The incident took place at the ground floor of a four-storied building, House 34/A on Road-4, Wednesday morning.
The injured have been rushed to the burn and plastic surgery unit of Dhaka Medical College Hospital (DMCH).
Injured Abdur Razzak, 50, Razzak, 23, security guard Abed Ali, 50, Saddam Hossain, 28, Masum Ali, 24, Shirin Akter, 27, have sustained 20% to 50% burn injuries, said Resident Surgeon Partha Shankar Paul.
Workers were cleaning the septic tank of the house in the morning. The tank exploded immediately after security guard Abed Ali lit a cigarette near the tank, leaving six injured, said a rescuer.
However, another rescuer Dalim, a tenant of the house said, the workers carried a lit bulb with them inside the tank. The bulb exploded at once, leading to a huge explosion that rocked the building.
The actual reason of the explosion could not be known as the injured are in critical condition.