LONDON -- Polonium first hit the headlines when it was used to kill KGB agent-turned-Kremlin critic Alexander Litvinenko in London in 2006.

This week, Yasser Arafat's widow has called for the late Palestinian leader's body to be exhumed after scientists in Switzerland found elevated traces of radioactive polonium-210 on clothing he allegedly wore before his death in 2004.

What is polonium and how dangerous can it be?

WHAT IS POLONIUM?

Polonium-210 is one of the world's rarest elements, discovered in 1898 by scientists Marie and Pierre Curie and named in honor of her country of origin, Poland. It occurs naturally in very low concentrations in the Earth's crust and also is produced artificially in nuclear reactors. In small amounts, it has legitimate industrial uses, mainly in devices to eliminate static electricity.

IS IT DANGEROUS?

Very. If ingested, it is lethal in extremely small doses. Less than 1 gram (0.04 ounces) of the silver powder is sufficient to kill. A 2007 study by radiation experts from Britain's Health Protection Agency concluded that once polonium-210 is deposited in the bloodstream, its potent effects are nearly impossible to stop. A poisoning victim would experience multiple organ failure as alpha radiation particles bombard the liver, kidneys and bone marrow from within. The symptoms shown by Litvinenko – nausea, hair loss, throat swelling and pallor – are also typical.

WHO CAN GET THEIR HANDS ON IT?

The good news – not too many people. The element can be a byproduct of the chemical processing of uranium, but usually is made artificially in a nuclear reactor or a particle accelerator. These nuclear facilities are monitored and tightly regulated under international agreements.

John Croft, a retired British radiation expert who worked on the Litvinenko case, said a dose large enough to kill would likely have to come from a government with either civilian or military nuclear capabilities. That category includes Russia – producer of the polonium believed to have killed Litvinenko – and Arafat's foe, Israel. But it also includes dozens of other nations, including the United States.

WHY WOULD IT BE ATTRACTIVE TO ASSASSINS?

Polonium makes a good weapon. Its large alpha particles of radiation do not penetrate the skin and don't set off radiation detectors, so it is relatively easy to smuggle across international borders. Polonium can be ingested through a wound or inhaled – but the surest method would be to have the victim consume it in food or drink. Litvinenko drank tea laced with polonium during a meeting at a luxury London hotel.

WHO HAS IT KILLED?

Polonium poisoning is so rare that it took doctors several weeks to diagnose Litvinenko's illness and security experts struggled to think of a previous case. More than five years after Litvinenko's death, no one has been arrested. British prosecutors have named ex-KGB agent Andrei Lugovoi as their chief suspect, but Russia refuses to hand him over.

Some speculate that the Curies' daughter Irene, who died of leukemia, may have developed the disease after accidentally being exposed to polonium in the laboratory.

Israeli author Michal Karpin has claimed that the cancer deaths of several Israeli scientists were the result of a leak at the Weizmann Institute of Science in 1957. Israeli officials have never acknowledged a connection.

CAN SCIENTISTS PROVE WHETHER ARAFAT WAS POISONED WITH POLONIUM?

Scientists caution that traces on Arafat's clothing aren't sufficient proof of poisoning. Exhuming his body would a surer method. Derek Hill, a radiological science expert at University College London, said eight years after Arafat's death in 2004, any polonium would have decayed and would be far less radioactive than it was at the time. But he says it would still be much higher than normal background levels and with an autopsy it should be possible to tell "with a pretty high confidence" whether Arafat had polonium in his body when he died.

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PHOTOS: THE WEEK IN SCIENCE HISTORY, JULY 2-8
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    After a "flying disk" crashed on a ranch in the New Mexico town of Roswell, rumors began to circle about possible extraterrestrial origins and a government cover-up. The Roswell Army Air Field (RAAF) public information officer in Roswell, New Mexico, issued a press release on July 8, 1947 about the event.

  • A Man And His Motorwagon

    German engine designer Karl Benz (1844-1929) unveiled the first automobile on July 3, 1886 in Mannheim, Germany. The Benz Patent Motorwagon could reach a then-astonishing speed of 10 mph.

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    Sir Isaac Newton (1642-1727) published <em>Philosophiae Naturalis Principia Mathematica</em> on July 5, 1687. Translated as "Mathematical Principles of Natural Philosophy," this work became one of the most important texts in the history of science, in which Newton explained his laws of motion and universal gravitation.

  • Hot Air

    Richard Branson and Per Lindstrand made aviation history on July 3, 1987 when they crossed the Atlantic Ocean in a hot air balloon. Their transatlantic flight took about 37 hours from Maine to Ireland--a distance of 3,075 miles.

  • Cable Across The Pacific

    On July 3, 1903, the first telegraph cable across the Pacific was completed. The Commercial Pacific Cable Company connected Hawaii, Midway, Guam and Manila. President Theodore Roosevelt was the first to send a message across the cable, beginning a new era in international communication. He wished "a happy Independence Day to the U.S., its territories and properties . . ."

  • Boom!

    On July 3, 1969, the Soviet N1 rocket exploded after stalling for 23 seconds on the launch pad, creating the biggest man-made non-nuclear explosion in history. The destruction was photographed by U.S. satellites.

  • Mars Arrival

    NASA's Mars <em>Pathfinder</em> spacecraft landed on July 4, 1997 after a seven-month journey to the Red Planet. It dispatched the first remote-control interplanetary rover, which brought back photographs and data about the composition of Mars' surface. The lander was renamed the Carl Sagan Memorial Station.

  • Comet Collision

    A NASA space probe collided with a comet on July 4, 2005. The probe, ejected by a larger spacecraft known as Deep Impact, was sent intentionally to crash into the comet 9P/Tempel. This allowed scientists to study the composition and surface of the comet close-up.

  • Happy Birthday, Dolly!

    Dolly, the first cloned sheep, was born on July 5, 1996 at the Roslin Institute in Scotland. Dolly sparked a heated debate about cloning ethics, and led the way for larger animals like horses to be cloned. She was euthanized in 2003 after developing lung cancer. Her taxidermied remains are now on display at the National Museum of Scotland.

  • Rabies Vaccine

    On July 6, 1885, French chemist and microbiologist Louis Pasteur tested his rabies vaccine on Joseph Meister, a young man who was in danger of contracting rabies after being bitten by a rabid dog. Meister recovered, and became the first of many to receive the groundbreaking vaccination.

  • Mysterious Disappearance

    On July 2, 1937, Amelia Earhart, the first woman to fly solo across the Atlantic Ocean, disappeared while flying over the Pacific Ocean along with her navigator Frederick Noonan. Their attempted trip around the world was tragically cut short somewhere en route to its destination at Howland Island. Searches never turned up a conclusive trace of the two aviators.