The accident occurred late Sunday night in the basement of the Siam Commercial Bank (SCB), one of the country's largest financial institutions.
SCB said contractors had been working to improve the building's chemical fire extinguishers but mistakenly set off the system, releasing chemicals that likely caused oxygen levels to plummet.
"The work may have triggered (the) Pyrogen aerosol which, once it works, will decrease oxygen, that could cause people's injuries and death," the bank said in a statement.
The company says on its website that its aerosol does not deplete oxygen. But it also advises against using it in occupied rooms and says "accidental exposure should be limited to five minutes".
Bangkok's Erawan emergency medical centre said five people were killed at the scene while three died in hospital.
Seven workers were injured and remained in hospital, the centre added.
One picture published by local media showed firefighters wearing masks and handkerchiefs over their faces extracting victims on stretchers from inside a building thick with haze.
"SCB would like to express its sympathy to those injured and killed," the bank said in its statement, adding it was cooperating with police.
The bank said the rest of the building was not affected and was open for business.
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