A stop action photograph of a man sneezing. A sneeze is a reflex from the inhalation irriitants of foreign particles, pollen, dander, dust in the nasal cavity. Allergies and viruses, like the common cold, are the principal causes of sneezing. The expulsion of air and mucus cleanse the nasal passages. If a person is infected with a cold, the virus can become airborne and transmitted to others. Violent sneezes have been clocked at over 100 mph and a sneeze can expel up to 40,000 droplets.