Causes of a yellow or green nasal mucus include bacterial or viral infection, nasal polyps or an allergic reaction to pollen, smoke, fumes or other environmental elements, says ZocDoc. Dry air and the presence of a cold can also cause yellow nasal mucus.
Inflammation of the sinuses with a viral, bacterial or fungal infection may lead to sinusitis, explains ZocDoc. In the case of a yellow discharge caused by an environmental factor, it is necessary to address an allergic attack right away. Once a persistent yellow mucus has set in, it can develop into rhinitis, and those membranes become susceptible to other viruses and bacteria. A yellow or green nasal discharge does not necessarily indicate a bacterial infection.
Different colors of mucus may be indications of various conditions, but other symptoms must be considered as well, notes WebMD. When someone has a cold, the immune system sends white blood cells called neutrophils to the area. These cells contain a greenish-colored enzyme that can turn mucus the same color. Even in the presence of an infection, mucus may still be clear, so other symptoms, such as congestion, fever, and pressure overlying the sinuses, help determine the diagnosis. The consistency of mucus also affects the color; thick mucus is often greenish.