Carbon monoxide (CO) is primarily caused by the incomplete combustion of carbon-containing fuels when there is insufficient oxygen to produce carbon dioxide (CO₂). This partial burning produces CO gas, which is colorless, odorless, and toxic. Common sources include fuel-burning appliances like gas stoves, boilers, water heaters, and fireplaces if they are faulty, improperly installed, or poorly ventilated. Vehicles and machinery powered by gasoline or diesel also produce CO through engine exhaust, especially when run in enclosed or poorly ventilated spaces. Additionally, burning wood, coal, oil, propane, or trash can generate CO, particularly when combustion is inefficient or incomplete. Even natural phenomena like wildfires and volcanic activity emit carbon monoxide into the atmosphere. Improper use of appliances, blocked chimneys, or inadequate ventilation further increase the risk of CO accumulation indoors. CO forms also from industrial processes involving high- temperature combustion or chemical reactions with carbon-containing materials. Overall, CO arises wherever carbon-based fuels burn without enough oxygen for full combustion, making it critical to ensure adequate ventilation and appliance maintenance to prevent exposure and poisoning.