Four fire engines and around 25 firefighters were called to a fire at a takeaway shop with flats above on Hale Lane in Edgware.
The alarm was raised by staff who heard a noise and saw smoke issuing from the fryers and the extraction system.
The ventilation system from the ground floor to the roof was damaged by fire and part of the ground floor takeaway was also damaged by fire. Three people left the building before the Brigade arrived. One man was treated on scene by London Ambulance Service crews suffering from smoke inhalation.
The Brigade's fire investigators believe the fire was caused by cooking oil overheating and igniting. It then spread via heavy grease within the extraction system.
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The Brigade was called at 1734 and the fire was under control by 1829. Fire crews from Mill Hill, Stanmore and Hendon fire stations attended the scene.