A small quantity of the radioactive isotope has been pinched from the back of a van. How worried should we be?

Age: Half-life of 73.83 days

Appearance: Platinum-coloured, but with a slight yellowish cast.

What is it? A radioactive isotope of the element iridium, which emits beta and gamma radiation as it decays.

Is it in the ground? Unlike more stable isotopes iridium-191 and 193, iridium does not occur in nature.

So where would I find something like that? In the back of a van in Bacup.

Are you sure? Yes. In fact, a small amount of iridium-192, enclosed in a lead-lined yellow canister, was stolen from a van parked in Bacup just last weekend.

I'm confused. Because you don't understand how highly radioactive material can be transported so insecurely in this day and age?

No, because I don't know where Bacup is. It's in Lancashire.

Ah. Finally all the pieces are beginning to fall into place. But who would want this stuff? That's unclear. Iridium-192 has industrial applications, and it's also used in medicine as a source of radiation to kill cancer cells. But it's also … now don't panic, will you?

Of course I won't. It's also one of the main lost or stolen commercial radioactive isotopes that could potentially be used in a dirty bomb.

Oh my God! What's a dirty bomb? A non-nuclear device using conventional explosives to spread radioactive material over a wide area.

We're all going to die! Calm down. There is a lot of unaccounted-for iridium-192 in the world – in the US particularly – but this a relatively small amount, and it is probable the thief or thieves don't even know what they have.

Is it dangerous? It should be safe as long as it remains in its lead-shielded casing. In the meantime, police are appealing to the public to keep an eye out for the missing canister.

Do say: "Don't worry. Unlike some radioactive isotopes, iridium-192 loses its potency quite quickly. No respectable dirty-bomb maker would touch it."

Don't say: "Bacup? Just take the A681 east through Rawtenstall, and turn left when you see the bright-green glow."