The seasonal allergens mainly responsible for causing hay fever are the very light pollen grains from wind-pollinated trees, grasses and weeds. More than 30 different pollens are in the air at different times of the year. The pollen from birch trees gets into the air as early as March and causes symptoms in about one-quarter of people who are already allergic to grass pollen. At the other end of the season, the pollen from weeds, particularly stinging nettle, can also affect some people causing their symptoms to carry on well into August, long after symptoms have finished for those allergic to grass pollen alone. The grass pollen season stretches from May to August with pollen counts being highest around mid-June to early July.
Most pollen is released from grass in the morning and rises into the air with the heat of the day.
It can be blown for many miles, so cities can still have lots of pollen in the air. When the air cools down in the evening, the 'pollen cloud' comes down again and pollen counts shoot upwards. Every type of pollen is a potential allergen, but it should be remembered that only the pollen from wind-pollinated plants becomes airborne. The pollen from brightly-coloured and fragrant flowers is sticky and is dispersed by insects, so it doesn't get into the air. However, flower lovers beware, sniffing an insect-pollinated flower particularly vigorously could lead to inhalation of its pollen or of any other pollen which had settled on it.