(COLORADO SPRINGS) — The Pikes Peak Region has officially seen its first snowfall of the winter season. As temperatures drop, the Colorado Springs Fire Department (CSFD) starts to see an increase in calls for carbon monoxide (CO). An odorless, colorless, poisonous gas, they call carbon monoxide the invisible killer. "Doors are closing, people are shutting up their windows. Maybe they're using their fireplace or appliances that are using fuel sources," said Kathy Hook with CSFD. "So any time that you are using a fuel that burns, you're at a risk for CO poisoning." She says every home should have a carbon monoxide detector outside the sleeping areas and on each level of the home. It is recommended that CO alarms are tested at least once a month. Hook said individuals should also change the batteries every year. "If you think that there is a chance you have carbon monoxide in your home, immediately get out. Sometimes the symptoms are more subtle. You might feel like, 'Oh, I'm dizzy, I have a headache'," said Hook. "We've certainly had carbon monoxide poisoning within the Pikes Peak region." Some common sources of CO include your furnace, water heater, dryer, stove, car, or a blocked vent or chimney among other things. Colorado Springs Utilities (Springs Utilities) offers advice on how to keep safe, including CO poisoning symptoms. Some poisoning symptoms include nausea, vomiting, fatigue, confusion, and a rapid heart rate. Jason Holmes, with Rocky Mountain Climate Heating and Cooling, says companies like his can come out and protect your home from potential CO leaks. "The service guys will go out and they'll have a little CO detector, we call it the sniffer. They'll go out there, and rub it around the vents, and up and down the vents, to make sure there's no leaks," said Holmes. "We get a lot of rodents or birds caught up in the vents, which will cause a backflow in venting, and cause carbon monoxide inside your house."