When we breathe in, the inhaled air can contain dust, chemicals and other irritants that can be harmful to the body, particularly to organs in the respiratory system like the lungs. While the tiny hairs inside the nostrils (cilia) trap many of these particles, some will often get through. To help you out, your body reacts to try and forcibly expel the offending particles via the sneeze reflex arc.
There are a number of other reasons why we sneeze, including to clear the nasal passages when you have a cold, to expel allergens if you are allergic to something, and interestingly even bright sunlight can cause some people to sneeze; this is specifically called photic sneezing.
When a stimuli is detected by the nerve endings in the nose, impulses are sent to the brain, which initiates a chain of physiological events that enable the body to rid itself of the unwelcome item.
2. Muscles contract – The brain tells the respiratory muscles – including throat, chest and diaphragm – to contract.
3. Intake of breath – Contraction of the diaphragm causes a sharp intake of breath.
4. Air pressure rises – The brain signals to the throat to close. This, combined with the contraction of the abdominal muscles, raises the air pressure inside the lungs.
5. Sneeze – The throat reopens suddenly, explosively forcing air out of the body, making the chest cavity contract sharply. The diaphragm relaxes once again.
6. Mucus – Together with the offending irritant, saliva and mucus from inside the mouth and nasal cavity are also expelled from the body at up to 160km/h (100mph).