The Most Toxic Elements on the Periodic Table

A list of six of the most lethal substances known to man

6 Deadly Elements

You might think the worst chemical elements might offer some sort of warning, like smoke or a radioactive glow. Nope! Most are invisible or innocuous-looking.
You might think the worst chemical elements might offer some sort of warning, like smoke or a radioactive glow. Nope! Most are invisible or innocuous-looking.

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There are 118 known chemical elements. While we require some of them in order to survive, others are downright nasty. What makes an element "bad"? There are three broad categories of nastiness:

  1. Radioactivity: The obviously dangerous elements are those which are highly radioactive. While radioisotopes can be made from any element, you'd do well to steer clear of any element from atomic number 84, polonium, all the way to element 118, oganesson (which is so new it was only named in 2016).
  2. Toxicity: Some elements are dangerous because of their inherent toxicity. The U.S. Environmental Protection Agency (EPA) defines a toxic chemical as any substance which can be considered harmful to the environment or hazardous to health if inhaled, ingested, or absorbed through the skin.
  3. Reactivity: Some elements present a risk due to extreme reactivity. The most reactive elements and compounds can ignite spontaneously—or even explosively, and generally burn in water as well as in the air.

Ready to meet the baddies? Take a look at this list of "the worst of the worst" to learn how to recognize these elements—and why you need to try your hardest to steer clear of them.

Polonium is One Nasty Element

Polonium isn't much worse than any other radioactive element, until it gets inside your body!
Polonium isn't much worse than any other radioactive element, until it gets inside your body!. Steve Taylor/Getty Images

Polonium is a rare, radioactive metalloid that occurs naturally. Of all the elements on the list, it's the one you're least likely to encounter in person unless you work at a nuclear facility or are a target for assassination. Polonium is used as an atomic heat source, in anti-static brushes for photographic film and industrial manufacturing, and as a nasty poison. Should you happen to see polonium, you might notice something is a bit "off" about it because it excites molecules in the air to produce a blue glow.

The alpha particles emitted by polonium-210 don't have enough energy to penetrate the skin, but the element emits a lot of them. 1 gram of polonium emits as many alpha particles as 5 kilograms of radium. The element is 250-thousand times more toxic than cyanide. So, one gram of Po-210, if ingested or injected, could kill 10 million people. Former spy Alexander Litvinenko was poisoned with a trace of polonium in his tea. It took 23 days for him to die. Polonium is not an element you want to mess around with.

The Curies Discovered Polonium

While most people are aware that Marie and Pierre Curie discovered radium, you might be surprised to learn that the first element the pair discovered was polonium.

Mercury is Deadly and Omnipresent

Mercury metal can be absorbed through your skin, but organic mercury is a much more common threat.
Mercury metal can be absorbed through your skin, but organic mercury is a much more common threat.

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There's a good reason you don't often find mercury in thermometers anymore. While Mercury is located right next to gold on the periodic table, you can eat and wear gold, you'd do best to avoid mercury.

Mercury is a toxic metal that's dense enough that it can be absorbed into your body directly through your unbroken skin. The liquid element has a high vapor pressure, so even if you don't touch it, you absorb it via inhalation.

Your biggest risk from this element isn't from the pure metal—which you can readily recognize on sight—but from organic mercury that works its way up the food chain. Seafood is the best-known source of mercury exposure, but the element is also released into the air from industries, such as paper mills.

What happens when you meet up with mercury? The element damages multiple organ systems, but the neurological effects are the worst. It affects memory, muscle strength, and coordination. Any exposure is too much, plus a large dose can kill you.

Liquid Mercury

Mercury is the only metallic element that is a liquid at room temperature.

Arsenic is a Classic Poison

Arsenic may be the element best-known as a poison.
Arsenic may be the element best-known as a poison.

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People have been poisoning themselves and each other with arsenic since the Middle Ages. In Victorian times, it was a poisoner's obvious choice, however, people were also exposed to it as it was used in paints and wallpaper.

