Furnaces, fireplaces, water heaters, dryers, and cars can make this gas you can’t smell or see. It can be deadly if it isn’t vented out and it builds up in your home. Install carbon monoxide detectors inside, and use appliances properly. The symptoms of carbon monoxide poisoning are a headache, shortness of breath, blurry vision, and feeling dizzy, confused, weak, and nauseated. If you have them, get fresh air right away, then call 911.