Mr Friedrich said it was not clear what had caused the explosion, which led to a fire starting in a tank containing solvents.
“Solvents were burned during the incident, and we do not know precisely what substances were released,” Friedrich added. “We are examining this with authorities, taking samples.”
Air quality tests by the environmental protection agency in the neighbouring city of Cologne in the early afternoon found there was no immediate danger to the population.
But drivers were advised to keep their windows closed and switch off their air conditioning as a precaution.
The explosion took place at around 9.40am local time (8.40am BST) in a hazardous waste processing facility at Chempark, one of the largest chemical plants in Europe on the banks of the river Rhine.