In the modern era, arsenic is not useful for homicide—unless you don't mind getting caught—because it's easy to detect. The element is still used in wood preservatives and certain pesticides, but the greatest risk is from groundwater contamination, most often resulting when wells are drilled into arsenic-rich aquifers. It's estimated that 25 million Americans and as many as 500 million people worldwide drink arsenic-contaminated water. In terms of public health risk, arsenic may well be the worst element of all.

Arsenic disrupts ATP production (the molecule your cells need for energy) and causes cancer. Low doses, which may have a cumulative effect, cause nausea, bleeding, vomiting, and diarrhea. A large dose causes death, however, it's a slow and painful demise that usually takes hours.

Arsenic Has Medicinal Uses

While deadly, arsenic was used to treat syphilis because it was vastly superior to the old treatment, which involved mercury. In the modern era, arsenic compounds show promise in treating leukemia.

Francium is Dangerously Reactive

Francium and other alkali metals react vigorously with water. The pure element would like explode on contact with skin.
Francium and other alkali metals react vigorously with water. The pure element would like explode on contact with skin.

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All of the elements in the alkali metal group are extremely reactive. If you put pure sodium or potassium metal in water the result will be a fire. The reactivity increases as you move down the periodic table, so cesium reacts explosively.

Not much francium has been produced, but if you had enough to hold the element in the palm of your hand, you'd want to wear gloves. The reaction between the metal and the water in your skin would make you a legend in the emergency room. Oh, and by the way, it's radioactive.

Francium is Extremely Scarce

Only about 1 ounce (20-30 grams) of francium can be found in the entire Earth's crust. The amount of the element that has been synthesized by mankind isn't even enough to weigh.

Lead is the Poison We Live With

Lead is used in or contaminates so many products, it's impossible to completely avoid exposure.
Lead is used in or contaminates so many products, it's impossible to completely avoid exposure.

Alchemist-hp

Lead is a metal that preferentially replaces other metals in your body, such as the iron, calcium, and zinc you need to function. In high doses, lead exposure can kill you, but if you're alive and kicking, you're living with at least some of it in your body.

There's no real "safe" level of exposure to the element, which is found in weights, solder, jewelry, plumbing, paint, and as a contaminant in many other products. The element causes nervous system damage in babies and children, resulting in developmental delays, organ damage, and reduced intelligence. Lead doesn't do adults any favors either, affecting blood pressure, cognitive ability, and fertility.

Lead Exposure is Toxic in Any Quantity

Lead is one of the few chemicals known with no safe threshold for exposure. Even minute quantities cause harm. There is no known physiological role played by this element. One interesting fact is that the element is toxic to plants, not just animals.

Plutonium is a Radioactive Heavy Metal

Plutonium may appear as a silver-colored metal, but it may oxidize in air (burn really) so that it appears like a glowing red ember.
Plutonium may appear as a silver-colored metal, but it may oxidize in air (burn really) so that it appears like a glowing red ember. Los Alamos National Laboratory

Lead and mercury are two toxic heavy metals, but they aren't necessarily going to kill you from across the room—although, mercury is so volatile it actually might. You can think of plutonium as the radioactive big brother to the other heavy metals. It's poisonous on its own, plus it floods its surrounds with alpha, beta, and gamma radiation. It's estimated that 500 grams of plutonium if inhaled or ingested, could kill 2 million people.

Like water, plutonium is one of the few substances that actually increases in density when melted from a solid into a liquid. While not nearly as toxic as polonium, plutonium is more abundant, thanks to its use in nuclear reactors and weapons. Like all of its neighbors on the periodic table, if it doesn't kill you outright, you may experience radiation sickness or cancer should you be exposed to it.

When Plutonium Heats Up

One way to recognize plutonium is that it's pyrophoric, which basically means it has a tendency to smolder in air. As a rule, never touch any metal that's glowing red. The color may indicate the metal is hot enough to be incandescent (ouch!) or it could be a sign that you're dealing with plutonium (ouch plus radiation).