Poisonous gas detectors dating back to the second world war was found at a house in Lyndhurst Gardens in Pinner.
The residential road was cordoned off for more than two hours while firefighters made the area safe.
Initial reports suggested the vials, which were discovered during a cleaning spree at the home yesterday at 2pm, contained deadly mustard gas.
However they were later revealed to be harmless detectors, used to indicate the presence of the deadly toxin.
Watch manager Adam Barnett said: "They were tiny vials of about 7cm each and were more than 60 years old.
"This doesn't happen very often, but if anyone finds anything like this they should contact the police and the fire brigade."
More than 16 firefighters and members of a scientific advisory team visited the house. The gas containers were removed by the fire brigade to be disposed of.