Pollens occur in the powder-like substance seen in some of the flowers. Pollen grains are very small in size. In a majority of cases, they are yellow in colour.
Only some of the pollens cause asthma. Out of thousands of different pollens, only about a hundred are known to cause this disease. A plant must have the following characteristic features before its pollens can be suspected of being the cause.
□ The pollen of this plant must be carried by wind for pollination purposes. (There are other plants which are pollinated by insects, water or by self-pollination).
□ The pollen should be sufficiently light for it to be carried over long distances by the wind.
□ The pollen must be produced in abundance.
□ The pollen must have chemical qualities to act as an allergen. Plants which pollinate through wind, have flowers which are usually unattractive in appearance, are comparatively small in size; rarely have any smell and contain no nectar. Their pollens are light, dry and abundant in quantity. They are carried over to long distance, sometimes even for hundreds of miles.
Pollens are released from the flowers very early in the morning, often before sunrise. Hence many patients of asthma, sensitive to pollens, have their symptoms at that time. Different asthma patients are allergic to different pollens; some only to one pollen but most of them to more than one. Pollens of common garden flowers such as roses, marigold, phlox, do not cause and do not float about in the air.
In order to establish whether an asthma patient is allergic to pollens, and if so to which pollens, detailed investigations are necessary.
First of all, it is necessary to find out which plants, trees, shrubs, grasses and weeds are present in the area. This needs surveying, identification and cataloguing of all the plants that grow throughout the year or in any one particular season.
The next step is to ascertain which pollens are present in the air in the area under study in different months of the year. This is done by exposing to the air vaseline-coated glass slides, placed in an instrument called pollen-catcher. The pollen-catcher is placed on top of a building, in different localities, where there is a free flow of air.
The pollens present in the air, on passing over the slides, get caught in the vaseline. These slides are then examined under a microscope and the type and number of different pollens are counted. This is done daily, throughout the year, and for three to five years. From these extensive observations, one comes to know :
□ The types of pollen present.
□ The month in which they are present.
□ Their respective quantities.
Thus, one can chart out a pollen calendar for the area.
Knowledge of the pollen calendar of an area is absolutely necessary for investigating a case of asthma. If, for instance, an asthma patient says that he gets symptoms in the month of March and April only, it is necessary that we know which pollens are present in the air during these two months.