OAKLAND — Three adults were taken to the hospital Monday night for treatment of suspected carbon monoxide exposure after firefighters went to their home because a malfunctioning furnace was reported.

Oakland Fire Chief Dave Coughlin said Tuesday that at 10:27 p.m. firefighters were called to the two-story single family home at 111 Axtell Drive when smoke was reported in the home.

Firefighters discovered a malfunctioning oil furnace in the basement. They also recorded carbon monoxide readings of 50 parts per million on the main level of the home before shutting down the furnace and ventilating the house. A carbon monoxide detector will sound when levels reach 30 parts per million, Coughlin said. There was no carbon monoxide detector in the home, he added.

Four adults were living in the home and three displayed signs and symptoms of carbon monoxide exposure, including headaches and nausea, Coughlin said. The three were taken to MaineGeneral Medical Center’s Thayer Center for Health in Waterville for evaluation.

Although the furnace started smoking Monday night, it could have been malfunctioning and spreading carbon monoxide in the home much earlier, according to Coughlin. Carbon monoxide is a colorless, orderless, poisonous gas, and exposure can lead to illness and death.

“You can’t tell it’s there until it’s too late,” Coughlin said. The homeowners have been advised to fix the furnace before restarting it, he added. The smoke from the furnace set off the smoke detectors.

Symptoms of carbon monoxide poisoning include headaches, dizziness, upset stomach and vomiting, and symptoms progressively get worse before people lose consciousness, Coughlin said.

The gas frequently comes from heating systems that aren’t working correctly, and homeowners are advised to make sure to get their furnaces serviced and to install carbon monoxide detectors in their homes, Coughlin said.