A tunnel under the River Thames had to be shut after seven pedestrians were taken ill following reports of an "unusual smell".
Emergency services closed off the Greenwich foot tunnel after several pedestrians reported breathing difficulties.
Police, fire, and ambulance crews were called to the scene shortly after 7pm on Friday following a call from a member of the public.
London Ambulance Service sent paramedics and its hazardous area response team.
Firefighters equipped with specialist equipment carried out a sweep of the area for any chemical spills.
The tunnel, which was built in 1902 to connect Greenwich to the docks on the Isle of Dogs, had to be shut for more than an hour and only reopened at 8.53pm.
A spokesman for London Ambulance Service said they had been called after reports of "patients taken unwell".
He said: "We sent an ambulance crew, a cycle responder, a duty manager and our hazardous area response team.
"Our staff checked over seven patients at the scene who complained of breathing difficulties. All seven patients declined further treatment."
A spokesman for London Fire Brigade said: "We carried out a sweep of the area with our detection, identification, and monitoring equipment. Once we established there were no elevated readings we left the scene."