When mucous membranes encounter an irritating stimulus or substance, they release a chemical called histamine. This involuntary reaction can also cause an inflammatory response and constriction of smooth muscle, which causes you to sneeze.

“Some people are more sensitive than others and tend to sneeze,” says Dr. Betty Miller, a founding director of Peninsula Allergy Associates in Daly City, California.

In the case of pepper, the irritant is piperine (an alkaloid that is also used in some insecticides).