What causes sneezing?

   Sneezing is the involuntary spasmodic expirations caused either by direct or reflex irritation of the sensory nerves of the nasal mucous membrane. The causes of excessive sneezing may be broadly divided into two classes, extrinsic and intrinsic. Extrinsic causes are those which act from without, e.g., tobacco snuff and dust. In hay fever, which is accompanied by periodic attacks of sneezing, the membranes are allergic to specific foreign substances, usually ragweed pollen. Sneezing may occur as a reflex from excessive stimulation of the optic nerve by strong light. Intrinsic causes are associated with such infections as colds, which produce inflammation of the mucous membranes.