In general, carbon monoxide is produced whenever something is burnt. Barbecues, fireplaces, radiant heaters but also cigarettes are possible sources of carbon monoxide. Carbon monoxide is produced by the incomplete combustion of carbon-containing substances such as natural gas, biogas, light petrol, heavy oil, coal or biomass with insufficient oxygen supply. The concentration in residential buildings is usually between 0.5 and 5 ppm. In rooms with gas burners, however, the CO concentration can be as high as 15 ppm. Even in rooms with chimneys, there should not normally be higher concentrations if there is sufficient chimney draught, as the gas evaporates with the warm air from the chimney. Improperly functioning heating and exhaust systems and defective or improperly maintained appliances can be sources of carbon monoxide. If the flue of a fireplace or stove is not able to transport the CO gas out, a backflow occurs and the room fills with carbon monoxide unnoticed. The flue does not even have to be defective for this, it is enough if, for example, a bird or wasp nest blocks the exhaust air duct. Carbon monoxide is also produced when barbecuing with charcoal and wood fires in general. Propane-fired radiant heaters and the like also emit carbon monoxide gas. In addition, gas cookers, the burning of wood pellets, car exhaust from attached garages and any kind of ventilation equipment and generators powered by combustion engines are sources of carbon monoxide. But cigarette and shisha smoke also contain carbon monoxide. Just ten smoked cigarettes increase the CO concentration in a 30 m³ room by 22 ppm. In an assessment, the Federal Ministry for Risk Assessment (BfR) explicitly points out a high CO concentration in shisha smoke.