Second, indoors, where we spend most of our time in the winter. Many of us are using woodstoves, coal stoves, gas stoves, fireplaces, and kerosene space heaters to keep warm without sending the fuel oil man's kids to college on one season's worth of our bills. If you use a woodstove, as we do, or a fireplace, you need to have your chimney cleaned annually. The cost is a small price to pay to save yourself the danger of a chimney fire or at the very least respiratory problems. You know the smoke and odors from these alternative heat sources get into the drapes, rugs, furniture, etc., but they also hang in the air long enough to give your family allergies, breathing problems, and headaches.
Proper ventilation is very important.
And a CO2 detector is a smart investment. A few things you may not have considered - being so preoccupied with the winter itch and what to do with the kids on snow days. If your garage is connected to the house, there should be a good air seal between. Fumes and dust get into the house through the ducts and cracks and who knows where else. Paint, strong cleaning solvents and other hazardous materials should be moved to a shed disconnected from the house.