Three Myanmar workers died after inhaling a toxic gas while cleaning out a 3 metre-deep wastewater pond at a factory in Chiang Rai's Chiang Saen district.
Sanga Phrommueang, owner of the Wandee Phuetphol rice mill, told police on Wednesday the pond held wastewater released from a process to heat corn, Matichon Online reported.
The pond usually held only a small amount of wastewater. It had filled up during recent frequent rainfall.
The victims were named as Lek Saithong, 41, Su Saengkaeo, 46, and Saikam Thaiyai, 35. They passed out while inside the tank.
Two men went who in to clean the pond had collapsed. A man who rushed to help them also died.
Rescuers who managed to reach and retrieve the bodies said the gas gave off a sulphurous smell.
The company trades in farm products.