Pollen collecting devices such as the ‘Hirst’ trap and the ‘Burkard’ apparatus provide information on average pollen content in the air during a certain time in a certain volume of air (e.g. 24 hr in 1 m3 air).

They give no idea, however, of the allergenic particles to which individual hay fever patients are exposed in the course of a day. An ‘Individual Pollen Collector’, attached to the patient's clothing, showed that the counts varied considerably in different persons in the same place. They differed—quantitatively and qualitatively—from the values with the ‘Burkard’ pollen and spore traps at two places in Switzerland. The slides from the ‘Individual Pollen Collector’ frequently show ‘clouds’ of certain kinds of pollen grains, which could be responsible for hay fever attacks in moderately sensitized subjects.