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Each spring, summer, and fall tiny particles are released
from trees, weeds, and grasses. These particles, known as pollen, hitch rides on currents
of air. Although their mission is to fertilize parts of other plants, many never reach
their targets. Instead, they enter human noses and throats, triggering a type of seasonal
allergic rhinitis called pollen allergy, which many people know as hay fever or rose fever
(depending on the season in which the symptoms occur). Of all the things that can cause an
allergy, pollen is one of the most widespread. Many of the foods, drugs, or animals that
cause allergies can be avoided to a great extent; even insects and household dust are
escapable. Short of staying indoors when the pollen count is high--and even that may not
help--there is no easy way to evade windborne pollen.
People with pollen allergies often develop sensitivities to
other troublemakers that are present all year, such as dust. For these allergy sufferers,
the "sneezin' season" has no limit. Year-round airborne allergens cause
perennial allergic rhinitis, as distinguished form seasonal allergic rhinitis.
What is pollen?
Plants produce microscopic round or oval pollen grains to reproduce. In some species, the
plant uses the pollen from its own flowers to fertilize itself. Other types must be
cross-pollinated; that is, in order for fertilization to take place and seeds to form,
pollen must be transferred from the flower of one plant to that of another plant of the
same species. Insects do this job for certain flowering plants, while other plants rely on
wind transport.
The types of pollen that most commonly cause allergic
reactions are produced by the plain-looking plants (trees, grasses, and weeds) that do not
have showy flowers. These plants manufacture small, light, dry pollen granules that are
custom-made for wind transport. Samples of ragweed pollen have been collected 400 miles
out at sea and 2 miles high in the air. Because airborne pollen is carried for long
distances, it does little good to rid an area of an offending plant--the pollen can drift
in from many miles away. In addition, most allergenic pollen comes from plants that
produce it in huge quantities. A single ragweed plant can generate a million grains of
pollen a day.
The chemical makeup of pollen is the basic factor that
determines whether it is likely to cause hay fever. For example, pine tree pollen is
produced in large amounts by a common tree, which would make it a good candidate for
causing allergy. The chemical composition of pine pollen, however, appears to make it less
allergenic than other types. Because pine pollen is heavy, it tends to fall straight down
and does not scatter. Therefore, it rarely reaches human noses.
Among North American plants, weeds are the most prolific
producers of allergenic pollen. Ragweed is the major culprit, but others of importance are
sagebrush, redroot pigweed, lamb's quarters, Russian thistle (tumbleweed), and English
plantain.
Grasses and trees, too, are important sources of allergenic
pollens. Although more than 1,000 species of grass grow in North America, only a few
produce highly allergenic pollen. These include timothy, Johnson, Bermuda, redtop,
orchard, sweet vernal, and Kentucky bluegrass. Trees that produce allergenic pollen
include oak, ash, elm, hickory, pecan, box elder, and mountain cedar.
It is common to hear people say that they are allergic to
colorful or scented flowers like roses. In fact, only florists, gardeners, and others who
have prolonged, close contact with flowers are likely to become sensitized to pollen from
these plants. Most people have little contact with the large, heavy, waxy pollen grains of
many flowering plants because this type of pollen is not carried by wind but by insects
such as butterflies and bees.
When do plants make pollen?
One of the most obvious features of pollen allergy is its seasonal nature--people
experience its symptoms only when the pollen grains to which they are allergic are in the
air. Each plant has a pollinating period that is more or less the same from year to year.
Exactly when a plant starts to pollinate seems to depend on the relative length of night
and day--and therefore on geographical location--rather than on the weather. (On the other
hand, weather conditions during pollination can affect the amount of pollen produced and
distributed in a specific year.) Thus, the farther north you go, the later the pollinating
period and the later the allergy season.
A pollen count, which is familiar to many people from local
weather reports, is a measure of how much pollen is in the air. This count represents the
concentration of all the pollen (or of one particular type, like ragweed) in the air in a
certain area at a specific time. It is expressed in grains of pollen per square meter of
air collected over 24 hours. Pollen counts tend to be highest early in the morning on
warm, dry, breezy days and lowest during chilly, wet periods. Although a pollen count is
an approximate and fluctuating measure, it is useful as a general guide for when it is
advisable to stay indoors and avoid contact with the pollen. |
