By Claire Foy-Smith BBC News Online

Osmium tetroxide is corrosive

It is thought the supposed plan involved detonating an explosive device and the compound.

Osmium tetroxide is used by scientists to stain materials and as a catalyst - something that speeds chemical reactions.

It is also a toxic substance with corrosive effects on eyes and skin, making it dangerous to touch or inhale.

'Volatile substance'

And it would also have the ability to cause panic if released in a closed environment.

But given its high cost and volatile nature, scientists say it makes an unlikely ingredient for an effective "dirty bomb".

Dr Steve Simpson, a senior lecturer in chemistry at the University of Salford works with the substance.

You would be in appreciable danger just opening a bottle Dr Steve Simpson

He told BBC News Online the yellow crystalline solid is a volatile substance - it turns from solid to gas at room temperature.

The resulting vapours corrode skin and eyes and fumes damage the lungs when inhaled. In the laboratory it is only safely handled inside a fume cupboard.

"If you get the vapour in your eyes, even a small amount, it can turn them brown or black and you could be permanently blinded," said Dr Simpson.

"It is so volatile, you would be in appreciable danger just opening a bottle.

'Massive panic'

"If you had 10g and you opened it up in a normal-sized room, within a couple of minutes it would cause people's eyes to stream.

"Some people might feel a bit tight in the chest. It would cause massive panic.

"You would certainly know it was there - it has an acrid smell."

It would be incredibly difficult to put it in a bomb Dr Simpson

Osmium tetroxide is not a fatal substance but, said Dr Simpson "could cause trouble" if swallowed.

However it is so quick to react it would be hard to take - most likely decomposing in the mouth.

Harmful effects aside, scientists believe it is an "extremely unlikely" candidate for use in a dirty bomb.

Given its reactive nature, it is likely to be destroyed in any actual explosion. It does cause elements to burn, but is not an explosive.

Handling the chemical would be difficult for any terrorist.

And it is expensive - one gram costs about �96-�130 from chemical suppliers.

Nerve agents

So expensive that working chemists like Dr Simpson buy it in "recyclable" form where the byproducts of using it can be sold back.

There is also a radioactive form - a byproduct of the nuclear industry - described as "astronomically expensive".

"It is one of the most unlikely things to put in a bomb," said Dr Simpson.

"You wouldn't get a lot of osmium for more than �30,000. You would be better off getting something for your money, some kind of nerve gas.

"You can cause an awful lot of havoc with gas rather than have a massive amount of a very expensive chemical."

"It would be incredibly difficult to put it in a bomb."

"You could buy 10g but it wouldn't be enough to cause damage over a wide area.