However, the Department of Industrial Works has still ordered the closure of the plant for 90 days.
Wichien Jungrungruang, director-general of the Pollution Control Department, said on Tuesday that his staff measured levels of air pollutants -- including volatile organic compounds benzene, styrene and sulphur dioxide -- in the vicinity of the factory and found nothing in breach of safety levels.
His department would continue to monitor air quality, he said. Staff from the Ministry of Natural Resources and Environment would also check the air quality, Mr Wichien said.
He said the explosion and fire on Monday evening were caused by a leaking hydrogen pipe at a vacuum gas oil hydrotreating unit in the IRPC complex.
IRPC president Sukrit Surabotsopon said most petrochemical plants and refineries in the area continued to function. The damage at the site was still being assessed. No one was injured in the accident, he said.
Three medical teams were deployed to examine the health of local people on Tuesday.