More than 40 people were taken to a hospital Tuesday after being exposed to high levels of carbon monoxide at a Northwest Washington drug rehabilitation facility, D.C. fire and EMS officials said.

An ambulance was called to evaluate a sick person in the 1300 block of Harvard Street at about 3:15 p.m., said Pete Piringer, a spokesman. Fire crews took readings and found elevated levels of carbon monoxide inside two adjacent buildings at Second Genesis rehabilitation facility and the buildings were evacuated, he said.

EMS units took 43 people to the hospital, including three people in serious condition, Piringer said.

Authorities believe a malfunctioning water heater in a basement caused the release of the gas.

Officials closed that block of Harvard Street and traffic was affected in the area.