THE death of former Russian spy Alexander Litvinenko from polonium poisoning has left a trail of radioactivity stretching from London to Moscow.

Traces of the element have been found at 10 UK addresses, some aircraft that flew between the two cities have been grounded, while an Italian who dined with Litvinenko is being treated for radiation poisoning. Nearly 3000 people have called NHS direct, the UK's health service helpline, worried that they might have ingested polonium. The investigation now involves the security services in UK and Russia and even the US Federal Bureau of Investigation.

The panic and the disruption triggered by a dash of polonium, probably less than a microgram, has led to fears that polonium-210 may become the latest weapon of choice for assassins and terrorists.

However, a careful look at this highly radioactive element reveals it to be less a weapon of mass destruction than one of mass distraction.

Firstly, polonium-210 is not easy to get hold of. Its only routine use these days is in commercial ionisation devices for removing static. And the polonium in these devices is sealed in ceramic microbeads, which are unlikely to linger in the body if ingested. Also an attempt to buy thousands of these devices would surely attract the attention of government security agencies.

"I think if you wanted to really go about this, you'd use a nuclear reactor to produce fresh polonium-210," says Steve Fetter of the University of Maryland at College Park, who has researched the threat from radiological terrorism for the US government.

Benn Tannenbaum, a nuclear physicist at the American Association for the Advancement of Science, agrees. "It's not the thing that a random terrorist organisation [will] be able to do," he says. "It would have to be state sponsored."

Undoubtedly, Po-210 is extremely lethal, as Litvinenko's death chillingly illustrates. If it is inhaled or ingested, the damage caused by its alpha radiation is severe (see "The Nuke Within"). And because Po-210 has a half-life of only 138 days it delivers its radioactivity very quickly. Also, Po-210 is easy to transport: a thin film of liquid, cloth, or merely the layer of dead cells on the surface of a person's skin can form an effective shield from the alpha radiation it emits, because alpha particles interact strongly with matter. This makes it relatively easy to carry dangerous quantities of Po-210 through airports or across borders without being detected - a single gram of polonium contains more than a million deadly doses.

Even so, Po-210 is a not an ideal choice of material for inflicting mass death. Other radioactive elements are easier to get hold of for making dirty bombs. For example, americium-241, also an alpha emitter with a half-life of 432 years, is widely used in smoke detectors and oil-well probes. Explode just 3 grams with half a kilo of TNT and about 1 per cent of the people within a block or two of the blast would eventually die of cancer, and contaminated buildings would have to be demolished within a fallout zone 60 blocks long. So claimed Henry Kelly, president of the Federation of American Scientists, in 2002 when he discussed "dirty bomb" scenarios at a US Senate hearing.

While Po-210 is much more radioactive, its shorter half-life means that any contamination, although significant, would be limited. Getting a large number of people to inhale Po-210 via the plume from a dirty bomb would not be easy. "The cross section of a person's nose versus the sidewalk or their skin or a building, is just so small," says Tannenbaum. "If it lands on the sidewalk, it's not a problem unless you're going to go lick the sidewalk."

Fetter notes that he has never seen a formal assessment of a Po-210 risk, but like others familiar with these issues he doesn't consider radiological weapons to be killers on the scale of 9/11. "It is relatively difficult to kill people with radioactivity," he says. "Biological terrorism is potentially far worse."

"It is relatively easy to carry dangerous quantities of Po-210 across borders. A single gram contains a million deadly doses"

From issue 2581 of New Scientist magazine, 09 December 2006, page 8-9
The nuke within
"My son died, and he was killed by a little nuclear bomb," said Walter Litvinenko on 25 November, mourning his son's death two days earlier. His analysis is a chillingly accurate summary of what happens when you swallow a microgram or less of polonium-210.

"It's a trauma to the whole body," says Al Keane, a biophysicist at the Argonne National Laboratory in Chicago, who specialises in the effects of radioactive isotopes in the body. "Polonium-210 gets distributed so widely that it's like a whole-body dose of radiation, and all tissue gets exposed," he says.

When Po-210 gets into the blood following inhalation or ingestion, the spleen, kidney and liver each soak up 10 per cent of the intake, and the remaining 70 per cent spreads throughout the body. Po-210's highly energetic alpha particles wreck any cells, DNA and tissue in their path. Cells that are not killed outright are so damaged they can no longer function at all.

"Above a certain threshold, it damages cells so much that you get multiple organ failure," says Derek Hill, a radiological sciences specialist at University College London.

No one yet knows how much polonium it took to kill Litvinenko within three weeks of exposure. Just a tenth of a microgram - less than a pinhead - is reckoned to be enough to kill someone. Keane estimates that the poisoner gave Litvinenko at least a microgram, but probably less than a milligram - between 10 and 10,000 times the lethal dose.

Keane says that it should be possible to work out the exact dose by scanning Litvinenko's body with a gamma ray detector. Although Po-210 primarily produces alpha particles, it also emits one gamma particle for every 100,000 alpha particles. So although alpha radiation can't be detected outside the body because it can't penetrate the skin, gamma rays can be detected. The gamma counts would allow radiologists to work backwards to calculate how much Po-210 was in Litvinenko's body to start with.

Concerns were mounting this week that many others had been exposed to potentially dangerous amounts of polonium. The saving grace is that Po-210 is only dangerous if ingested. So although traces of radiation have been found at several locations visited by Litvinenko, and on several airliners, people are unlikely to have taken in harmful amounts.

"If you touched it and licked your fingers, you might get a small amount, but otherwise, there can't be any harm through touching it," says Keane.

Philip Walker, a physicist at the University of Surrey in Guildford, UK, thinks it is likely that the traces of polonium all came from the same source, probably spilt by the person handling the solution in which Po-210 had been dissolved.

Polonium facts
DISCOVERY

Polonium was the first element to be discovered by Marie and Pierre Curie in 1898.

About 25 isotopes of polonium are known. Only Po-208, Po-209 and Po-210 have significant half-lives and decay by emitting alpha particles. One milligram of Po-210 emits as many alpha particles as five grams of radium-226.

NATURE

Polonium-210 is the most abundant isotope, and is formed during the decay of uranium-238.

Uranium ores contain only about 1 milligram of Po-210 per tonne of ore.

INDUSTRY

Po-210 can be made in a nuclear reactor by bombarding bismuth-209 with neutrons. It is used in devices designed to eliminate static electricity. Alpha particles from the polonium ionise the air, and the ions neutralise any static electricity on surfaces in contact with the air.

ENVIRONMENT

Po-210 is present in soil in concentrations of about one trillionth of a curie per gram. Because Po-210 is produced by the decay of radon-222 gas, Po-210 can be found in the air. It can settle on tobacco plants, so smokers have higher levels of Po-210 in their lungs compared to non-smokers.

Source: Argonne National Laboratory