The common experience of having only one nostril blocked while sick is often frustrating. This sensation is not random, but the result of a constant physiological process within your nose that is dramatically amplified by a virus or allergen. The inability to breathe out of one nostril is a complex interaction between your body’s normal function and its immune response.
The Normal Nasal Cycle
The nasal passages are never truly symmetrical in terms of airflow, even when a person is healthy. This subconscious, alternating process is known as the nasal cycle, where the body deliberately shifts the bulk of the airflow from one side to the other. The cycle lasts for an average of two to three hours before the roles of the nostrils reverse.
This alternation is regulated by the autonomic nervous system. The system directs blood flow to the turbinates, which are curled bony projections covered by erectile tissue inside the nose. When the parasympathetic nervous system is dominant on one side, it causes the blood vessels in the turbinate to engorge with blood, resulting in partial congestion.
The congestion phase allows the lining of that nostril to rest from the constant airflow, preventing it from drying out. This helps maintain the health of the cilia and mucus layer. The narrowed passage works more efficiently to humidify and warm the air, while the other side focuses on maximizing airflow.
How Illness Causes Tissue Swelling
When a virus, bacteria, or allergen enters the body, the immune system launches a protective response resulting in inflammation. This reaction attempts to flush out the invader and prevent it from moving deeper into the respiratory system. The body releases chemical mediators, such as histamine, which act on the local tissues.
These chemicals cause the blood vessels lining the nasal passages and the turbinates to dilate. This increased blood flow causes tissue swelling, known as congestion, which physically narrows the airway. The immune response also triggers the submucosal glands to increase mucus production.
This combination of engorged tissue and excess fluid accumulation causes the feeling of a complete blockage. The swelling is a defense mechanism, creating a barrier to trap pathogens while the runny nose helps wash them away. This inflammation transforms the normally subtle turbinate swelling into a pronounced obstruction.
Why One Nostril Blocks More Than the Other
The intense swelling caused by illness does not override the body’s pre-existing nasal cycle; instead, it amplifies it. The congestion felt is the inflammatory response layered on top of the natural, alternating congestion of the nasal cycle.
The side of the nose already in its natural resting phase, with its turbinate slightly engorged with blood, becomes the target of the most severe blockage. When inflammatory chemicals cause the blood vessels to swell further, the naturally congested side is pushed into a state of near-total closure.
The other nostril, currently in its active, decongested phase, remains relatively open despite the inflammation. The sensation of being completely blocked in a single nostril is thus the amplification of a healthy physiological phase by the presence of infection. The blockage will eventually switch sides when the autonomic nervous system naturally flips the nasal cycle.