Any number of allergens present in an indoor environment can contribute to allergic reactions in sensitive people. These include dust mite droppings, mold, animal dander, cockroach debris and more. Dust allergies are among the most common allergic disorders. Dust is made up of tiny particles that include pollen, mold, fabric fibers and dander.

The Most common indoor allergens are:

Dust Mites
Molds (Fungi)
Animals & Insects
Pollens

DUST MITES

Dust mites are microscopic arachnids (simmilar to tiny little spiders). They live indoors, have eight legs, and are blind. The dust mites at the ends of their legs have tiny "sticky" pads. These pads help them firmly attach to fibers, which allows them to live deep within fabrics, upholstery, mattresses and carpeting. Most of the dust mites found in the homes are from the Dermatophagoides family with pteronyssinus and farinae being the most common species. (Dermatophagoides comes from Latin and means skin eating. Pteronyssinus comes from Latin and means feather loving. Farinae is Latin for flour.)
Dust mites thrive in warm, humid places.The ideal temperature for dust mites ranges from 65 to 80 degrees F. Since dust mites have no means of drinking, they are totally dependent on the humidity surrounding them for water. They live best at a relative humidity above 55%.

The dust mites do not bite, spread disease, or actually live on humans but they can still make you allergic. The digestive enzymes that are discharged into the mite feces are the most bothersome of the dust mite allergens. Less potent allergens are found in the mite bodies. The mite's tiny fecal pellets disintegrate to form a very fine powder, which can easily float into the air when disturbed. This commonly occurs during vacuuming, making the bed, turning in bed while sleeping, or walking on the carpet. When an allergic person inhales these particles, asthma or nasal allergy symptoms may occur. There is also evidence that allergic eczema can be aggravated by this exposure.

Each dust mite lives for approximately 30 days and produces about 20 fecal pellets per day. During that time, females may have added 30 new dust mites to the population.

Almost 10% of the population is allergic to dust mites and about 80% of asthmatic children are allergic to dust mites.

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MOLDS (FUNGI)

Molds are organisms that may be found indoors and outdoors. They are part of the natural environment and play an important role in the environment by breaking down and digesting organic material, such as dead leaves. Also called fungi or mildew, molds are neither plants nor animals; they are part of the kingdom Fungi.

Molds can multiply by producing microscopic spores (2 - 100 microns [µm] in diameter), similar to the seeds produced by plants. Many spores are so small they easily float through the air and can be carried for great distances by even the gentlest breezes. The number of mold spores suspended in indoor and outdoor air fluctuates from season to season, day to day, and even hour to hour.
Mold spores are ubiquitous; they are found both indoors and outdoors. Mold spores cannot be eliminated from indoor environments. Some mold spores will be found floating through the air and in settled dust; however, they will not grow if moisture is not present.

Mold is not usually a problem indoors -- unless mold spores land on a wet or damp spot and begin growing. The potential human health effects of mold are also a concern. It is important, therefore, to prevent mold from growing indoors.To grow indoors, mold needs moisture and food. Moisture is the most important factor influencing mold growth indoors. Controlling indoor moisture helps limit mold growth.Moisture control is the key to mold control.Mold does not need a lot of water to grow. A little condensation, in a bathroom or around a window sill, for example, can be enoughBesides moisture, mold needs nutrients, or food, to grow. Mold can grow on virtually any organic substance. Most buildings are full of organic materials that mold can use as food, including paper, cloth, wood, plant material, and even soil. In most cases, temperature is not an issue; some molds grow in warm areas, while others prefer cool locations such as bread stored in a refrigerator. Often, more than one type of mold can be found growing in the same area, although conditions such as moisture, light, and temperature may favor one species of mold over another.

Buildings that have been heavily damaged by flood waters should be assessed for structural integrity and remediated by experienced professionals.
Inhalation exposure to mold indoors can cause health effects in some people. Molds produce allergens (substances that can cause allergic reactions), irritants, and, in some cases, potentially toxic substances or chemicals (mycotoxins). Inhaling or touching mold or mold spores may cause allergic reactions in sensitive individuals. Mold does not have to be alive to cause an allergic reaction. Dead or alive, mold can cause allergic reactions in some people.

Allergic reactions to mold are common and can be immediate or delayed. Repeated or single exposure to mold, mold spores, or mold fragments may cause non-sensitive individuals to become sensitive to mold, and repeated exposure has the potential to increase sensitivity.

