Q. Why do I sneeze when I feel cold?
A. The explosive release of air through the nose and mouth that is a sneeze is a neurological process that usually starts with physical stimulation of the wide-ranging trigeminal nerve. This nerve's branches are responsible for sensation in the face and surrounding skull areas.
The branches that terminate in the facial skin are sensitive to chemical, mechanical and tactile stimulation, including sensations of pain and temperature. Branches also serve the sensitive lining of the nasal passages.
While irritation of the nasal passages is the most common trigger for a sneeze, the presence of cold, dust-bearing air is not necessary. Simply being cold and shivering, or even moving from one temperature zone to another, can jar the nerve.
Eventually the impulses converge on the sneezing center in the brain's lateral medulla, and when they reach a threshold, the nerves that control inhalation and explosive exhalation are recruited.
In a comprehensive review of what is known about the sneeze, published in June 2009 in the journal Therapeutic Advances in Respiratory Disease, sneezing is described as a protective reflex that is not completely understood but that expels irritants and keeps nasal passages open in conditions like nasal congestion.
Many other conditions besides rhinitis can produce a sneeze, including sudden exposure to bright light (called the photic response); a particularly full stomach (the satiation response); central nervous system diseases like epilepsy; and sexual excitement or orgasm.
C. CLAIBORNE RAYscience
This article originally appeared in The New York Times.