An intense updraft of smoke and hot fumes turned the staircase of a walk-up building into a "death trap" for residents trying to escape the deadly 2011 Mong Kok fire, an inquest heard yesterday.
The staircase was like "the flue of a chimney" between its ground floor entrance and rooftop exit, senior government chemist Bobbie Cheung Kwok-keung told the Coroner's Court.
Residents had little chance of escaping down the staircase if they could not get out within the first 20 minutes of the fire, Cheung said. "The hot fumes and dense smoke generated from the shops on the ground and mezzanine floors … vented out on the rooftop," he said.
"[The staircase] sucked the dense smoke produced from the fierce fire in the hawkers' stalls through its main entrance."
The fumes, dense with carbon monoxide and other toxic substances, killed nine people, eight of whom were found on the main staircase. Another 34 were injured.
The inferno, a No 4 fire on a scale of five, broke out at stall No 268 in the market street at 4.40am on November 30, 2011. It reached No 266, which was packed with flammable plastic goods, where it flared up and spread into the adjacent 192-194 Fa Yuen Street.
Cheung said the fire was "highly unlikely to be the result of a deliberate act, [but] the possibility that it was caused by an electrical fault could not be eliminated".
Metal electricity boxes were found in the debris of stall No268 after the fire. An electrician moved them from outside to inside the stall some five years ago, but the inquest did not hear if that played a part in the incident.
The cause of the blaze will be further explained by experts from the Electrical and Mechanical Services Department next week.
The flames took only about 10 minutes to spread from the stalls to the residential building, Cheung estimated. It moved rapidly over plastic and canvas awnings connecting the stalls to the apartment building.
Cheung said the heat of the fire built up under the stall's galvanised iron roof before bursting out into other nearby stalls.
"The fire extended out from the front and the back of the roof, igniting the structures hanging out from the building - in particular the canvas and plastic awnings connecting the stalls to the building," he said.
The inquest heard that stall No266 - directly in front of 192-194 Fa Yuen Street - had a large stock of highly flammable plastic shoe soles and handbags, making the fire abruptly more powerful.
Giving a detailed explanation of the fire's progress, Cheung said the hot fumes accumulated under the overhanging mezzanine-floor porches at numbers 192-194 and 196-198. The mezzanine windows were larger at 192-194, and when the heat shattered the glass, the fire moved more easily into that building.
"The breaking of the windows [from the heat] allowed the blaze to enter the mezzanine floor without check," Cheung said.
Large quantities of fabric stored in the shops there quickly caught fire, and spread to other shops on the ground floor through their own internal staircase. The inquest continues on Monday.