What is a common cause of yellow, green, or brown sputum?

The presence of yellow, green, or brown sputum is typically indicative of older secretions in various stages of decomposition. This coloration often suggests that there is an infection or inflammation within the respiratory system. Infections can lead to the accumulation of mucus, which becomes discolored as it interacts with immune cells, bacteria, or environmental contaminants.

In the case of yellow or green sputum, the color is frequently associated with the presence of white blood cells, particularly neutrophils, which play a critical role in the body's immune response to infections. Brown sputum may result from older blood or other contaminants that have been present in the airways for an extended period.

The other options do not adequately account for the significant coloration and implications of sputum production. Recent exercise does not tend to change the color of sputum, excessive mucus production can occur without any discoloration, and environmental allergens primarily lead to clear or white mucus rather than colored sputum. Thus, identifying older secretions as the cause aligns most closely with the understanding of sputum characteristics associated with respiratory conditions.