The explosion happened in a part of the site where waste is incinerated, Chempark operator Currenta said. Chempark, an industrial park for chemicals companies including Bayer and Lanxess, said on Twitter it was not yet clear what caused the explosion, Reuters reported.
Police said that it was not yet clear how much damage was caused and that a large number of emergency staff were on site.
The German government's civil protection agency warned residents in the area of "extreme danger", and police said on Twitter they should go indoors and keep doors and windows closed.
Several nearby motorways were closed, police said.
More than 30 companies operate at the Chempark site in Leverkusen, including Covestro, Bayer, Lanxess and Arlanxeo, according to its website.
Bayer and Lanxess in 2019 sold Chempark operator Currenta to Macquarie Infrastructure and Real Assets for an enterprise value of 3.5 billion euros ($4.12 billion).