As molds grow, some (but not all) of them may produce potentially toxic byproducts called mycotoxins under some conditions. Some of these molds are commonly found in moisture-damaged buildings. More than 200 mycotoxins from common molds have been identified, and many more remain to be identified. The amount and types of mycotoxins produced by a particular mold depends on many environmental and genetic factors. No one can tell whether a mold is producing mycotoxins just by looking at it. Some mycotoxins are known to affect people, but for many mycotoxins little health information is available. Research on mycotoxins is ongoing. Exposure to mycotoxins can occur from inhalation, ingestion, and skin contact. It is prudent to avoid unnecessary inhalation exposure to mold.
Molds come in many colors including white. "Black mold" is not a species or specific kind of mold, and neither is "toxic mold."

Some compounds produced by molds have strong smells and are volatile and quickly released into the air. These compounds are known as microbial volatile organic compounds (mVOCs). Because mVOCs often have strong or unpleasant odors, they can be the source of the "moldy odor" or musty smell frequently associated with mold growth. A moldy odor suggests that mold is growing in the building and should be investigated.

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ANIMALS & INSECTS

Cats

The most well known indoor allergy is probably due to the domestic cat. The major cat allergen is a protein (Fel d); it is found in cat saliva, dander from sebaceous (sweat) glands in the skin, fur, and anal sebaceous glands. The other allergens are found in cat urine, saliva and blood.
The cat allergens are particularly widespread because they are stickier, smaller and lighter than other allergens, which make them easily airborne. As a result, a person could be exposed without being in the presence of cats.

The reason why cat allergens may have a wider impact is that these small airborne particles, which can be a quarter of the size of dust mites, are much more likely to have access to the lung than other allergens, and can be inhaled deep into the lungs.
Cat dander is therefore a common cause of allergic asthma, and cat owners who are allergic to cats are more prone to the development of asthma symptoms.

Even with a past history of tolerance to cats, it is possible for a person with an allergic tendency to develop sensitivity after constant exposure.

Kittens produce less dander than adult cats which is why people with cat allergies, especially children, may be able to tolerate kittens but not adult cats.
Cat ownership varied by country, ranging from as low as 10 percent of participants in Spain to as high as 35 percent among the British.
An estimated 30% of households in North America have at least one cat.
About 6% of the population is allergic to cats.
About 3 million Americans are allergic to cat.

Dogs

Domestic dogs are found in over 40% of homes in North America; about 30% of allergic individuals are sensitive to dog allergen. The major dog allergen is a protein called Can f 1, it is primarily found in dog saliva.
Dog hair itself is not a significant allergen, but it can trap pollen, dust, mold, and other allergens that you can then become exposed to. Regular bathing of the dogs, would be expected to reduce much of the allergens released from the animal.

Allergy to dogs is relatively common, as dog allergen can be found in public places, including daycare facilities, schools and offices.
Recent studies show that kids exposed to two or more indoor pets were half as likely to develop common allergies to such things as dust mites, cats and dogs, grass, ragweed, and molds.

Researchers believe that children living with these animals are probably exposed to higher levels of endotoxins, the breakdown products of Gram-negative bacteria commonly found in the mouth of a cat or dog. Exposure to endotoxins is thought to force the body's immune system to develop a different pattern of response that makes you less likely to become allergic.
In people who have already developed allergy or asthma symptoms exposure to endotoxins may worsen their condition.

Allergies affect approximately 15% of the population, and while all cats and dogs are allergenic to people allergic to animals, cats tend to be more allergenic than dogs. There are no "non-allergenic" breeds of dogs or cats; even hairless breeds may be highly allergenic.

Birds

Allergy to birds is more common among bird breeders where the exposure is highest. People who are sensitive to the feathers of chickens, geese, turkeys, and ducks can still eat their meat or eggs. They may well react, however, to the feathers in down comforters, pillows, and duvets. You should also remember that dust mites hide in these bedding accessories.

Rodents

This intriguing family includes hamsters, rabbits, guinea pigs, gerbils, ferrets, mice, and rats. Most allergic reactions are caused by exposure in laboratories, but these animals are also common pets. Mouse urine is an especially potent allergen for personnel that handle laboratory animals. The urine of rats and guinea pigs also contains allergens as do the saliva and fur of rabbits. Rabbit hair can be found in fur coats, glove linings, slippers, foot muffs, pillows, and quilts. The fur of the Angora rabbit is said to be 10 times warmer than that of sheep wool. The soft yarn spun from Angora rabbit can be found in hand-knitted trimming, crochet work, gloves, hosiery and knee pads. Alone or mixed with silk, it is also used in sportswear. And, of course, rabbits often appear in schools as the classroom pet.

