Asbestos, which can still be found in homes, schools and offices, remains a health threat. Even people who have already been exposed may not find out until the year 2005 or later.

According to the American Lung Association, symptoms of asbestos- induced disease typically do not appear for 15 to 35 years after the initial exposure. After that time, X-rays may be able to detect exposure. In later stages a dry, crackling sound can be detected with a stethoscope when breathing.

"That's one of the dangers of asbestos -- you don't know if you've been exposed," said Dr. Clifford Zwillich, professor of medicine and chief of the divisions of pulmonary and critical care at the University's Hershey Medical Center.

Asbestos is the general name used to describe a group of mineral fibers that are resistant to heat, fire, acids and alkalis. Asbestos fibers are usually combined with a binding compound, but when this compound ages and crumbles, the fibers become airborne and can be inhaled.

Once the needle-like fibers become imbedded in the lungs they remain there for life. As fibers accumulate, they cause chronic irritation and eventually asbestosis.

"Asbestosis, which is lung fibrosis, results from long-term high levels of exposure," Zwillich said.

The stiffening of the lung tissue in fibrosis causes difficulty in breathing. The lungs' reduced oxygen capacity makes it difficult to pump blood through the lungs. The resulting backup of blood will cause swollen ankles and may eventually cause heart failure, he said.

Asbestos exposure may also result in cancer of the lung tissue itself and mesothelioma, a rare cancer of the pleura, or chest lining. The development of mesothelioma, from the time of exposure, may take up to 30 years. Mesothelioma is fatal, Zwillich added.

"Mesothelioma can occur with relatively infrequent and low exposure to asbestos," he said.

According to the American Lung Association, studies have shown that an asbestos worker who smokes is 50 to 70 times more likely to get lung cancer as a non-smoker who has not been exposed.

"Asbestosis and cigarette smoking will cause lung cancer in the majority of people with both factors," Zwillich said.

Of people diagnosed with lung cancer, 80 percent die within five years. The other 20 percent can be cured surgically but that often involves complications, Zwillich said.

During surgery the tumor is removed from the diseased lung. Sometimes only a portion of the diseased lung can be safely removed. The remaining diseased tissue causes shortness of breath, coughing and the inability to get enough oxygen, Zwillich added.

Insulation, construction and other industrial workers exposed to asbestos are at the greatest risk for developing lung cancer. Asbestos- related maladies have also been found in the families of asbestos workers and people living near factories or sites using asbestos, according to the American Lung Association.

The properties of asbestos make it a highly valued construction material. Asbestos is used in a variety of materials including insulation, textured paints, cements, brake linings and pot holders.