Nasal mucus (snot) is a complex fluid composed primarily of water, proteins, and antibodies, continuously produced by cells lining the nasal passages. Its main function is to act as a physical barrier, trapping inhaled foreign particles like dust, pollen, viruses, and bacteria before they enter the respiratory system. Mucus also humidifies the air we breathe and contains antimicrobial substances that neutralize harmful microorganisms.
The Color Spectrum of Mucus
When the body is healthy, nasal discharge is typically clear, thin, and watery. The first sign of an infection, such as a common cold, causes the mucus to become thicker and sometimes turn white or cloudy. This change occurs because inflamed nasal tissues slow the movement of mucus, causing it to lose moisture. As the immune response ramps up, the mucus often progresses to a yellow color. This yellowish tinge results from the accumulation of initial immune cells (white blood cells) arriving at the site of infection.
Why Snot Turns Green
The specific green color in nasal mucus is a direct result of the immune system’s intense response to an infection. The primary immune fighters involved are neutrophils, the most common type of white blood cell and the body’s first line of defense. Neutrophils contain a specific enzyme called myeloperoxidase. Myeloperoxidase is a heme-containing protein that contains iron, giving it a distinctive green pigment. When large numbers of neutrophils gather, fight the infection, and eventually die, they release this enzyme into the mucus, staining it green.
Green Snot and Bacterial Infections
A common misconception is that green snot automatically signals a bacterial infection requiring antibiotics. The color change simply indicates a high concentration of neutrophils and the resulting myeloperoxidase, which happens during nearly all significant infections, including the common cold. Green snot is most frequently observed in the later stages of a typical viral infection, as the body’s immune response peaks before recovery. Since antibiotics are only effective against bacteria and have no effect on viruses, the color of the mucus alone is an unreliable indicator for deciding on a course of treatment. Instead of color, the duration and severity of the illness are much more important factors in determining the type of infection.
When to Seek Medical Attention
While green mucus is common during a cold, certain accompanying symptoms suggest the need for professional medical evaluation. If cold-like symptoms, including colored discharge, last for more than 10 to 12 days without improvement, it may indicate a bacterial infection has set in. This duration is a better indicator than color alone for potential bacterial sinusitis. Seek care if the green mucus is accompanied by severe, persistent facial pain and pressure, or a high fever, especially one over 102°F. Other warning signs, such as sudden changes in vision, swelling around the eyes, or a stiff neck, can indicate a more serious infection requiring specific treatment.