Mucus, often called snot or phlegm, is a protective, gel-like substance produced by the mucous membranes lining the respiratory and digestive tracts. This fluid acts as a first line of defense against the outside world, constantly safeguarding delicate internal tissues. The color of this discharge often changes when the body is fighting an illness. However, the color of mucus alone cannot diagnose the presence of the SARS-CoV-2 virus, as it typically follows a pattern common to many respiratory illnesses.
The Purpose of Mucus Production
Mucus is primarily composed of water (about 95%), along with a complex mix of proteins, salts, lipids, and specialized glycoproteins called mucins. These mucins give the fluid its sticky, viscoelastic texture, which is necessary for its function. The body produces this substance continuously to keep the airway linings moist and protect the epithelial cells from drying out.
The primary role of this fluid is to trap inhaled foreign particles, such as dust, pollutants, and pathogens. Once trapped, the mucus layer is constantly swept away by tiny, hair-like structures called cilia in a process known as mucociliary clearance. This mechanism moves the contaminated fluid toward the throat, where it is typically swallowed and neutralized by stomach acid. Mucus also contains immune components, such as antibodies and antimicrobial enzymes, which help disable invaders before they cause infection.
What Mucus Color Reveals About Respiratory Infections
The color of mucus during a respiratory illness reflects the immune response, not the specific pathogen involved. Mucus is typically clear or white when a person is healthy or experiencing a new, mild irritation, such as an early-stage viral infection or allergen. Clear mucus signifies a high water content, often indicating the body is attempting to flush out an irritant or nascent infection.
As the infection progresses, the immune system increases its activity, impacting the mucus color. Yellow or green coloration results from the influx of white blood cells, specifically neutrophils, which contain the greenish enzyme myeloperoxidase. When these immune cells die, their accumulated presence tints the mucus. This yellow or green discharge is not unique to bacterial infections; viral illnesses like COVID-19, the common cold, or influenza can all cause this color change.
The color transition from clear to yellow or green indicates a robust immune response is underway, regardless of whether the cause is viral or bacterial. Therefore, mucus associated with COVID-19 usually falls within the clear, white, or yellowish-green spectrum, making it non-diagnostic. Less common are pink, red, or brown tints, which are caused by the presence of blood. This happens due to irritation and inflammation of the nasal passages or throat lining from excessive coughing or dryness.
Symptoms That Matter More Than Mucus Color
Since mucus color does not reliably distinguish between COVID-19, the flu, or a common cold, diagnosis relies on a cluster of other symptoms and clinical testing. Reliable indicators of a potential COVID-19 infection include systemic symptoms that affect the entire body. These often involve fever, significant fatigue, and muscle or body aches that feel more pronounced than those experienced with a typical cold.
One distinct symptom that can occur with COVID-19 is a new loss of taste or smell, though this is less frequent with newer variants. Other common symptoms include a sore throat, headache, and shortness of breath. Gastrointestinal issues like nausea, vomiting, or diarrhea have also been reported.
If an individual experiences any combination of these symptoms, the most responsible step is to take a clinical test for SARS-CoV-2. The presence of a fever or difficulty breathing indicates the need for immediate action, such as isolation and consultation with a healthcare provider. Paying attention to the duration and severity of the symptom cluster is far more informative than using mucus color as a diagnostic tool.