Allergens & Asthma

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Dust Mites
Furry Pets
Molds & Fungus


How were household allergens discovered?

It had been known since the turn of the century that house dust made people allergic. What puzzled scientists was how could dust found in different people's houses, from different parts of the country, give similar kinds of allergic reactions? At first, it was thought that chemical reactions occurring in the dust produced "the house dust allergen". Scientists in the 1940's and 50's tried to isolate the allergen from vacuum cleaner bags of house dust!

This puzzle was finally solved in the 1960's when two Dutch scientists examined house dust samples under the microscope. They discovered that the dust contained lots of microscopic creatures, which were found to be house dust mites. The scientists (Drs. Voorhorst and Spieksma) found that allergic reactions were related to the numbers of mites in the house dust. Dutch patients had symptoms most commonly in the fall when there were large numbers of mites in their houses. Later, house dust mites were found in countries all over Europe, in the United States, Australia, Japan and all over the world. In all these countries, it was found that asthma patients were often allergic to house dust mites.

At about the same time that mites were discovered, an allergist working in New York found that several of his asthma patients were allergic to cockroaches. Similar patients were found in Boston, Chicago and other cities on the East coast. When cockroach allergen was blown into the patient's lungs in a allergy clinic, they developed asthma-like symptoms. To complete the story, it had been known since the 1920's that allergic reactions could be caused by animals such as cats, dogs, and horses. By the 1980's, it was clear that house dust allergen was biological, and not chemical, in origin. House dust allergy, therefore, is caused by biologic contaminants or pollutants in the home.


Allergic reactions usually develop in early childhood. Typically, children first become allergic to foods such as milk and egg. In the second and third years of life, they begin to develop allergic reactions to indoor allergens and also to outdoor allergens, such as grass, tree and ragweed pollens. The reason why some individuals develop nasal symptoms whereas others develop asthma or skin diseases are not very well understood. Some people can develop an allergic reaction to indoor allergens and yet not have any clinical symptoms (they are asymptomatic). But many children and adults who develop asthma are more likely to have asthma if they have allergic reactions to indoor allergens.

Having the allergic reaction is a risk factor for asthma in the same way that high levels of cholesterol increase the risk of heart disease and smoking increases the risk of getting lung cancer. The risks of developing asthma if you are exposed to indoor allergens are comparable to these well known risk factors for other diseases. Therefore, it is important to try to reduce allergen levels in the home. First, to prevent people becoming allergic and, second, to reduce the symptoms of people who are already sensitive to the allergen.

Dust Mites

House dust mites are 8 legged microscopic creatures that are closely related to spiders and ticks (they are not insects). The scientific order to which they belong is the Acari and the study of mites is called acarology.

Dust mites are about 1/3 of a millimeter long. They are barely visible to the naked eye but can be seen with a low power microscope. Mites develop from eggs through various larval stages and become adult at 20-30 days. Adult mites live for 8-10 weeks.

The main species of dust mites are called Dermatophagoides pteronyssinus (which means "skin eating feather mite") and Dermatophagoides farinae (the "flour mite," so called because it infests stored foods and grains). House dust mites are designed to live with humans. They feed mainly on human skin scales but can also feed on animal skin scales and debris found in dust. Humans shed approximately 5 grams of skin scales per week, which is enough to feed many thousands of mites! Mites thrive at temperatures 70-72°F and a relatively humidity of 75%. These warm humid conditions are exactly the same as those favored by most humans.

And guess what - mites love to sleep with you! Large populations of mites are found in beds, pillows, bedding (blankets, comforters etc.) and bedroom carpets. Even teddy and other soft toys are good homes for house dust mites. Fitted carpets and soft furnishings (sofas and chairs) are other common sites of mite infestation. Mites burrow down into carpet pile and into padded furniture. Carpets fitted onto concrete slabs in basements often become damp and harbor large numbers of mites.

