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Top 5 Deadliest Substances on Earth

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Five hundred micrograms — less than a pinch of salt — is all it takes to end a human life, and that’s not even the deadliest toxin nature has to offer.

In “The Deadliest Toxins on Earth,” SciShow host Hank Green skips past the usual suspects — arsenic, cyanide, industrial chemical weapons — to rank the natural compounds that kill at doses too small to see. The episode opens with a real Cold War murder case and ends with a bacterial protein so lethal it makes ricin look almost mild by comparison, yet gets injected into foreheads by the millions every year under a different name.

• Green opens with the 1978 assassination of Bulgarian dissident journalist Georgi Markov, killed on London’s Waterloo Bridge by a modified umbrella that fired a ricin-laced pellet into his leg.
• Ricin, extracted from castor bean seeds, kills an adult human at a dose of roughly 500 micrograms — about five ten-thousandths (0.0005) the weight of a standard paperclip.
• Botulinum toxin, produced by the bacterium Clostridium botulinum, is identified as the most acutely lethal substance known to science, working at nanogram-scale doses — yet heavily diluted versions of it are used medically as Botox.

The Umbrella on Waterloo Bridge

Green frames the whole episode around one Cold War killing. In September 1978, Georgi Markov — a Bulgarian writer and BBC broadcaster who had defected and become an outspoken critic of the communist regime in Sofia — was walking across Waterloo Bridge when he felt a sharp jab in his thigh. He’d been struck by an umbrella rigged to fire a tiny metal pellet, drilled with microscopic holes and packed with ricin. Markov died days later; the case remains one of the most notorious poison assassinations of the 20th century, and Green uses it as the entry point into how plant-derived toxins actually kill.

Ricin Toxin Mechanisms Shut Down Organs

Ricin comes from the seeds of the castor bean plant, Ricinus communis — the same plant used to produce castor oil. Once inside the body, the protein blocks ribosomes from making new proteins inside cells, and without that machinery running, cells simply stop functioning and die. Green’s number for the lethal dose is stark: about 500 micrograms, or roughly five ten-thousandths of the weight of a paperclip, is enough to kill an adult. That’s the scale that made an umbrella-fired pellet a viable murder weapon.

The Bacterium That Beats Ricin

For all of ricin’s notoriety, Green is clear that it isn’t even close to the top of the list. That distinction goes to botulinum toxin, produced by the anaerobic bacterium Clostridium botulinum. It’s regarded as the most acutely toxic substance known to science, working at nanogram doses — a fraction of ricin’s already tiny lethal threshold. The toxin binds irreversibly to neuromuscular junctions and blocks the release of acetylcholine, the neurotransmitter muscles need to fire, which produces progressive flaccid paralysis. Death comes from respiratory failure once the toxin reaches the muscles that control breathing.

A dose of botulinum toxin measured in nanograms can kill an adult — a scale even smaller than the microgram doses of ricin that ended Georgi Markov’s life.

The Botox Paradox

What makes botulinum toxin such a strange case study is what happens to it once it’s diluted down to medical concentrations. The same neurotoxin behind some of history’s worst food-poisoning outbreaks is injected, in vanishingly small and controlled doses, as Botox — used to treat muscle spasms and chronic migraines as well as for cosmetic wrinkle treatment. Green’s point isn’t that the toxin got safer; it’s that dose is everything, and a compound capable of shutting down a diaphragm can also be precise enough, in the right amount, to relax a single overactive muscle.

Green doesn’t rank the two by drama — Markov’s umbrella makes for a better story than a botulism outbreak — but by the numbers, and the numbers say botulinum toxin wins by a wide margin. Ricin needs 500 micrograms to do its damage; botulinum toxin does its work in nanograms, thousands of times smaller. That gap is the whole point of the episode: the deadliest things on Earth aren’t the ones with headlines attached, they’re the ones a bacterium makes by accident.