Pollen grains are the male reproductive cells of the plant that are produced in the anthers (the male part of the flower). They carry the male genes to the female flowers to fertilise them so they can set seed. Most plants use insects such as bees to carry their pollen from flower to flower, but in other plants such as grasses or some trees pollen blown from plant to plant by the wind. Because the pollen grains have a low chance of reaching the female flowers when blown in the wind, they are produced in vast quantities.

The amount of pollen in the air makes such a big difference to hay fever sufferers that pollen counts are published daily in many newspapers and with the weather forecasts on television during the hay fever season. The amount of pollen in the air on one day can be used to predict how much pollen is likely to be in the air the following day if the weather conditions remain the same.

Some people may start to react when the pollen count is as low as 10 grains per cubic metre, and others would only react to a much higher count of, say, 50 or even higher cubic grains per cubic metre. However, in June and July, the count could rise up to 200 grains per cubic metre. Hay fever sufferers may become more sensitive to pollen as the season ends, so even a small pollen count then could cause a problem.