The investigation is ongoing, but London fire officials have told AM980 a blaze at an apartment on Adelaide Street near Central Avenue was likely the result of careless smoking.
Emergency crews were called to the scene at around 10:40 Thursday morning.
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The roadway was blocked from Central Avenue to Princess Avenue while crews battled the blaze.
Fire officials say a male tenant was taken to hospital for treatment of smoke inhalation.
Damage is estimated at $20,000.