To assess the level of mite infestation, acarologists count mites present in a house dust sample. A low level is less than 20 mites per gram of dust. Allergies develop when people are exposed to approximate 100 mites per gram (or more). Heavy mite infestation is greater than 500 mites per gram dust. Allergic individuals are likely to have symptoms if they are continually exposed to dust containing more than 500 mites/g. Some highly sensitive patients may have symptoms when exposed to dust with lower mite counts.

Approximately 90-95% of the mites found in house dust belong to the Dermatophagoides group. D. pteronyssinus prefers a more humid environment than D. farinae. In drier parts of the U.S., D. farinae predominates. These include areas such as Northeastern States and the Midwest (Ohio Valley). In moist humid areas, such as Washington State, the Southern States and Florida, one finds large numbers of D. pteronyssinus. From an allergic point of view both species produce similar allergens and both of these mites are used to diagnose and treat allergic reactions. There are two other species that are also found in house dust. Blomia tropicalis is found in tropical and subtropical areas of the world: Central and South America, Singapore and S.E. Asia. In the US, B. tropicalis is found in Florida, Texas and Southern California.

Another species that can also occur in house dust is Euroglyphyus maynei, which closely related to Dermatophagoides mites.

House dust mites themselves are too large to become airborne. They cannot be inhaled into the nose or into the lungs. However, mites produce many fecal particles that are about the same size as pollen grains. The feces are about 10 to 40 microns in diameter. During domestic activity, the fecal particles are disturbed, become airborne, and can be inhaled by people living in the home. When these particles enter the nose or the lungs, the allergens they contain are rapidly released.

Mites produce about 20-30 fecal particles a day and these can accumulate in large amounts in the home. Continuously inhaling small amounts of highly concentrated allergen may explain why mites cause asthma.

Ventia™ is a home-based rapid allergen test for detection of house dust mite allergen which is designed for use by allergic patients and other consumers. The Ventia™ kit enables the presence of mite allergen to be detected in a dust extract within ten minutes.  Order Ventia™.


Cockroaches are arguably the oldest insects on the earth. Fossil records indicate that cockroaches were present from 30 million to 300 million years ago. They are highly adaptable insects and are found in many different environments on the earth. Cockroaches are well known as household pests. However, it is important to note that the main disease associated with cockroaches in houses is asthma. People living in cockroach infested houses frequently develop asthma. This is the main public health problem associated with cockroaches.

There are over 50 cockroach species in the US, but the species that are most important in terms of allergy are the German cockroach (Blattella germanica) and the American cockroach (Periplaneta americana).

German cockroaches are about 3/4 of an inch long. They reproduce more rapidly than American cockroaches and cause most allergic reactions. Cockroach infestation develops in poor quality, substandard housing, with lots of cracks and crevices in the walls, and gaps behind baseboards, where cockroaches can hide. Typically, cockroaches are found in kitchens when food is left out and where they obtain water from leaking faucets or sinks. Cockroach allergies were first discovered in New York and in large metropolitan cities such as Detroit, Boston, Chicago, Baltimore and Washington DC.

In the US, asthma has increased among people living in urban or inner city areas, who are more likely to live in housing that contains cockroaches. Over the past 10-20 years, the increase in asthma has occurred particularly among African American and Hispanic families living in this kind of housing. However, cockroach allergy is not confined to inner city areas, but occurs wherever substandard housing permits cockroach infestation.

Cockroaches have a strong characteristic odor which is apparent in heavily infested houses. As with dust mites, cockroach allergens are secreted by cockroaches in the feces. Cockroaches also produce secretions that may contain allergens. Cockroach feces are larger than those produced by dust mites. The allergen could be inhaled as fragments of feces or as "flakes" of secretions. Cockroach secretions may become attached to dust particles, or may form small droplets that become airborne and are inhaled by individuals living in cockroach infested apartments or homes.

