DUST MITES, which under a microscope resemble eight-legged sci-fi monsters, can bring on asthma attacks in some people, and asthma sufferers are often advised to buy special mattress and pillow covers -- even high-cost dust-mite sucking vacuum cleaners -- to keep the tiny critters at bay.

They may be wasting their money. "We can conclude with confidence that there is no need to buy expensive vacuum cleaners or mattress covers or to use chemical methods against dust mites, because these treatments do not work," says Dr. Peter Gotzsche, director of the Nordic Cochrane Centre in Copenhagen and lead author of a review, published last week by the Cochrane Collaboration, of 54 studies of 3,000 asthma patients. Even if the cleaning and covering measures reduce exposure to dust mites by half to as much as 90%, the review found, the level of allergens is still high enough in most homes to cause an asthma attack.

Instead, "figure out what kicks off asthma symptoms," says Noreen Clark, director of the Center for Managing Chronic Disease at the University of Michigan. If it's a cat or dog, limit exposure to the pet. And effective use of medicine, Clark says, is the first line of defense against asthma.

susan.brink@latimes.com