As part of its research, the DSTL produces small quantities of chemical and biological agents, which are securely stored and disposed of safely when no longer required.
The Porton Down site also disposes of old chemical weapons when found in the UK.
Dr Michelle Carlin, a toxicologist at Northumbria University, told Forces News after the second nerve agent attack in Wiltshire that the incident was "far more serious" than routine cases, which is why Porton Down was used.
"Levels of personal protective equipment is going to be far greater [at Porton Down] than we would have in a university lab, or an analytical testing lab, or a pharmaceutical lab, because, obviously, the compounds could be far more dangerous," Dr Carlin said.
Following the Salisbury incident, it is likely a process called gas or liquid chromatography was used to test for the nerve agent Novichok, which was used in the attack.
Dr Carlin said: "It would separate out the components and we would use mass spectrometry in order to identify particular irons that were specific to those compounds.
"So we separate and we identify."
After the second nerve agent incident in Wiltshire in 2018, Dr Michelle Carlin told Forces News why Porton Down is used in those types of cases: