Allergies generally cause skin problems in cats however they can also cause ‘hay fever’ like symptoms such as a runny nose, itchy eyes, and sneezing. Allergies can relate to something in the diet, something in the environment such as household cleaning products or more natural allergens such as pollen. If the sneezing occurs at a particular time of the year it may be the result of a seasonal allergy. i.e. pollen
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.
Sneezing accompanied by blood may be a sign of nasal cancer or fungal infection.