IEAT said chlorine leaked from the Aditya Birla Chemicals (Thailand) factory in the eastern province of Rayong on Sunday after an emergency shutdown of the plant's operations.
The smell of the gas reportedly spread through surrounding areas for a short time, making those who inhaled it feel dizzy.
Forty-two persons were sent to Map Ta Phut Hospital, with seven remaining hospitalised.
IEAT ordered temporary suspension of the plant's operations until the investigation of the accident is complete.
Aditya Birla Chemicals is a leading manufacturer of bulk and speciality chemicals and viscose filament yarn. The business is spread across multiple manufacturing locations in three countries – India, Thailand and China.
The incident occurred just one day after an explosion and fire at the Bangkok Synthetics Co (BST) petrochemical factory in the nearby Map Ta Phut industrial estate on Saturday.
The public health ministry confirmed 12 people were killed in Saturday’s accident. Twenty-four of the 142 injured remain hospitalised.
The industrial estate agency ordered the company to stop butadiene rubber production in order to find the cause of the explosion and to assess damage from the incident as well as to equipment in nearby areas.