Preliminary results suggest it was carbon monoxide poisoning, said Greg MacLeod, general manager of Sunny Corner Enterprises Inc., the manufacturing company that owns the building where the leak occurred.
The cause and origin of the leak at the Sunny Corner assembly plant have not yet been determined, he said.
All of the employees are recovering and are expected to be released from hospital by Wednesday night, said MacLeod.
The building will remain closed on Thursday — and possibly longer — as WorkSafeNB continues to investigate, he said.
There is no danger to the general public and the company will continue to monitor the building to ensure the conditions are safe before workers return, officials said.
Alerted by worker passing out
Kirk Mullin, a company official, said the company was alerted to the problem when one of the assembly-line workers passed out.
The employees were assembling drill rig components.
Responding emergency crews detected the odour of a foreign substance and ordered the building evacuated.
Police said 36 people were in the building at the time; 34 of them were taken to hospital.
Some employees needed medical attention more urgently than others, but all of them exhibited the same symptoms, said Const. Todd Chadwick, of the Miramichi Police Force.
"Various ailments, which would be nausea, respiratory, headaches, confusion — all of which would be related to breathing in some sort of substance," he said.
Ambulance New Brunswick officials said 13 of the employees were transported by ambulance.
Of those, seven were deemed in "critical condition," which means there was a "risk to life and limb" and they were rushed to hospital with the ambulance lights and sirens activated.
The other six were taken by ambulance without the lights and sirens activated.
The remaining 21 people were transported by a Miramichi City Transit bus, officials said.
Initial reports suggested propane
The Miramichi Police Force said it was called to the business at 9:40 a.m. local time.
"Upon arrival at the scene, it was quickly determined that a noted odour of a foreign substance was detected," said Const. Todd Chadwick.
He said in a statement that the city's fire department "checked for air quality and determined a concern, which required the building to be ventilated."
Police initially believed it was a propane leak.
Sunny Corner is a construction, fabrication and industrial sales company. Its head office is located at 259 Dalton Ave. in Miramichi.
The company also has a fabrication office on Water Street and a branch office in Saint John.