The Missouri Department of Natural Resources (DNR) issued a statement on Friday that recent tests had shown methane gas levels at the St. Louis Demolition Landfill are “dangerously high.” DNR officials warned that methane levels are well above the regulatory limit and, in some cases, are concentrated enough to be explosive. The landfill is located in the northern part of St. Louis and is near several residential areas.

Subsequent testing by firefighters did not indicate elevated methane levels in houses in the nearby Baden neighborhood, according to the St. Louis Post-Dispatch. Testing continues and residents are being offered methane detectors as a precaution.

The St. Louis Demolition Landfill was built 50 years ago, predating current landfill regulations. It has not accepted waste for 12 years. The city of St. Louis (which purchased the landfill 30 years ago) has been working with DNR to try and cap the site.