the body's internal systems at the cellular level. Understanding this distinction is crucial for recognizing the different threats posed by various toxic substances. For more information on radiation safety and related health topics, consult the official website of the Centers for Disease Control and Prevention.

Frequently Asked Questions

Polonium-210's toxicity comes from its radioactivity, not its chemistry. It emits alpha particles that cause concentrated, widespread cellular damage once inside the body. In contrast, arsenic and mercury are chemical poisons that interfere with specific biological functions.

No, external contact with polonium-210 is not a health hazard because the alpha particles it emits cannot penetrate the dead outer layer of skin or clothing. The danger is exclusively from internal exposure via ingestion, inhalation, or through an open wound.

A lethal dose of polonium-210 typically requires access to industrial-grade sources and specific, intentional manufacturing processes. Natural sources contain only minuscule amounts that are not harmful in regular environmental contexts.

Long-term exposure to low levels of polonium-210, such as from tobacco smoke, can increase the risk of developing cancer, particularly lung cancer. This is due to the cumulative damage from alpha radiation over time.

There is no cure for severe radiation poisoning from polonium-210 once the symptoms have progressed to the point of organ damage. Treatment focuses on managing radiation exposure symptoms, and once the effects become apparent, they are often fatal.

Because the initial symptoms can be misleading, detection often requires a high degree of suspicion from doctors. Specialized analysis of blood or urine samples using gamma-ray spectroscopy is necessary to confirm the presence of polonium-210.

Besides industrial uses like static eliminators, the most common source of low-level exposure is tobacco smoke. Tobacco plants absorb polonium from the soil, which is then inhaled when a person smokes.