New London — High carbon monoxide readings led to the evacuation of a Truman Street apartment building on Friday, and a utility company employee was taken to the hospital for carbon monoxide exposure.

New London firefighters responded to 36 Truman St. on Friday morning where an Eversource gas employee had encountered higher than normal carbon monoxide readings. Known as the "invisible killer," carbon monoxide is odorless, colorless and produced by the incomplete burning of various fuels, including wood, oil, propane, and natural gas.

Firefighters evacuated the four-unit building and while wearing breathing masks removed two unharmed adults and three pets from various apartments.

The Eversource employee was treated at the scene and taken to Lawrence + Memorial Hospital. The employee had obtained carbon monoxide meter readings of more than 600 parts per million during his tests. Any reading above 101 parts per million is considered dangerous.

Carbon monoxide readings were elevated throughout the building but highest in the basement. Firefighters ventilated the building, which was temporarily condemned until repairs were made and an inspection completed.

The source of the carbon monoxide was not immediately clear.