Also, change furnace filters regularly and have your heating system annually checked and tuned up to make sure combustion is clean and complete. Don't use natural gas ovens as a heating source. And never use charcoal indoors for either cooking or heating.
Charcoal releases carbon monoxide gas.
Warming up your car in an attached garage allows a large amount of carbon monoxide to enter your home. Never idle a car in an attached garage. Back the car completely out of the attached garage and close the garage door while the vehicle warms up. Consider purchasing CO detectors that are designed to identify traces of carbon monoxide after several minutes and with memory to show the highest-achieved concentration levels. Purchase a detector for each level of the home that has bedrooms.