Dust Mites: The Unwanted Houseguest

What dustmites are
Where dust mites are found
Health problems associated with dust mites
Decreasing exposure to dust mites

What Dust Mites Are

Dust mites are living, spider-like microscopic insects that can travel through the air and are too small to be seen. They feed off dead skin cells from humans and animals as well as mold spores, so they are almost everywhere. Dust mites, like other biological contaminants, contribute to poor indoor air quality (IAQ) and may be a major cause of days lost from work and school because they can make you sneeze, trigger allergic reactions, cause rashes, watery eyes, coughing, dizziness, lethargy, breathing difficulties, and digestive problems. Exposure to dust mites, animal dander, cat saliva and mold causes about 200,000 emergency-room visits a year for people with asthma.

Where Dust Mites Can Be Found

Dust mites are found everywhere there are humans or animals, warm temperatures, and humidity levels above 60 percent. Dust mites have been reported worldwide, including the continental United States and Hawaii, southern Canada, most of Europe including the Scandinavian countries, Russia, Asia, the Middle East, parts of Australia, South America, and Africa. At least 13 species of dust mites have been found in house dust. Six of these are commonly found throughout the world.

In humid environments, dust mites thrive and produce waste pellets (feces). The dust mite's life spans about 30 days and the female lays approximately one egg each day. In less than ideal conditions, they can go into dormancy. Indoor levels of dust mites are higher than outdoor levels since the conditions are more favorable there with an ample food source. House dust mite populations peak in the summer in humid climates and are lowest in the winter months. Several studies in the US have demonstrated a seasonal rise in dust mite numbers that correlate with increases in humidity.¹ Live dust mites can be seen by microscope when dust from carpet, upholstery, and bed sheets of the infested houses is analyzed. However, the body parts of dead mites and feces, which are most responsible for causing allergic reactions, are difficult to identify. For this reason, the best way to estimate how bad a mite problem is in a residence is to analyze dust samples and look for presence of mite allergens.

Dust mites are commonly found in:

• House dust

• Bedding: There are thousands of dust mites in your bed every night.

• Carpets: Nearly 100,000 dust mites can live in one square yard of carpet.²

• Sofas

• Stuffed chairs

Health Problems Associated with Dust Mites

In 1921 researchers first hypothesized that house dust may contain allergens that may cause asthma episodes. Allergens are biological or chemical substances that can cause the human body to develop a specific immune response with each exposure, even when exposure levels are low. However, house dust was not actually analyzed until 1964 when a group of investigators led by Voohorst suggested that a mite may be responsible for dust allergies.³ The allergens shed by the dust mite are chemicals called glycoproteins, which came from dried body parts and feces. Every single day just one dust mite produces about 20 of these droppings. ² Over a lifetime, a dust mite produces 200 times its body weight in fecal droppings. Dust mite allergy is as high as 25 percent in the humid Southeast and as low as five percent in the drier Mountain states.⁴ Exposure to dust mite allergens may be a major reason why asthma and other allergic diseases are experienced at increasing rates worldwide.

Health problems caused by exposure to dust mites are primarily allergic and include:

• Eye irritation (conjunctivitis)

• Running nose and sneezing (rhinitis)

• Cough and other respiratory problems (asthma)

• Skin problems, such as rash and itching (dermatitis)

Decreasing Exposure to Dust Mites

Because dust mites are everywhere, it is almost impossible to eliminate them or their waste products; however, there are things that you can do to minimize their growth:

• Maintain relative humidity between 30 percent and 50 percent. Dust mites need humid conditions in order to be able to live and thrive. At lower humidity levels, dust mites die of dehydration. In order to keep home humidity levels down, do the following:

  • Repair leaks and seepage. These contribute to a humid environment.

  • Install and use exhaust fans in the kitchen and bathrooms since they also add to indoor humidity levels.

  • Install a dehumidifier, but be sure to empty out the drain pans and disinfect these regularly so that mold will not grow there.

• Maintain indoor temperatures to less than 77° F since dust mites do not reproduce well below this temperature.

• Keep the house clean: dust mites can be reduced, although not eliminated, through regular cleaning. In cleaning, use bleach-based solutions-bleach kills dust mites. Wet mop or wet-wipe hard surfaces such as floors, walls and ceilings. Clean up surface dust as often as possible. Do not use aerosols or spray cleaners in the bedroom. People who are allergic to dust mites should not be the ones doing the cleaning. Proper gloves and other protective equipment should be worn by the person doing the cleaning.

