Experts investigating the death of Russian ex-spy Alexander Litvinenko say there is no radiation risk to those who had contact with him.
The Health Protection Agency says it is testing staff and monitoring radiation levels at the two London hospitals, Barnet and University College, where Mr Litvinenko was treated.
The radioactive substance responsible, polonium-210, is naturally found at very low traces in some foods.
It is poisonous only at higher doses when eaten or inhaled.
HPA experts stressed that polonium-210 cannot pass through the skin so should not be caught by direct physical contact.
Professor Roger Cox of the HPA said: "Having contact with Mr Litvinenko physically does not pose any risk.
"These short alpha particles cannot penetrate the skin, they cannot provide a dose from the outside.
"However, if that polonium-210 is ingested into the body by eating, inhalation or perhaps by a wound, then it will rapidly track through the body and go to most organs."
Alpha particles are stopped by a sheet of paper and cannot pass through unbroken skin
Beta particles are stopped by an aluminium sheet
Gamma rays are stopped by thick lead
If the dose was high enough, as in Mr Litvinenko's case, it would kill.
Professor Cox said the high levels of polonium-210 found in Mr Litvinenko's urine suggested the poison was man-made.
He said there was a theoretical risk that anyone who had come into contact with the urine, faeces, and possibly even sweat, of Mr Litvinenko could ingest a small amount of the polonium.
As well monitoring hospital staff, the HPA will also be contacting those who may have had very close contact with him when he was ill, including his family.
The Metropolitan Police are said to be looking for any residual material at a number of locations where Mr Litvinenko has been, including house in north London.
Polonium-210 levels have been found in the sushi restaurant in West London where Mr Litvinenko had dined just before he fell ill.
The HPA said it would be assessing what risk if any this might pose to any of the other diners, stressing that it was likely to be insignificant.
In a statement dictated before he died, the 43-year-old accused Russian President Vladimir Putin of involvement in his death.
The Kremlin has denied Mr Litvinenko's claims that it was involved.
The Home Office said anybody concerned should contact NHS Direct on 0845 4647, whose staff have been briefed about this issue.