April 15, 2008 -- You can fight dust mites, but you can't win.
Peter C. Gotzsche, MD, DrMedSci, director of the Nordic Cochrane Center in Copenhagen, Denmark, and colleague H.K. Johansen, MD, DrMedSci, analyzed 54 clinical trials involving more than 3,000 asthma patients.
In each trial, researchers took on house dust mites with physical methods such as mite-proof mattress casings and vacuums, mite-killing chemicals, or a combination of physical and chemical methods to fight the mite.
"We were unable to demonstrate any clinical benefit to mite-sensitive patients with asthma," Gotzsche and Johansen write. "We conclude that the trials of current chemical and physical methods aimed at reducing exposure to house dust mite allergens failed to find an effect."
The finding may come as a surprise to people allergic to dust mites, many of whom have spent -- and are spending -- serious time, money, and effort in their battle against the bugs.
Dust Mites Small in Size, Great in Number
House dust mites fall into the category of things we'd rather not think about. They are as ugly as they are small. And they are very small. Ranging in size from a hundredth to a thousandth of an inch, they're too small to see without magnification.
Like spiders, dust mites are eight-legged arachnids. The good news is they don't bite. The bad news is they do eat the dried, dead skin that sloughs from our bodies every day. It's a rich source of food, as we shed about a fifth of an ounce of skin a day. It makes up about 80% of the dust you see in an indoor sunbeam.
As many as 19,000 dust mites can be found in a ball of dust the weight of a 1 gram paper clip, although the typical dust ball contains only 100 to 500 mites. Contrary to popular belief, dust mites don't live in your heating/air ducts. But they love living in your mattress and in your pillow, where your body provides them not only food but also the moisture and warmth they crave.
Hideous to behold under the microscope, dust mites themselves aren't really a problem. But their shells, their feces, and their corpses break down into potent allergy-causing proteins or allergens. That's why just killing dust mites isn't enough. Dust-mite control means getting rid of dust-mite dust, and -- because you can't totally get rid of them -- keeping dust-mite populations low.
Mite Resistance Not Futile, Allergists Say
Even though researchers were able to significantly reduce dust-mite allergens in several studies, Gotzsche and Johansen note that this did not help people's asthma.
"The explanation that we find most plausible for the lack of effect of the interventions is, therefore, that the methods we have reviewed do not adequately reduce mite antigen levels," they write. "Mite-sensitive asthmatic patients are usually sensitive to other allergens, so that successful elimination of only one allergen may have limited benefit, whatever its success."
That last phrase is key, says allergist Leonard Bielory, MD, director of the asthma research center at UMDNJ-New Jersey Medical School, Newark.
"Getting rid of dust mites alone is not the answer," Bielory tells WebMD. "When we discover that patients have dust-mite allergy, we tell them it is rarely just dust mites."
Allergist Jonathan A. Bernstein, MD, of the University of Cincinnati, bristles at the suggestion that mite resistance is futile for allergic people.
"You have to recognize that people can be sensitive to multiple allergens -- as well as to non-allergic triggers such as odorants, irritating chemicals, tobacco smoke, mildew, and things of that nature," Bernstein tells WebMD. "So these studies with just one or even two or three interventions are fraught with limitations. Just to target dust mites and then to say these interventions don't work is out of context with patients' real lives."
Over time, Bielory and Bernstein insist, reducing allergens in the home and in the office will help patients suffering from dust-mite allergy and asthma. They say reducing dust mites is a good place to start.
How to Fight the Mite
It's really not feasible to permanently rid your house of dust mites, says Greg Baumann, senior scientist for the National Pest Management Association.
"They are here to stay for sure," Baumann tells WebMD. "The pest control industry really does not deal with dust mites because there are so few products out there professionals can use. They are very, very hard to control. You do not have any magic wand to wave."
Entomologist Ron Harrison, PhD, director of training for the pest-control firm Orkin Inc., agrees with Baumann.
"You don't want dust mites in your home, but there may not be techniques that are helpful," Harrison tells WebMD. "We typically don't target dust mites because they are so small and don't fit under Orkin's goals for eradication. But if people ask us to come in, we don't shy away from saying, 'Here are some things that might help.'"
Bernstein says acaricides -- pesticides that target mites -- aren't particularly helpful in reducing dust-mite allergens. Baumann suggests this is because poison simply turns live mites into dead mites, which continue to cause allergies while dust mite populations rebound.
But all of the pest and allergy experts who spoke with WebMD agree that expensive efforts aren't worth the cost.
"People don't need to go out and spend an exorbitant amount of money to do a good intervention," Bernstein says. "It is simply a matter of reasonably good hygiene and what you would do anyway. You would control humidity and vacuum and tidy up the room and not have a lot of dust-gathering collectibles and hang things up and put things in drawers."
"Just taking better care of yourself takes care of your allergy," Bielory says. "You keep yourself in a cleaner, healthier environment, and your allergy will improve."
Each of the experts disagrees over details. Bernstein says that mite-proof mattress and pillow covers are a good idea. Bielory says these products make the bed uncomfortable and that poorer sleep is a factor that makes allergy worse. But here's a list of what many experts say will -- by and large -- reduce your home exposure to dust-mite allergens:
- Get white sheets and pillowcases for your bed and wash them every week in very hot water. Wash the mattress pads and blankets, too.
- Vacuum regularly, but not within two hours of bedtime. A HEPA filter isn't necessary and may not help. If you're allergic to dust mites, wear a dust mask while vacuuming or get out of the house while someone else vacuums. Vacuums raise residual dust, and you should wait for it to settle.
- Remove stuffed animals and dusty clutter from the bedroom.
- Remove upholstered furniture from the bedroom and from other rooms in which you spend a lot of time.
- Get a dehumidifier. Dust mites love humidity. Keeping humidity in the 30% to 50% range helps control dust mites.
- Consider putting mite-blocking covers on your mattress and pillows.
- Consider removing fabric window coverings and replacing them with plastic ones.
- Get rid of tapestries or fabric wall hangings.
- Consider replacing carpeting with tile or wooden floors.
- Do not hire a duct-cleaning service. Dust mites do not live in working heating and air-conditioning ducts.
- Clean the air filters on your furnace/air conditioner at least once a month.
Gotzsche and Johansen argue that these steps are not proven to help asthma.
"Reviews and guidelines do not reflect the fact that measures designed to reduce the patients' exposure to mite antigen in the home are ineffective," they write. "The recently published, very extensive  U.S. guidelines for asthma control were also misleading. ... Reviews and guidelines should reflect the facts."
But Bielory says patients should not give up.
"Patients need to be empowered -- they can't just take a pill and have their allergy go away," he says. "If you clean your home, even if it is a placebo effect, it is something positive. People feel better because they are doing something."
The Gotzsche review appears in the April 16 issue of Cochrane Database of Systematic Review.