Allergies can be different for different people, but common allergens include pollen (which causes hayfever), house dust mites and pets (which cause perennial allergic rhinitis), chemicals and certain foods.

Hayfever

Also known as seasonal allergic rhinitis, hayfever occurs when a person has an allergic reaction to an airborne substance, such as grass or tree pollen. This reaction normally takes place in the upper respiratory tract, which includes the nose, sinuses and throat. The eyes can also be affected, becoming itchy, red and watery.

Some of your customers may experience hayfever-type symptoms all year around because they are allergic to other things, such as house dust mites or pets (see later). This is known as perennial allergic rhinitis.

Pollen is produced at different times of the year, depending on its source, and a person can be allergic to more than one type of pollen:

• Tree: February-June
• Grass: May-August
• Weeds and other plants: August-October.

The weather can also affect the amount of pollen in the air and a pollen index forecast is often given during the hayfever season to let people know how high the pollen count for that day will be. Pollen levels tend to be higher on warm, sunny days and often rise in the evening.

Hayfever usually affects people in their early teens and peaks in the 20s. It seems that many people become less sensitive to pollen as they get older, and by the time they reach their mid-40s, they might no longer get hayfever at all.

The exact reason why some people get hay fever and others don't is not known. However, there do seem to be some hereditary factors involved.