Cockroach

Over the past 25 years, cockroaches have become recognized as a powerful indoor allergen. Cockroach allergy can be a major factor in serious asthma and nasal allergy. Cockroaches tend to be very troublesome in inner city areas, multifamily dwellings, and around areas of food preparation.
Cockroaches are among the oldest of all living species (about 350 million years old). The three species of cockroaches that are commonly found in the North America are Blatella germania , Periploneta americana , and Blatella orientalis .
A recent study has shown that 37% of inner-city children in North America were allergic to cockroaches. The study showed that 50% of the rooms had high levels of cockroach allergen.
There is strong scientific evidence that exposure to cockroach allergens can cause asthma in children and adults.

Asthma is a chronic, inflammatory disease cause by allergic reactions that results in narrowed airways in the lungs. Symptoms range from mild shortness of breath to severe airway obstruction that can result in wheezing or, in about 5,000 cases a year, death.

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POLLENS

Pollen grains are an outrageous invention of the seed plants, which first appeared over 300 million years ago.
Pollen grains contain the male reproductive cells of a plant.
They can come in a wide variety of shapes, sizes, and surface markings characteristic of the species. Most are spherical. The smallest pollen grain is around 6 µm (0.006 mm) in diameter. This transfer can be mediated by the wind, in which case the plant is described as anemophilous (literally wind-loving). Anemophilous plants typically produce great quantities of very lightweight pollen grains, sometimes with air-sacs.
Generally pollens that cause allergies are those of anemophilous plants, which can be carried for great distances and are easily inhaled, bringing it into contact with the sensitive nasal passages.
The chemical composition of pollen is the basic factor that determines whether it is likely to cause allergy.
Allergy to pollen is called hay fever. One of the most obvious features of pollen allergy is its seasonal nature. Most species of pollen have some level of allergenicity but some are particularly notorious for inducing symptoms of hay fever. One of the most allergenic species on an international level is the wind-pollinated ragweed. Most late summer and fall pollen allergies are probably caused by ragweed, a widespread anemophilous plant. It produces a huge amount of pollen - up to 8,000 million pollen grains can be released in just 5 hours from the giant ragweed species. Scientists have collected samples of ragweed pollen 400 miles out at sea and 2 miles high in the air.
Ragweed allergies affected 15 million people in the United States and Canada alone. Although the plant is native to North America, the nuisance appears to be spreading. Researchers say the plant has invaded China, Japan and parts of Australia, and is now moving rapidly across Europe as well.
During the spring months the main problem for hay fever sufferers is tree pollen. Some types such as alder and hazel can be in the air as early as January. These types are followed by elm, willow and ash in March. Pollen from these trees affects only a small percentage of hay fever sufferers but can cause symptoms in some allergic people. The pollen from silver birch trees is the most important tree pollen type for hay fever sufferers and usually occurs in April. This pollen type affects about 25% of hay fever sufferers. The season usually lasts for about four weeks and counts will be high on dry, warm days with some wind in areas where there are birch trees. The birch tree pollen season is followed by the oak tree pollen in late April and early May, although oak affects fewer sufferers than birch.
Trees are the earliest pollen producers, releasing their pollen as early as January in the Southern states and as late as May or June in the Northern states.
Trees can aggravate your allergy whether or not they are on your property, since trees release large amounts of pollen that can be distributed miles away from the original source.
Of the 50,000 different kinds of trees, less than 100 have been shown to cause allergies.
Although the ragweed pollen season runs from August to November, ragweed pollen levels usually peak in mid-September in many areas in the country.
Among North American plants, weeds are the biggest producers of allergenic pollen. Other important sources of allergenic pollen are sagebrush, redroot pigweed, lamb's quarters, Russian thistle (tumbleweed), and English plantain.
The pollen count is high during extended warm, dry periods, and is low when it is rainy and damp. The count also goes up when it is windy.
Daily weather can strongly influence pollen levels. On hot, dry and windy days, pollen is dispersed in greater quantities and stays suspended in the air longer. This increases the suffering of those allergic to the pollen. Rainy and windless days tend to lessen the pollen distribution in the air and lessen the effect of pollen on human health.
It is surprising, considering how frequently outdoor pollens cause allergies, that few indoor plants are troublesome. Indoor plants are more leafy than flowering and do not pollinate as much. Some offending indoor plants are the weeping fig (Ficus benjamina), which can cause eye allergies, and the flowering maple (Alutilon hybridum), which can cause asthma.

Indoor plants, especially those that are kept in damp wicker baskets, are a source of molds. Dried flowers and live Christmas trees should also be avoided, as they are common carriers of molds.

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