People allergic to outdoor molds may have symptoms from spring to late fall.
The mold season often peaks from July to late summer. Unlike pollens, molds
may persist after the first killing frost. Some can grow at subfreezing temperatures,
but most become dormant. Snow cover lowers the outdoor mold count dramatically
but does not kill molds. After the spring thaw, molds thrive on the vegetation
that has been killed by the winter cold.
In the warmest areas of the United States, however, molds thrive all year
and can trigger year-round (perennial) asthma problems. In addition, molds
growing indoors can trigger asthma even in the coldest climates.
What is mold?
There are thousands of types of molds and yeast, the two groups of plants
in the fungus family. Yeasts are single cells that divide to form clusters.
Molds consist of many cells that grow as branching threads called hyphae. Although
both groups can probably cause allergic reactions, only a small number of molds
are widely recognized offenders.
The seeds or reproductive particles of fungi are called spores. They differ
in size, shape, and color among species. Each spore that germinates can give
rise to new mold growth, which in turn can produce millions of spores.
What is mold allergy?
When inhaled, microscopic fungal spores or, sometimes, fragments of fungi
may cause allergic rhinitis. Because they are so small, mold spores may evade
the protective mechanisms of the nose and upper respiratory tract to reach
In a small number of people, symptoms of mold allergy may be brought on or
worsened by eating certain foods, such as cheeses, processed with fungi. Occasionally,
mushrooms, dried fruits, and foods containing yeast, soy sauce, or vinegar
will produce allergic symptoms. There is no known relationship, however, between
a respiratory allergy to the mold Penicillium and an allergy to the drug penicillin,
made from the mold.
Where do molds grow?
Molds can be found wherever there is moisture, oxygen, and a source of the
few other chemicals they need. In the fall they grow on rotting logs and fallen
leaves, especially in moist, shady areas. In gardens, they can be found in
compost piles and on certain grasses and weeds. Some molds attach to grains
such as wheat, oats, barley, and corn, making farms, grain bins, and silos
likely places to find mold.
Hot spots of mold growth in the home include damp basements and closets, bathrooms
(especially shower stalls), places where fresh food is stored, refrigerator
drip trays, house plants, air conditioners, humidifiers, garbage pails, mattresses,
upholstered furniture, and old foam rubber pillows.
Bakeries, breweries, barns, dairies, and greenhouses are favorite places for
molds to grow. Loggers, mill workers, carpenters, furniture repairers, and
upholsterers often work in moldy environments.
Which molds are allergenic?
Like pollens, mold spores are important airborne
allergens only if they are abundant, easily carried by air currents,
and allergenic in their chemical makeup. Found almost everywhere, mold spores
in some areas are so numerous they often outnumber the pollens in the air.
Fortunately, however, only a few dozen different types are significant allergens.
In general, Alternaria and Cladosporium (Hormodendrum) are the
molds most commonly found both indoors and outdoors throughout the United States. Aspergillus, Penicillium, Helminthosporium, Epicoccum, Fusarium, Mucor, Rhizopus,
and Aureobasidium (Pullularia) are also common.
Are mold counts helpful?
Similar to pollen counts, mold counts may suggest the types and relative quantities
of fungi present at a certain time and place. For several reasons, however,
these counts probably cannot be used as a constant guide for daily activities.
One reason is that the number and types of spores actually present in the mold
count may have changed considerably in 24 hours because weather and spore dispersal
are directly related. Many of the common allergenic molds are of the dry spore
type -- they release their spores during dry, windy weather. Other fungi need
high humidity, fog, or dew to release their spores. Although rain washes many
larger spores out of the air, it also causes some smaller spores to be shot
into the air.
In addition to the effect of day-to-day weather changes on mold counts, spore
populations may also differ between day and night. Day favors dispersal by
dry spore types and night favors wet spore types.
Are there other mold-related disorders?
Fungi or microorganisms related to them may cause other health problems similar
to allergic diseases. Some kinds of Aspergillus may cause several different
illnesses, including both infections and allergy. These fungi may lodge in
the airways or a distant part of the lung and grow until they form a compact
sphere known as a "fungus ball." In people with lung damage or serious underlying
illnesses, Aspergillus may grasp the opportunity to invade the lungs
or the whole body.
In some individuals, exposure to these fungi also can lead to asthma or to
a lung disease resembling severe inflammatory asthma called allergic bronchopulmonary
aspergillosis. This latter condition, which occurs only in a minority of people
with asthma, is characterized by wheezing, low-grade fever, and coughing up
of brown-flecked masses or mucus plugs. Skin testing, blood tests, x-rays,
and examination of the sputum for fungi can help establish the diagnosis. Corticosteroid
drugs are usually effective in treating this reaction; immunotherapy (allergy
shots) is not helpful.
Created by the National Institutes of Health. Illustrations copyright A.D.A.M.,
Review Date: 5/16/2007
Reviewed By: Alan Greene, M.D., F.A.A.P., Department of Pediatrics, Packard Children's Hospital, Stanford University School of Medicine; Chief Medical Officer, A.D.A.M., Inc.
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