Police, who said they are investigating the circumstances after an incident in Salisbury on Sunday, have reassured people there is no risk to the wider public.
A major incident has been declared at Salisbury District Hospital but patients are advised to attend appointments as normal unless advised otherwise.
A man and woman are being treated at the hospital after police were called to The Maltings on Sunday shortly after 4pm.
Wiltshire Police said: “At this stage it is not yet clear if a crime has been committed and a multi-agency response has been co-ordinated.
“Police are carrying out a full investigation and working with partner agencies, to clarify the exact circumstances.
“At this stage, Wiltshire Police does not believe there is any risk to the wider public.”
The force said a number of areas in Salisbury had been cordoned off in relation to their investigation.
A spokesman for Public Health England (PHE) said anyone exposed to the unknown substance had been decontaminated “as is standard practice in situations like this”.
He added: “Scientists from PHE’s Centre for Radiation, Chemical and Environmental Hazards, will continue to assist the response and review information as it becomes available.”