BERLIN—An explosion at an industrial park for chemical companies shook the German city of Leverkusen on Tuesday, sending a large black cloud rising into the air. At least 16 people were injured and five remain missing. Germany’s Federal Office for Civil Protection and Disaster Assistance classified the explosion as “an extreme threat” and asked residents to stay inside, turn off ventilation systems, and keep windows and doors closed, German news agency dpa reported. Later on Tuesday, however, the Cologne fire department said on Twitter 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 at the Chempark site, about 20 kilometers (13 miles) north of Cologne on the Rhine river, occurred in storage tanks for solvents. It said …