An explosion at an industrial park for chemical companies in Germany has killed at least one person, with 31 others injured and four missing.

Fire officials who tested the air said there did not appear to be a danger to nearby residents after authorities initially urged people to shelter inside.

The explosion at the waste management facility of the Chempark site in the city of Leverkusen, near Cologne, sent a large black cloud into the air.

It took firefighters almost four hours to extinguish the fire that took hold after the explosion.

Germany’s Federal Office for Civil Protection and Disaster Assistance initially classified the incident as “an extreme threat”.

Later on Tuesday, however, the Cologne fire department tweeted that measurements of the air’s pollution “do not show any kind of abnormality”.

They said the smoke had gone down but that they would continue to measure the air for toxins.

The city of Leverkusen said in a statement that the explosion occurred in storage tanks for solvents.

Police said on Tuesday afternoon that 31 people were injured, with five of them in intensive care.

“We are deeply concerned about this tragic accident and the death of our employee,” the head of Chempark, Lars Friedrich, said in a written statement.

City officials asked all residents to stay inside until the late afternoon and warned people from outside of Leverkusen to avoid the region.

City officials later also warned people not to let children play outside, use outside pools or eat fruit and vegetables from their gardens in the coming days. They said experts would only be able to tell in a few days how toxic the soot of the explosion would be.

Currenta, the company operating the chemical park, said the explosion happened at 9.40am local time and then developed into a fire. They said three big tanks were affected by the explosion, but that it was too early to know the cause of the explosion.

“Sirens were operated to warn residents and warning alerts were sent,” Currenta said in the statement.

Police in nearby Cologne said a large number of officers, firefighters, helicopters and ambulances from across the region had been deployed to the scene. They asked all residents to stay inside and warned people from outside of Leverkusen to avoid the region.

They also shut down several nearby major roads.

The mayor of Leverkusen, Uwe Richrath, called the blast “a tragic moment for Leverkusen”.

“It was a huge detonation felt in all over the entire city,” he said. “I hope the four missing people are still alive.”