Sergei Skripal and his daughter Yulia are thought to have been targeted with a small amount of the deadly substance in a liquid form.
The highest concentration was found on Mr Skripal’s front door, but nine locations are being decontaminated across the city as experts believe it could still be present.
The decontamination process will cost millions of pounds and take months to complete.
When asked whether the chemical remains at the same lethal level as the day it was put on Mr Skripal’s door, Department for Environment, Food and Rural Affairs (Defra) chief scientific adviser said there could be levels in very specific locations which could still be toxic.
Ian Boyd said We have to make an assumption that in certain circumstances there will be relatively high concentrations, probably in very, very specific locations, which could be at levels that could be toxic to individuals.
That’s an assumption, it’s also one we’ve tested in some circumstances and we do know that there are hot spots like that around, so we have to make those assumptions that some of the hot spots we’ve still got to find.
But those hot spots will still be in the locations we are talking about.
In these locations, there may well be higher concentrations that we still have to find, but we already know there are some high concentrations within those locations.
He made the comments at a meeting addressing the clean-up process, which involves replaces police cordons around the Maltings area of the city centre, the Zizzi restaurant and the Mill pub with more robust hoardings.
Salisbury’s police station and the council buildings are among those which will be closed for eight weeks from Friday.
Russia has been spying on Skripals for five years, security adviser says
Decontamination work will focus on the evidence room and two lockers inside the station, which were sealed off after the 4 March attack.
Deputy Chief Constable Paul Mills of Wiltshire Police said it would be business as usual as operations move to other sites around the city.
Two ambulance stations, a car compound and poisoned police officer Nick Bailey’s home will also be cleaned.
As Mr Skripal’s home is still part of the police investigation, his is the last on the list to be cleaned.
It comes after Russian investigators claimed they had retraced Ms Skripal’s steps to the airport to visit her father and found no trace of the military grade agent.
They released CCTV and said they had identified everyone on her flight, and none of the other passengers displayed symptoms of poisoning.
Earlier this week, it was revealed that the nerve agent was delivered in liquid form, and although there are nine areas to be cleaned, the headstone of Mr Skripal’s son was not targeted.
A police cordon at the London Road cemetery was lifted on Tuesday – the first area to be reopened to the public after extensive testing determined it was not contaminated with the poison.
Yulia Skripal abducted by Britain to blame us, says Russia
The 66-year-old former Russian double agent and his 33-year-old daughter were attacked with novichok and found slumped on a bench at The Maltings shopping centre on 4 March.
In an analysis of samples taken from the home of the Skripals, the highest concentrations of toxic chemical novichok, thought to be responsible for their poisoning, was found on door handles.
Russia has continued to deny any involvement in the poisoning and accused Britain of abducting Ms Skripal, who has been discharged from hospital and taken to a secret secure location.
Mr Skripal, who is no longer in a critical condition, is said to be making good progress and is also expected to be discharged in due course.
News Source SkyNews