The strange death of Alexander Litvinenko has never lacked the necessary elements to keep the media entranced. The revelation that the former Russian spy was poisoned with polonium-210, however, took the story to another level. The radioactive isotope is thought never to have been used before as a murder weapon, and the difficulty of obtaining it added fresh intrigue to conspiracy theories.

Its discovery also opened an entirely new angle: there was now an opportunity to talk up a public health scare. The Health Protection Agency (HPA) was appropriately cautious when announcing that it was tracing Litvinenko’s contacts to assess their risk, insisting that it was low. But that was not always reflected in the headlines. The idea that an assassin had been roaming London carrying radioactive poison was too seductive to resist.

“How many more were poisoned?” asked the Daily Mail. “An estimated 21,000 of my fellow Londoners have had their lives put in danger by this deadly poison,” answered Tony Parsons, in the Daily Mirror. The scare progressed this week as 49 hospital workers were sent for tests and traces of radiation were found on four British Airways jets. Litvinenko, it was suggested, might be only the first victim.

The HPA was right to investigate. Given the toxicity of polonium when swallowed or inhaled, a precautionary approach was justified. The media line that thousands were at serious risk, however, was a gross exaggeration. The hazards were never likely to be great, and it takes only a loose familiarity with GCSE-level physics to understand why.

Polonium-210 is highly radioactive, but all radiation is not alike. The isotope emits alpha particles, one of the three main kinds of ionising radiation produced by radioactive decay. These are dangerous inside the human body, carrying enough energy to destroy cells, but they do not get inside easily. As school textbooks say, alpha particles are stopped by paper or the dead outer layer of the skin, and travel only a few centimetres in air.

Left in the environment, an alpha source such as polonium is pretty harmless. It must be ingested, inhaled or enter an open wound to become dangerous. Even for Litvinenko’s family, with whom he would have had intimate contact, the risk was small. For ordinary readers of alarmist headlines, it was virtually non-existent.

That so many people worried about so slim a threat highlights the way radiation hazards are so commonly misunderstood. Like most risks to health, the dose and circumstances of exposure are all-important, but radiation is seen as a uniquely terrifying killer. Its reputation stems at once from ignorance and familiarity. We cannot see or feel radiation, it is hard to control our exposure and few of us understand how it works. Yet we have all seen graphic images of what it can do:

Hiroshima, deformed children, lingering deaths from cancer, and now Litvinenko. The very word invokes fear.

The price of this was spelt out in a recent public lecture by Wade Allison, an Oxford University physicist. The fear factor means that radiation safety limits are set too low, at great cost to business. It makes the public unreasonably suspicious of nuclear power, despite its potential role in tackling the much greater risk of global warming. It may even play into the hands of terrorists: the chief effect of a “dirty” radiological bomb would be to spread panic, rather than to kill.

The polonium scare raises another danger. Though the element has a half-life of just 138 days, and the small traces found are almost harmless, the Itsu sushi bar and the Millennium Hotel will struggle to lift the stain of “radioactive contamination”.

Radiation can obviously be harmful, but we have to learn to be realistic about its effects. Scare stories based on misunderstandings can be just as damaging.

Mark Henderson is Science Editor of The Times