What is one of the key characteristics of flue gases?

Flue gases are the byproducts of combustion processes, commonly emitted from furnaces, boilers, and other heating appliances. One of the critical characteristics of flue gases is that they contain a variety of combustion products, which can include carbon monoxide, nitrogen oxides, sulfur dioxide, particulate matter, and unburned hydrocarbons. Many of these components can be harmful to human health and the environment, making proper ventilation and emission control essential. Understanding this aspect reinforces the importance of safety measures when dealing with gas appliances and exhaust systems.

The other options incorrectly represent the composition and safety of flue gases. For instance, they do not consist mainly of methane, and asserting that they are completely safe to breathe ignores the toxic nature of many of their components. While oxygen is present in flue gases, it is not the primary constituent, as the gases are predominantly made up of combustion byproducts. Recognizing the hazardous nature of flue gases is crucial for professionals in plumbing and HVAC industries to ensure proper handling and safety protocols.