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Pollen Allergy
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 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 on 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 in all year, such
as dust mites. For these allergy sufferers, the “sneezin’ season” has no
limit. Year-round airborne allergens cause perennial allergic rhinitis,
as distinguished from 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 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 the 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 a highly allergenic pollen. These
include timothy grass, Kentucky bluegrass, Johnson grass, Bermuda grass,
redtop grass, orchard grass, and sweet vernal grass. Tress that produce
allergenic pollen include oak, 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 it symptoms only when the pollen
grains to which they are allergic in the air. Each plant has a
pollinating period that is more or less that 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 during 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 pollen.
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