Police have described the “extraordinary” work of scientists and officers working on sites in Amesbury and Salisbury as part of the nerve agent investigation.
Metropolitan Police Assistant Commissioner Neil Basu said high temperatures meant experts could only work in their suits for 15 minutes at a time.
Mr Basu, Britain’s most senior counter-terrorism police officer, said they had been able to work for six hours in March following the poisoning of Sergei and Yulia Skripal.
Speaking at a public meeting on Tuesday night, he stressed that cordoning off a site did not mean it had been contaminated with Novichok.
He said a bus in which Dawn Sturgess and Charlie Rowley travelled was seized, tested and was clear, so had been released.
“We will be doing the same thing as quickly as we possibly can,” Mr Basu told members of the public at the meeting.
“It’s probably difficult for you to understand why this is all taking so much time.
“I haven’t talked about this before but I have come straight from the forensic management team to this.
“I can explain to you why it takes a long time. There are multiple scenes, there are only limited numbers of scientists and forensic medical officers – all of whom are volunteers by the way – to do what is a really, really dangerous piece of work for obvious reasons.
“It takes them about 40 minutes to get changed into the suits. Before they get changed the temperature inside the forensic tents two days ago was 40 degrees – before they put the suits on.
“When they put the suits on, they walk into a scene where you can’t see or detect the agent that you’re looking for.”
Mr Basu said the scientists and officers worked in teams of six to eight people, who were looking after each other within the scene.
“They have about 15 minutes to be in there before they can’t operate, they physically can’t operate any more,” he said.
“Enough time to do maybe one, two, three swabs, which then need to be taken out and sent to a defence laboratory for testing.
“When they come out, it takes them about 40 minutes to de-robe.
“Before they go in, their bloods are taken and when they come out their bloods are taken to check that there’s not been anything.
“That’s how long it takes.”
Mr Basu said he was “astonished” by the bravery and physical resilience of those working in such conditions.
“When they take the suits off, they literally pour water out of their respirators and the suits because it is so hot,” he said.
“We had dehydration problems in March, when it was cold, you can imagine how they’re feeling now and they were spending about six hours in the suits in March – that’s down to 15 minutes.
“That is extraordinary and I just wanted you to know that as the public.”