Furry Pets

Most people know whether they are allergic to cats or dogs because if they go into houses that contain these pets they have allergy symptoms. The cat allergen is produced in the sebaceous glands of cat skin and in saliva. This allergen becomes coated onto the fur and is secreted by cats into the house in large quantities. Similarly, dog allergen is produced in saliva, becomes coated on the fur and is distributed around the house. There are approximately 55 million cats and the same number of dogs in the US Not surprisingly, it is difficult to avoid these allergens! Approximately 5% of the population is allergic to cats or dogs.

Other small furry mammals also cause allergic reactions. Rats and mice often cause allergic reactions in animal handlers working in laboratories. These reactions can be severe, and may require that the individual stop working with these animals because they develop asthma. House pets such as rabbits, guinea pigs and gerbils can occasionally cause allergies but cats and dogs are the most common cause of animal allergies. Cat and dog allergens are present on hair and skin flakes (or dander) and in furnishings. Large quantities of cat and dog allergens accumulate in the baskets, bedding and blankets used by the pets.

The amount of cat or dog allergen in the air can be very high and the allergen stays airborne for several hours. That is the reason why allergic patients have symptoms immediately on entering a house which contains a cat or dog. It has been estimated that a cat carries about 100 milligrams of allergen on its coat and sheds this allergen at a rate of about 0.1 milligram per day (a milligram is one thousandth of a gram). The allergen accumulates in carpets and sofas, on beds, in rugs and in chairs. The concentration of cat or dog allergen in dust from houses that contain cats or dogs ranges from 0.1 - 3 milligrams per gram of dust. To put these allergen levels in perspective, an allergic person can react to much less than one millionth of a gram of cat allergen present in the air.

Animal allergen contaminates the environment quite effectively. Cat allergen is regularly found in houses that do not contain cats. This suggests that the allergen is transported from house to house, most likely on people's clothes. There are certainly patients allergic to cats who have symptoms simply by sitting or standing next to someone who lives in a house with several cats. Reports of patients who developed cat allergy but who never kept a cat are quite common. These patients probably became allergic by visiting homes that contained cats or by being exposed to cat allergen in public places (schools, day nurseries, offices, etc.).

This makes avoidance of cat and, to a lesser extent, dog allergen quite difficult. For both cats and dogs, the allergen seems to be produced by many different strains and species. Patients often say that they do not get symptoms when they are exposed to their own cat, but do when they visit other houses which contain different cats. So far there is no scientific basis for explaining these kinds of observations.

Molds & Fungus

House dust from most homes contains mold or fungal spores. In damp houses, the spores germinate and the mold grows on walls, carpets and furnishings. Mold growth produces more spores which contaminate the air. Damp basements, kitchens and bathrooms are usual sites of mold growth.

Fewer people are allergic to mold than to other indoor allergens. In the US, the fungi that most often cause allergy are Alternaria, Cladosporium, Aspergillus and Penicillium. There are many other species that have been reported to cause allergies and allergists usually test patients using a mixture of molds. Alternaria sensitivity is common in arid regions of the country, such as the Midwest and desert regions of Arizona, Nevada, and California. Both Alternaria and Cladosporium occur at high levels in outdoor air and it is likely that most patients are sensitized following outdoor exposure.

Alternaria spores are large and elongated (about the same size as pollen grains), but most fungal spores are small and are readily breathed into the lungs. Allergic reactions to fungi usually cause rhinitis or asthma. Some fungi grow inside the lungs and cause other kinds of allergic diseases, such as aspergillosis, but this is rare. Infection of the sinuses by fungi can also occur and cause allergic fungal sinusitis, but again this condition is rare.

It is important to distinguish fungi that cause allergic reactions from fungi that may cause other illnesses. Fungi such as Stachybotrys produce certain kinds of toxins, but these species do not cause allergies. Allergies to fungi are complicated because there are many potential fungal allergens and different types of allergic and non-allergic symptoms. It may be difficult to decide which of the fungi present in the home cause the allergy. Your allergist can help answer these questions.
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