File Photo: An image from the chemical factory were an ammonium tank caused the blast. Photo courtesy of IRNA
The blast at a paint factory in the Shahid Salimi industrial zone in East Azerbaijan province was caused by a gas leak, official news agency IRNA said.
It said "22 people were treated on the spot and the rest were transferred to medical centres."
Vahid Shadinia, a spokesman for the province's emergency services, told IRNA more than 20 of those injured had been taken to "specialised hospitals" in the provincial capital Tabriz.
Shahid Salimi, located in the city of Azarshahr, is the largest industrial zone in northwestern Iran.
It hosts 790 factories and other facilities employing nearly 28,000 workers, according to state media.