Sneezing is a natural body mechanism in cats. There are a lot of causes for sneezing, some of which are covered below:-
Dust and airborne particles
Similar to humans, dust can cause a sinus irritation in cats. So if you are cleaning and dusting, renovating, using a heater or air conditioner for the first time in the season, using a wood fire place or there is generally more dust or airborne particles in the air, then this could be a cause.
Cigarette and tobacco smoke is a common cause of respiratory problems in cats and can cause sneezing.
The roots of the teeth are located very closely with the sinuses and when the root of a tooth becomes infected or inflamed, it can cause drainage into the sinuses and lead to sneezing.
Your cat may have an upper respiratory viral or bacterial infection from another cat. Respiratory infections can have effects much like human colds and sinus infections and can be treated.
If the sneezing is persistent your cat may need a veterinary examination to ensure there is no underlying health problem such as an infection or allergy.