Two volunteers have been transferred to the UK after potential contact with the Ebola virus in separate incidents.
Public Health England said the patients' risk of having the virus was low and the measure was a precaution.
Authorities said the individuals - one of whom is Australian - had not been diagnosed with Ebola and did not currently have symptoms of the disease.
Experts emphasised the risk to the public was low and said the volunteers would be monitored for 21 days.
Australian authorities confirmed an Australian nurse was flown to the UK after a low risk clinical incident while working in Sierra Leone.
The Australian department of Foreign Affairs and Trade said in a statement: "The individual, who for privacy reasons has not been named, has not been diagnosed with Ebola, and the transfer to the UK for a 21-day observation period is a precautionary step.
"The Australian-funded Ebola treatment centre has strict infection prevention protocols in place, and the safety of staff and patients is paramount."
Public Health England (PHE) said one volunteer arrived on an RAF flight after damage to personal protective equipment.
They have left hospital and will now be monitored in private accommodation for 21 days.
The second volunteer had potential exposure through contact with a healthcare worker who recently died and was subsequently diagnosed with Ebola.
The individual will undergo full checks in hospital.
Dr Jenny Harries, of PHE, said: "The risk to the public posed by these and indeed any of the returning workers is extremely low.
"We are confident that all appropriate public health actions have been taken, and will continue to be taken, to support these individuals and to protect the public's health.
"It is important to remember, in choosing to volunteer, that these individuals have taken a courageous step not only to help those affected in West Africa, but also prevent the spread of Ebola any wider."