Allergens that cause hay fever are the pollens of grass, trees and a few weed species, all of which are wind-pollinated. Only the small, light wind-borne pollens cause the allergic reaction in the nose.
Occasionally fungal spores are implicated, but as these usually have a year-round distribution, seasonal flare-ups are very unlikely.
Certain tasks such as mowing the lawn during the pollen season will aggravate symptoms of hay fever.
Perennial allergic rhinitis (that occurs all year round) is mainly caused by house dust mite, animals and moulds.
Pollen levels are usually highest in the mid morning, and larger amounts are present on hot windy days. During hot humid weather – especially after thunderstorms – pollen grains tend to rupture. These release fine starch granules into the air, which are highly allergenic and can penetrate deeply into the lungs. Early springtime hay fever is most often caused by pollens of common trees. Late springtime pollens come mostly from grasses.
Colourful or fragrant flowering plants, such as many garden plants, rarely cause allergy, because their pollens are too heavy to be airborne.
Unlike grasses, trees and weeds, which are wind-pollinated, garden plants are pollinated by insects, which carry their heavy, sticky pollen.
Pollinating seasons for grasses, trees and weeds are fairly consistent from year to year. These seasons vary according to geographic region. Hay fever season extends beyond the period of late summer into early autumn, as this is the time when many weeds release their pollen.
In South Africa with its warm climate, grass seasons are very long and may continue for nine months, resulting in symptoms for long periods.
Revised and reviewed by Professor Sharon Kling, Clinical Unit Head, General Paediatrics, Intensivist at Tygerberg Hospital and Associate Professor, Department of Paediatrics and Child Health at Stellenbosch University. February 2015.