Toxic “hotspots” of the nerve agent used to poison a former spy and his daughter could still be present around Salisbury, a government scientist says.
Ian Boyd was addressing a public meeting in the city, during which people expressed frustration at the continued closure of several sites.
A small amount of Novichok in liquid form is believed to have been used to target Sergei and Yulia Skripal.
Decontamination work, expected to take months, has begun at nine locations.
More robust hoardings have begun to replace police cordons around the Maltings area in the city centre, the Zizzi restaurant and the Mill pub.
Former Russian double agent Sergei Skripal and his daughter Yulia were found unconscious on a bench in the Wiltshire city on 4 March.
Yulia, 33, left hospital earlier this month. Her 66-year-old father is said to be recovering more slowly but is expected to eventually be discharged.
The British government says a military-grade Novichok nerve agent of a type developed by Russia was used in the attack although Moscow denies any involvement.
Officials from the Department for Environment, Food and Rural Affairs (Defra), as well as council, police and health representatives, met to update residents of the Wiltshire city on Thursday evening.
Asked whether the chemical remains at a “lethal” level, Defra’s chief scientific adviser Mr 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.”
He said the nerve agent was still present at “hotspots” around the city.
“We already know there are some high concentrations within those locations,” he said.
Residents were told the Bourne Hill building, home to Salisbury’s police station and Wiltshire council’s offices, will close for up to eight weeks from Friday.
The evidence room and two lockers inside the station, sealed off after the 4 March attack, will be decontaminated.
Deputy Chief Constable Paul Mills said it would be “business as usual” because they have moved to other sites in the city.
Other areas that will be decontaminated include two ambulance stations, a car compound and the home of Det Sgt Nick Bailey, who was left seriously ill after responding to the attack.
Mr Skripal’s home – where the highest concentration of military grade Novichok was found – will be the last site to be decontaminated.
The nerve agent does not evaporate or disappear over time, experts have said, and intense cleaning with caustic chemicals is required to get rid of it.
Almost 200 military personnel will help with the decontamination work, which is expected to cost millions of pounds.
What are Novichok agents?
The name Novichok means “newcomer” in Russian, and applies to a group of nerve agents developed by the Soviet Union in the 1970s and 1980s.
Novichok’s existence was revealed by chemist Dr Vil Mirzayanov in the 1990s, via Russian media. He says the nerve agents were designed to escape detection by international inspectors.
Novichok agents are liquids, although others are thought to exist in solid form and could be dispersed as an ultra-fine powder.
Some of the agents are also said to be “binary weapons”, meaning the nerve agent is typically stored as two less toxic chemical ingredients that are easier to handle.
When these are mixed, they react to produce the active toxic agent which can cause convulsions, shortness of breath, profuse sweating and nausea.