Bloomington, IL- At first, Doug Matthews didn’t want to pay attention to the alarm that started sending an alert at 1:05 a.m. on a recent Saturday morning.

“It is one of those with the voice and it said the carbon monoxide levels were 131 (parts per million) in quite a strident tone,” he said. “The first thing I thought was that it needed new batteries, but I knew I had just changed the batteries about a month earlier.”

Knowing that levels above 70 ppm were dangerous, he changed the batteries and set the alarm on the bedroom dresser. Five minutes later, it went off again.

“So, this time, I knew it was the real deal. I called the fire department and the dispatcher said to get out of the house immediately and call 911,” said Matthews of Bloomington.

Both he and his wife, Mary Lee, felt fine and didn’t have any of the symptoms of carbon monoxide poisoning, but fire officials finished the investigation and told them they’d made a wise decision.

Further investigation revealed the levels to be more than 500 ppm in the basement. The problem was traced to a faulty heater on a spa.

Last week, the Normal Fire Department responded to two separate medical calls involving illness and fainting due to carbon monoxide issues that become more pronounced with the onset of cold weather, and leading the department to urge homeowners to check that their alarms are working properly and to install an alarm if one is not already in the home.

“We really feel like we dodged a bullet twice, but we may not be as lucky the next time,” said Fire Chief Mick Humer.

Carbon monoxide is a colorless, odorless and tasteless gas byproduct of burning fossil fuels. At elevated levels, it causes sickness and, if not detected, death.

“Your body can’t see or smell carbon monoxide, so it has earned the nickname ‘the silent killer,’ ” said Normal Fire Department Public Information Officer Matt Swaney. “If carbon monoxide levels are high enough, you may go to sleep and never wake up.”

Last month, the Bloomington Fire Department also responded to a home with extremely high levels of carbon monoxide.

“They were outside their home complaining of headaches,” said Stuart Blade, the agency’s public information officer. “The son woke up in the morning, was able to walk to his parent’s bedroom, complaining of a headache. All family members had symptoms of carbon monoxide poisoning.”

The first responding unit arrived to find readings of more than 100 ppm in the garage and as high as 550 ppm in the basement. The family went to the hospital for precautionary measures and the residence was cleared of carbon monoxide. A faulty heater was found to be the cause.

Fossil fuels such as natural gas are used to heat the vast majority of American homes and faulty heating equipment accounts for nearly one-third of accidental monoxide deaths, said fire officials. These can include home heating systems, improperly vented gas appliances, kerosene or propane space heaters, charcoal grills and Sterno-type fuels.

The health effects of carbon monoxide poisoning depend on the concentration and length of exposure, as well as each individual’s health condition.

Most people will not experience any symptoms from prolonged exposure to carbon monoxide levels of 1 to 70 ppm, but some heart patients might experience an increase in chest pain.

As carbon monoxide levels increase and remain above 70 ppm, symptoms become more noticeable and can include headache, fatigue and nausea. At sustained carbon monoxide concentrations above 150 to 200 ppm, disorientation, unconsciousness, and death are possible, according to the U.S. Consumer Product Safety Commission.