Unsafe levels of a carcinogen have been discovered at a nuclear missile base in Montana after the area experienced a spike in cancer case numbers, the Air Force Global Strike Command announced Monday.

A carcinogen is a “substance, organism or agent capable of causing cancer,” a category that includes naturally occurring things like UV rays or those created by human production, like cigarette smoke, according to the National Human Genome Research Institute.

The announcement said an immediate cleanup effort was ordered after bioenvironmental experts reported polychlorinated biphenyls was found in samples taken from Malmstrom Air Force Base Aug. 4.

Polychlorinated biphenyls, often called simply PCBs, are hazardous, manmade substances that were banned in the U.S. in 1977 for their potential ability to cause cancer, per the Centers for Disease Control and Prevention.

The Associated Press said the samples taken found PCB levels in two facilities located at the Malmstrom Air Force Base in central Montana exceeded thresholds recommended by the U.S. Environmental Protection Agency.