Officers and hazard response teams were called to Newham Hospital in Glen Road, Plaistow around 7.58pm yesterday (February 27).
The hospital was evacuated and a cordon was erected while specialist officers checked for chemical substances.
No higher readings of chemical substances were found and the incident was over around 10pm.
Four people were treated at the hospital according to the fire service, but nobody was harmed.
Two fire engines and two fire rescue units attended and ensured the area was safe before leaving.
A spokesperson from Newham Hospital Trust said: “We evacuated patients, families and staff from the paediatric A&E department to protect their safety following reports of an unusual smell.
“We worked with emergency services to investigate the area, and by 10pm, the area was declared safe. No one was harmed during the incident.”
A London Fire Brigade spokesperson said: “We were called at 1958 yesterday (27 February) to a report of an unknown chemical smell at a hospital on Glen Road in Plaistow.
“The area was evacuated and a cordon was implemented as a precaution.
“Specialist officers carried out a monitoring sweep and found no elevated readings of chemical substances.
“Crews ensured the area was safe before leaving the scene. Four people were treated by staff at the hospital.
“Two fire engines and two fire rescue units attended the scene.”
A London Ambulance Service spokesperson said: “We sent resources to the scene, including an incident response officer, a paramedic from our tactical response unit and members of our hazardous area response team (HART).
“We responded alongside colleagues from London Fire Brigade but we were later stood down.”
