Long despised by housekeepers, the lowly cockroach is now being targeted by scientists who are finding it to be one of the most serious indoor causes of asthma and allergies, especially in the inner city.

The cockroach "is a risk factor people haven't considered to date, in part because it's not a socially acceptable topic for discussion. But it's one we have to consider now," said Dr. Robert G. Hamilton of the Johns Hopkins School of Medicine.

Although cockroaches have been accepted as a source of human allergies for the last five years, he said, "only recently have we been able to prove it, and prove to patients that they may have a problem in their own environment."

Without proof, people with cockroach sensitivity tend to react emotionally and deny the problem exists in their homes. That's because, more than almost any other pest, cockroaches elicit feelings of guilt, revulsion and dirtiness.

Now, by testing their homes for cockroach allergens -- chemicals that trigger an allergic reaction -- people can make decisions "in a more rational way" about whether and how to get rid of the pests, Dr. Hamilton said.

This advance has been made possible by new techniques, developed through bioengineering, that allow scientists to isolate, identify and measure the presence of such cockroach allergens.

Using these "immunochemical" techniques, Drs. Hamilton, N. Franklin Adkinson Jr., Peyton A. Eggleston and Martin Chapman at Hopkins say they have proven the previously suspected link between the presence of cockroaches and the development of allergies.

Skin tests on 73 asthmatic children in the Baltimore area found that 49 percent were allergic to cockroach allergens. Of those children, 61 percent were found to be living in homes with cockroach allergens present, enough to establish a cause-and-effect relationship, Dr. Hamilton said.

"People's reactions can range from the very mild -- watery eyes and a runny nose -- to . . . wheezing and respiratory problems that can be severe in some cases," Dr. Hamilton said.

Described by Dr. Hamilton last week at a conference of the American Academy of Allergy and Immunology in Orlando, Fla., the study also has shown how cockroach allergens are distributed across a community, and within a home.

Asthma, afflicting an estimated 10 million Americans, grew by 29 percent between 1980 and 1987, and its death rate climbed 31 percent, according to the National Institutes of Health. Some 4,300 people died from it in 1987 alone. Growth in the incidence of asthma may be due in part to better diagnoses, Dr. Hamilton said.

In asthma attacks, breathing passages are obstructed by the inflammation and swelling of tissues in an allergic response to environmental stimuli. The illness cost Americans more than $4 billion in health care expenditures in 1988, the latest data available.

Only mold spores and the droppings of dust mites -- microscopic spider-like creatures that consume the dead cells of human skin -- rank higher as indoor risks to allergy and asthma sufferers, Dr. Hamilton said. That's chiefly because they exist everywhere.

Cats and dogs are potent sources of allergens, too, but because they are easily removed from the home, they are a less serious problem than mites, mold and cockroaches.

Dr. Hamilton's research focused on two proteins released by German cockroaches (Blatella germanica), the most "cosmopolitan" of eight species common in American homes. "We don't know yet exactly what [the proteins] are, or what their functions are," he said.

University of Virginia scientists say the substances seem to be secreted continually as the cockroaches walk about, perhaps as sex lures, or as a sort of territorial marker. They are not limited to cockroach parts or droppings.

After being released by the cockroaches, the allergens cling to dust particles, which are then blown around the house and inhaled. Once in the human airways, they can touch off allergic reactions among sensitive individuals.

Allergists conduct tests to discover patients' sensitivity to cockroach allergens and can arrange for Hopkins to analyze house dust -- the only way to be verify the presence of cockroach allergens in a home.

To determine where the allergens are most common, Dr. Hamilton and his team collected dust samples from the homes of 79 asthmatic children in Baltimore and Baltimore, Harford, Howard and Prince George's counties.

The samples were then ground up, filtered and subjected to "immunoassay" tests to isolate and measure the various allergens.

They found the cockroach allergens in the homes of 81 percent of the asthmatic children in Baltimore. The presence of the allergens faded to 30 percent in suburban neighborhoods and to 15 percent in rural areas. The average was 30 percent, somewhat higher than the average of 23 percent in a nationwide sampling from 244 homes of people both with and without allergies.

The rural/urban differences, Dr. Hamilton said, "are probably due to the fact that rural homes tend to be more distant from the influence of their neighbors." No matter how hard city rowhouse residents work to keep their homes roach-free, the bugs won't disappear if they are thriving next door.

Within the home, Dr. Hamilton's study found slightly more cockroach allergens in kitchens. However, contamination occurred throughout the house. "It is not isolated in sites where they breed," Dr. Hamilton said.

Have allergies? Here are steps you should take

* If you have allergy symptoms -- watery eyes, runny nose, wheezing -- go to an allergist for skin tests to identify which allergens are causing your problems.

* Determine which of those allergens are present in your home. The Johns Hopkins Division of Allergy and Clinical Immunology performs house dust analyses for allergists. You can call 1-800-344-3224 for information.

* If you are allergic to cockroaches, and the house dust analysis detects cockroach allergens, getting rid of them may be cheaper than medical treatment.

* Cockroaches breed in dark, warm, damp places like the walls of bathrooms, kitchens or basements near central heating systems. Licensed exterminators may be needed to fumigate inaccessible places. Neighbors in rowhouses and other multifamily buildings must cooperate to kill all the roaches in the building.

* Thorough, regular cleaning to remove residual food particles, and control of stored and waste food (including pet food), will deprive roaches of their food sources. Here, too, the cooperation of neighbors is vital.

* Homes with cats have fewer cockroaches, but before you get one, be sure you're not allergic to cats.