• In humid climates, late winter and early spring are the best times of the year to clean mattresses and carpets very aggressively to kill the few mites that survived the winter. This should reduce the chances of having a large infestation during the summer months.¹ Cleaning mattresses is important since the average mattress will double its weight in 10 years as a result of being filled with dead dust mites and their excrement.

• Allergic individuals should leave the house while it is being vacuumed. Though vacuuming can remove the dust and dust mite feces, and keep them from getting in the air, vacuuming will not actually remove the mites from furniture; the mites attach themselves to upholstery fibers and most vacuums have inadequate suction to pull them away. ¹ Also vacuuming may temporarily increase the concentration of dust mite allergens in the air. A central vacuum-cleaner system is the best option because it exhausts outside. Although there is no clinical proof of the benefits of HEPA bags in vacuum cleaners, they probably are more effective at trapping allergens than regular bags.

• Launder bedding in hot water that is at least 130° F to 140° F. This is one of the most important things you can do to battle dust mites. Cold water will not kill all the dust mites. Sheets and pillowcases should be ideally washed every week. Pillows, comforters, and spreads should be washed every one-to two months. There is no good data about the effectiveness of antimicrobial pillows against dust mites. Replace synthetic pillows every two to three years.

• Replace comforters and pillows made with down feathers, kapok, and cotton with ones made with synthetic fibers such as Dacron and Orlon. Do not use fuzzy wool blankets, feather or wool-stuffed comforters, and feather pillows.

• Use a periodic carpet cleaning system that removes greater than 99 percent of allergens.

• Electric blankets are effective in reducing the relative humidity and the concentration of house dust mites on the surface of mattresses. ¹ However, some people are concerned about the possible health problems that may be caused by the electromagnetic forces given off by electrical objects?especially so close to one's body.

• Encasing mattresses, pillows, and blankets in rubber or vinyl plastic covers is an important strategy in avoiding dust mite allergens.

• Remove stuffed furniture and anything under the bed.

• Do not put bookshelves and other things in the bedroom that gather dust.

• Choose washable stuffed toys, wash them often in hot water, and dry them thoroughly. Keep stuffed toys off beds.

• If possible, avoid basement apartments. Bedrooms should be upstairs.

• Dry cleaning kills all mites and is very effective at physically removing dust from items such as bedding.

• Use washable rugs instead of carpet, on hard-surface, easy-to-clean flooring such as hardwood, linoleum, or terrazzo floors. Wash rugs regularly.

• Closets need extra care. They should hold only the clothing that is needed for that season. Putting clothes in a plastic garment bag may help, but do not use the bags that cover dry cleaning.

• Do not carpet high-humidity areas such as bathrooms and kitchens. In addition, carpeting laid over concrete floors tends to have more dust mites because of increased humidity. Dust mites live deep inside carpet and cannot be removed by vacuuming. If you do have carpeting, use superheated steam to clean it.

• Minimize room décor that collects dust and cannot be washed. Open shelves, fabric wallpaper, knickknacks, draperies, and Venetian blinds are sources of dust mites. Window coverings attract dust. Use window shades or curtains made of plastic or other washable material for easy cleaning.

• Consider using leather-covered furniture

• High-efficiency air filters installed in air-handling systems may help reduce the levels of dust mites. However, many doctors say the filters fill up quickly and may have little impact on dust mites, who are not usually airborne. In addition, air-duct cleaning may not be effective since there is no fabric for dust mites to hide in, few skin scales for them to feed on, and low humidity.

Be wary of products that are designed to kill dust mites because they might have other side effects. On January 13, 2000, some well-known carpet powders and sprays were recalled because of more than 400 incidents from consumers reporting health problems associated with the use of these products. They contained low levels of pesticides to kill dust mites, and people reported having asthma attacks, respiratory problems, burning sensations, and skin irritation from being exposed to this product. The manufacturer said that it was not the active ingredient but the fragrance in the product that caused reactions in some asthma sufferers. However, products likes these only temporarily reduce the dust mite population. The best thing to do is prevent the conditions that make them thrive like high humidity and moisture.

References

1. Fernández-Caldas E, Trudeau WL. Environmental control of indoor biologic agents. J Allergy Clin Immunol. 1994;(issue 2, part 2): 404-412.

2. USA Today. Winter months bring allergen invasion. USA Today. October, 1996;125:2617.

3. Blanco LR. Dust Mites: A Primer. www.ozemail.com.au/~lblanco/0mites.htm

4. Parker-Pope T. Mighty mite isn't easy to eradicate. Wall Street Journal. March 5, 2000.