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Something in the Air: Airborne
Allergens
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 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
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 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 grass, Kentucky bluegrass, Johnson grass, Bermuda
grass, redtop grass, orchard grass, and sweet vernal grass.
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 it 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.
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