The death of the Russian ex-spy Alexander Litvinenko has been linked to the presence of a "major dose" of radioactive polonium-210 in his body.
Traces of the radioactive substance have since been discovered at various locations in London visited by Mr Litvinenko as well as in Russia and on two British Airways (BA) flights.
What is polonium-210?
It is a naturally occurring radioactive material that emits highly hazardous alpha (positively charged) particles.
It was first discovered by Marie Curie at the end of the 19th century.
There are very small amounts of polonium-210 in the soil and in the atmosphere, and everyone has a small amount of it in their body.
But at high doses, it damages tissues and organs.
However the substance, historically called radium F, is very hard for doctors to identify.
Philip Walker, professor of physics, University of Surrey said: "This seems to have been a substance carefully chosen for its ability to be hard to detect in a person who has ingested it."
What is the risk to other people from the dose Mr Litvinenko received?
It cannot pass through the skin, and must be ingested or inhaled into the body to cause damage.
And because the radiation has a very short range, it only harms nearby tissue.
However, there is a theoretical risk that anyone who came into contact with the urine, faeces, and possibly even sweat, of Mr Litvinenko could ingest a small amount of the polonium.
Where does polonium-210 usually occur?
It has industrial uses such as static control and as a heat source for satellite power supplies, but is not available in these areas in a form conducive to easy poisoning.
It is also present in tobacco.
Professor Dudley Goodhead, Medical Research Council Radiation and Genome Stability Unit, said: "To poison someone much larger amounts are required and this would have to be man-made, perhaps from particle accelerator or a nuclear reactor."
Where would someone obtain polonium-210 from?
Although it occurs naturally in the environment, acquiring enough of it to kill would require individuals with expertise and connections.
It would also need sophisticated lab facilities - and access to a nuclear reactor.
Alternatively, it could have been obtained from a commercial supplier.
Polonium-210 can either be extracted from rocks containing radioactive uranium or separated chemically from the substance radium-226.
Production of polonium from radium-226 would need sophisticated lab facilities because the latter substance produces dangerous levels of penetrating radiation.
Is the public in danger from the traces of polonium-210 found in London, and on the BA flights?
The Health Protection Agency (HPA) said monitoring in aircraft, hotels, offices and other venues had shown nothing of public health concern.
It said polonium-210 can only represent a radiation hazard if it is taken into the
body, by breathing it in, taking it into the mouth or if it gets into a wound.
After seven workers at the Millennium Hotel's Pine Bar, in London, tested positive for low levels of radioactive polonium-210, the HPA said the risk to the general public was likely to be "very low".
Nevertheless, more than 200 people known to have visited the Pine Bar on the day the bar staff were working - 1 November - are to be offered tests for radiation.
As far as the bar staff were concerned there was "no health risk in the short term and in the long term the risk is judged to be very small on the basis of initial tests", the HPA said.